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<aside>April 17, 2025</aside>
<a href="https://gto76.github.io" rel="author">Jure Šorn</a>
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<div><h1 id="comprehensivepythoncheatsheet">Comprehensive Python Cheatsheet</h1><p class="banner"><sup><a href="https://raw.githubusercontent.com/gto76/python-cheatsheet/main/README.md">Download text file</a>, <a href="https://github.com/gto76/python-cheatsheet">Fork me on GitHub</a>, <a href="https://github.com/gto76/python-cheatsheet/wiki/Frequently-Asked-Questions">Check out FAQ</a> or <a href="index.html?theme=dark3">Switch to dark theme</a>.
</sup></p><p class="banner" style="margin-bottom: 20px; padding-bottom: 7px;"><img src="web/image_888.jpeg" alt="Monty Python"></p><script>
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</script><pre style="border-left: none;padding-left: 1.9px;"><code class="hljs bash" style="line-height: 1.327em;"><strong>ToC</strong> = {
<strong><span class="hljs-string"><span class="hljs-string">'1. Collections'</span></span></strong>: [<a href="#list">List</a>, <a href="#dictionary">Dictionary</a>, <a href="#set">Set</a>, <a href="#tuple">Tuple</a>, <a href="#range">Range</a>, <a href="#enumerate">Enumerate</a>, <a href="#iterator">Iterator</a>, <a href="#generator">Generator</a>],
<strong><span class="hljs-string"><span class="hljs-string">'2. Types'</span></span></strong>: [<a href="#type">Type</a>, <a href="#string">String</a>, <a href="#regex">Regular_Exp</a>, <a href="#format">Format</a>, <a href="#numbers">Numbers</a>, <a href="#combinatorics">Combinatorics</a>, <a href="#datetime">Datetime</a>],
<strong><span class="hljs-string"><span class="hljs-string">'3. Syntax'</span></span></strong>: [<a href="#function">Function</a>, <a href="#inline">Inline</a>, <a href="#imports">Import</a>, <a href="#decorator">Decorator</a>, <a href="#class">Class</a>, <a href="#ducktypes">Duck_Type</a>, <a href="#enum">Enum</a>, <a href="#exceptions">Except</a>],
<strong><span class="hljs-string"><span class="hljs-string">'4. System'</span></span></strong>: [<a href="#exit">Exit</a>, <a href="#print">Print</a>, <a href="#input">Input</a>, <a href="#commandlinearguments">Command_Line_Arguments</a>, <a href="#open">Open</a>, <a href="#paths">Path</a>, <a href="#oscommands">OS_Commands</a>],
<strong><span class="hljs-string"><span class="hljs-string">'5. Data'</span></span></strong>: [<a href="#json">JSON</a>, <a href="#pickle">Pickle</a>, <a href="#csv">CSV</a>, <a href="#sqlite">SQLite</a>, <a href="#bytes">Bytes</a>, <a href="#struct">Struct</a>, <a href="#array">Array</a>, <a href="#memoryview">Memory_View</a>, <a href="#deque">Deque</a>],
<strong><span class="hljs-string"><span class="hljs-string">'6. Advanced'</span></span></strong>: [<a href="#operator">Operator</a>, <a href="#matchstatement">Match_Stmt</a>, <a href="#logging">Logging</a>, <a href="#introspection">Introspection</a>, <a href="#threading">Threading</a>, <a href="#coroutines">Coroutines</a>],
<strong><span class="hljs-string"><span class="hljs-string">'7. Libraries'</span></span></strong>: [<a href="#progressbar">Progress_Bar</a>, <a href="#plot">Plot</a>, <a href="#table">Table</a>, <a href="#consoleapp">Console_App</a>, <a href="#guiapp">GUI</a>, <a href="#scraping">Scraping</a>, <a href="#webapp">Web</a>, <a href="#profiling">Profile</a>],
<strong><span class="hljs-string"><span class="hljs-string">'8. Multimedia'</span></span></strong>: [<a href="#numpy">NumPy</a>, <a href="#image">Image</a>, <a href="#animation">Animation</a>, <a href="#audio">Audio</a>, <a href="#synthesizer">Synthesizer</a>, <a href="#pygame">Pygame</a>, <a href="#pandas">Pandas</a>, <a href="#plotly">Plotly</a>]
}
</code></pre></div>
<div><h2 id="main"><a href="#main" name="main">#</a>Main</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">if</span> __name__ == <span class="hljs-string">'__main__'</span>: <span class="hljs-comment"># Skips next line if file was imported.</span>
main() <span class="hljs-comment"># Runs `def main(): ...` function.</span>
</code></pre></div>
<div><h2 id="list"><a href="#list" name="list">#</a>List</h2><pre><code class="python language-python hljs"><list> = [<el_1>, <el_2>, ...] <span class="hljs-comment"># Creates new list. Also list(<collection>).</span>
</code></pre></div>
<pre><code class="python language-python hljs"><el> = <list>[index] <span class="hljs-comment"># First index is 0. Last -1. Allows assignments.</span>
<list> = <list>[<slice>] <span class="hljs-comment"># Also <list>[from_inclusive : to_exclusive : ±step].</span>
</code></pre>
<pre><code class="python language-python hljs"><list>.append(<el>) <span class="hljs-comment"># Appends element to the end. Also <list> += [<el>].</span>
<list>.extend(<collection>) <span class="hljs-comment"># Appends elements to the end. Also <list> += <coll>.</span>
</code></pre>
<pre><code class="python language-python hljs"><list>.sort() <span class="hljs-comment"># Sorts elements in ascending order.</span>
<list>.reverse() <span class="hljs-comment"># Reverses the list in-place.</span>
<list> = sorted(<collection>) <span class="hljs-comment"># Returns new list with sorted elements.</span>
<iter> = reversed(<list>) <span class="hljs-comment"># Returns reversed iterator of elements.</span>
</code></pre>
<pre><code class="python language-python hljs"><el> = max(<collection>) <span class="hljs-comment"># Returns largest element. Also min(<el_1>, ...).</span>
<num> = sum(<collection>) <span class="hljs-comment"># Returns sum of elements. Also math.prod(<coll>).</span>
</code></pre>
<pre><code class="python language-python hljs">elementwise_sum = [sum(pair) <span class="hljs-keyword">for</span> pair <span class="hljs-keyword">in</span> zip(list_a, list_b)]
sorted_by_second = sorted(<collection>, key=<span class="hljs-keyword">lambda</span> el: el[<span class="hljs-number">1</span>])
sorted_by_both = sorted(<collection>, key=<span class="hljs-keyword">lambda</span> el: (el[<span class="hljs-number">1</span>], el[<span class="hljs-number">0</span>]))
flatter_list = list(itertools.chain.from_iterable(<list>))
</code></pre>
<ul>
<li><strong>For details about sort(), sorted(), min() and max() see <a href="#sortable">Sortable</a>.</strong></li>
<li><strong>Module <a href="#operator">operator</a> has function itemgetter() that can replace listed <a href="#lambda">lambdas</a>.</strong></li>
<li><strong>This text uses the term collection instead of iterable. For rationale see <a href="#collection">Collection</a>.</strong></li>
</ul>
<pre><code class="python language-python hljs"><int> = len(<list>) <span class="hljs-comment"># Returns number of items. Also works on dict, set and string.</span>
<int> = <list>.count(<el>) <span class="hljs-comment"># Returns number of occurrences. Also `if <el> in <coll>: ...`.</span>
<int> = <list>.index(<el>) <span class="hljs-comment"># Returns index of the first occurrence or raises ValueError.</span>
<el> = <list>.pop() <span class="hljs-comment"># Removes and returns item from the end or at index if passed.</span>
<list>.insert(<int>, <el>) <span class="hljs-comment"># Inserts item at index and moves the rest to the right.</span>
<list>.remove(<el>) <span class="hljs-comment"># Removes first occurrence of the item or raises ValueError.</span>
<list>.clear() <span class="hljs-comment"># Removes all items. Also works on dictionary and set.</span>
</code></pre>
<div><h2 id="dictionary"><a href="#dictionary" name="dictionary">#</a>Dictionary</h2><pre><code class="python language-python hljs"><dict> = {key_1: val_1, key_2: val_2, ...} <span class="hljs-comment"># Use `<dict>[key]` to get or set the value.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><view> = <dict>.keys() <span class="hljs-comment"># Collection of keys that reflects changes.</span>
<view> = <dict>.values() <span class="hljs-comment"># Collection of values that reflects changes.</span>
<view> = <dict>.items() <span class="hljs-comment"># Coll. of key-value tuples that reflects chgs.</span>
</code></pre>
<pre><code class="python language-python hljs">value = <dict>.get(key, default=<span class="hljs-keyword">None</span>) <span class="hljs-comment"># Returns default if key is missing.</span>
value = <dict>.setdefault(key, default=<span class="hljs-keyword">None</span>) <span class="hljs-comment"># Returns and writes default if key is missing.</span>
<dict> = collections.defaultdict(<type>) <span class="hljs-comment"># Returns a dict with default value `<type>()`.</span>
<dict> = collections.defaultdict(<span class="hljs-keyword">lambda</span>: <span class="hljs-number">1</span>) <span class="hljs-comment"># Returns a dict with default value 1.</span>
</code></pre>
<pre><code class="python language-python hljs"><dict> = dict(<collection>) <span class="hljs-comment"># Creates a dict from coll. of key-value pairs.</span>
<dict> = dict(zip(keys, values)) <span class="hljs-comment"># Creates a dict from two collections.</span>
<dict> = dict.fromkeys(keys [, value]) <span class="hljs-comment"># Creates a dict from collection of keys.</span>
</code></pre>
<pre><code class="python language-python hljs"><dict>.update(<dict>) <span class="hljs-comment"># Adds items. Replaces ones with matching keys.</span>
value = <dict>.pop(key) <span class="hljs-comment"># Removes item or raises KeyError if missing.</span>
{k <span class="hljs-keyword">for</span> k, v <span class="hljs-keyword">in</span> <dict>.items() <span class="hljs-keyword">if</span> v == value} <span class="hljs-comment"># Returns set of keys that point to the value.</span>
{k: v <span class="hljs-keyword">for</span> k, v <span class="hljs-keyword">in</span> <dict>.items() <span class="hljs-keyword">if</span> k <span class="hljs-keyword">in</span> keys} <span class="hljs-comment"># Filters the dictionary by keys.</span>
</code></pre>
<div><h3 id="counter">Counter</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> Counter
<span class="hljs-meta">>>> </span>counter = Counter([<span class="hljs-string">'blue'</span>, <span class="hljs-string">'blue'</span>, <span class="hljs-string">'blue'</span>, <span class="hljs-string">'red'</span>, <span class="hljs-string">'red'</span>])
<span class="hljs-meta">>>> </span>counter[<span class="hljs-string">'yellow'</span>] += <span class="hljs-number">1</span>
<span class="hljs-meta">>>> </span>print(counter.most_common())
[(<span class="hljs-string">'blue'</span>, <span class="hljs-number">3</span>), (<span class="hljs-string">'red'</span>, <span class="hljs-number">2</span>), (<span class="hljs-string">'yellow'</span>, <span class="hljs-number">1</span>)]
</code></pre></div>
<div><h2 id="set"><a href="#set" name="set">#</a>Set</h2><pre><code class="python language-python hljs"><set> = {<el_1>, <el_2>, ...} <span class="hljs-comment"># Use `set()` for empty set.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><set>.add(<el>) <span class="hljs-comment"># Or: <set> |= {<el>}</span>
<set>.update(<collection> [, ...]) <span class="hljs-comment"># Or: <set> |= <set></span>
</code></pre>
<pre><code class="python language-python hljs"><set> = <set>.union(<coll.>) <span class="hljs-comment"># Or: <set> | <set></span>
<set> = <set>.intersection(<coll.>) <span class="hljs-comment"># Or: <set> & <set></span>
<set> = <set>.difference(<coll.>) <span class="hljs-comment"># Or: <set> - <set></span>
<set> = <set>.symmetric_difference(<coll.>) <span class="hljs-comment"># Or: <set> ^ <set></span>
<bool> = <set>.issubset(<coll.>) <span class="hljs-comment"># Or: <set> <= <set></span>
<bool> = <set>.issuperset(<coll.>) <span class="hljs-comment"># Or: <set> >= <set></span>
</code></pre>
<pre><code class="python language-python hljs"><el> = <set>.pop() <span class="hljs-comment"># Raises KeyError if empty.</span>
<set>.remove(<el>) <span class="hljs-comment"># Raises KeyError if missing.</span>
<set>.discard(<el>) <span class="hljs-comment"># Doesn't raise an error.</span>
</code></pre>
<div><h3 id="frozenset">Frozen Set</h3><ul>
<li><strong>Is immutable and hashable.</strong></li>
<li><strong>That means it can be used as a key in a dictionary or as an element in a set.</strong></li>
</ul><pre><code class="python language-python hljs"><frozenset> = frozenset(<collection>)
</code></pre></div>
<div><h2 id="tuple"><a href="#tuple" name="tuple">#</a>Tuple</h2><p><strong>Tuple is an immutable and hashable list.</strong></p><pre><code class="python language-python hljs"><tuple> = () <span class="hljs-comment"># Empty tuple.</span>
<tuple> = (<el>,) <span class="hljs-comment"># Or: <el>,</span>
<tuple> = (<el_1>, <el_2> [, ...]) <span class="hljs-comment"># Or: <el_1>, <el_2> [, ...]</span>
</code></pre></div>
<div><h3 id="namedtuple">Named Tuple</h3><p><strong>Tuple's subclass with named elements.</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> namedtuple
<span class="hljs-meta">>>> </span>Point = namedtuple(<span class="hljs-string">'Point'</span>, <span class="hljs-string">'x y'</span>)
<span class="hljs-meta">>>> </span>p = Point(<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>); p
Point(x=<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>)
<span class="hljs-meta">>>> </span>p[<span class="hljs-number">0</span>]
<span class="hljs-number">1</span>
<span class="hljs-meta">>>> </span>p.x
<span class="hljs-number">1</span>
<span class="hljs-meta">>>> </span>getattr(p, <span class="hljs-string">'y'</span>)
<span class="hljs-number">2</span>
</code></pre></div>
<div><h2 id="range"><a href="#range" name="range">#</a>Range</h2><p><strong>Immutable and hashable sequence of integers.</strong></p><pre><code class="python language-python hljs"><range> = range(stop) <span class="hljs-comment"># range(to_exclusive)</span>
<range> = range(start, stop) <span class="hljs-comment"># range(from_inclusive, to_exclusive)</span>
<range> = range(start, stop, ±step) <span class="hljs-comment"># range(from_inclusive, to_exclusive, ±step_size)</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[i <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">3</span>)]
[<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>]
</code></pre>
<div><h2 id="enumerate"><a href="#enumerate" name="enumerate">#</a>Enumerate</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">for</span> i, el <span class="hljs-keyword">in</span> enumerate(<coll>, start=<span class="hljs-number">0</span>): <span class="hljs-comment"># Returns next element and its index on each pass.</span>
...
</code></pre></div>
<div><h2 id="iterator"><a href="#iterator" name="iterator">#</a>Iterator</h2><pre><code class="python language-python hljs"><iter> = iter(<collection>) <span class="hljs-comment"># `iter(<iter>)` returns unmodified iterator.</span>
<iter> = iter(<function>, to_exclusive) <span class="hljs-comment"># A sequence of return values until 'to_exclusive'.</span>
<el> = next(<iter> [, default]) <span class="hljs-comment"># Raises StopIteration or returns 'default' on end.</span>
<list> = list(<iter>) <span class="hljs-comment"># Returns a list of iterator's remaining elements.</span>
</code></pre></div>
<div><h3 id="itertools">Itertools</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> itertools <span class="hljs-keyword">as</span> it
</code></pre></div>
<pre><code class="python language-python hljs"><iter> = it.count(start=<span class="hljs-number">0</span>, step=<span class="hljs-number">1</span>) <span class="hljs-comment"># Returns updated value endlessly. Accepts floats.</span>
<iter> = it.repeat(<el> [, times]) <span class="hljs-comment"># Returns element endlessly or 'times' times.</span>
<iter> = it.cycle(<collection>) <span class="hljs-comment"># Repeats the sequence endlessly.</span>
</code></pre>
<pre><code class="python language-python hljs"><iter> = it.chain(<coll>, <coll> [, ...]) <span class="hljs-comment"># Empties collections in order (figuratively).</span>
<iter> = it.chain.from_iterable(<coll>) <span class="hljs-comment"># Empties collections inside a collection in order.</span>
</code></pre>
<pre><code class="python language-python hljs"><iter> = it.islice(<coll>, to_exclusive) <span class="hljs-comment"># Only returns first 'to_exclusive' elements.</span>
<iter> = it.islice(<coll>, from_inc, …) <span class="hljs-comment"># `to_exclusive, +step_size`. Indices can be None.</span>
</code></pre>
<div><h2 id="generator"><a href="#generator" name="generator">#</a>Generator</h2><ul>
<li><strong>Any function that contains a yield statement returns a generator.</strong></li>
<li><strong>Generators and iterators are interchangeable.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">count</span><span class="hljs-params">(start, step)</span>:</span>
<span class="hljs-keyword">while</span> <span class="hljs-keyword">True</span>:
<span class="hljs-keyword">yield</span> start
start += step
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>counter = count(<span class="hljs-number">10</span>, <span class="hljs-number">2</span>)
<span class="hljs-meta">>>> </span>next(counter), next(counter), next(counter)
(<span class="hljs-number">10</span>, <span class="hljs-number">12</span>, <span class="hljs-number">14</span>)
</code></pre>
<div><h2 id="type"><a href="#type" name="type">#</a>Type</h2><ul>
<li><strong>Everything is an object.</strong></li>
<li><strong>Every object has a type.</strong></li>
<li><strong>Type and class are synonymous.</strong></li>
</ul><pre><code class="python language-python hljs"><type> = type(<el>) <span class="hljs-comment"># Or: <el>.__class__</span>
<bool> = isinstance(<el>, <type>) <span class="hljs-comment"># Or: issubclass(type(<el>), <type>)</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>type(<span class="hljs-string">'a'</span>), <span class="hljs-string">'a'</span>.__class__, str
(<<span class="hljs-class"><span class="hljs-title">class</span> '<span class="hljs-title">str</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">str</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">str</span>'>)
</span></code></pre>
<div><h4 id="sometypesdonothavebuiltinnamessotheymustbeimported">Some types do not have built-in names, so they must be imported:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> types <span class="hljs-keyword">import</span> FunctionType, MethodType, LambdaType, GeneratorType, ModuleType
</code></pre></div>
<div><h3 id="abstractbaseclasses">Abstract Base Classes</h3><p><strong>Each abstract base class specifies a set of virtual subclasses. These classes are then recognized by isinstance() and issubclass() as subclasses of the ABC, although they are really not. ABC can also manually decide whether or not a specific class is its virtual subclass, usually based on which methods the class has implemented. For instance, Iterable ABC looks for method iter(), while Collection ABC looks for iter(), contains() and len().</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> collections.abc <span class="hljs-keyword">import</span> Iterable, Collection, Sequence
<span class="hljs-meta">>>> </span>isinstance([<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>], Iterable)
<span class="hljs-keyword">True</span>
</code></pre></div>
<pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┓
┃ │ Iterable │ Collection │ Sequence ┃
┠──────────────────┼────────────┼────────────┼────────────┨
┃ list, range, str │ ✓ │ ✓ │ ✓ ┃
┃ dict, set │ ✓ │ ✓ │ ┃
┃ iter │ ✓ │ │ ┃
┗━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┛
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-keyword">from</span> numbers <span class="hljs-keyword">import</span> Number, Complex, Real, Rational, Integral
<span class="hljs-meta">>>> </span>isinstance(<span class="hljs-number">123</span>, Number)
<span class="hljs-keyword">True</span>
</code></pre>
<pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┓
┃ │ Number │ Complex │ Real │ Rational │ Integral ┃
┠────────────────────┼──────────┼──────────┼──────────┼──────────┼──────────┨
┃ int │ ✓ │ ✓ │ ✓ │ ✓ │ ✓ ┃
┃ fractions.Fraction │ ✓ │ ✓ │ ✓ │ ✓ │ ┃
┃ float │ ✓ │ ✓ │ ✓ │ │ ┃
┃ complex │ ✓ │ ✓ │ │ │ ┃
┃ decimal.Decimal │ ✓ │ │ │ │ ┃
┗━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┛
</code></pre>
<div><h2 id="string"><a href="#string" name="string">#</a>String</h2><p><strong>Immutable sequence of characters.</strong></p><pre><code class="python language-python hljs"><str> = <str>.strip() <span class="hljs-comment"># Strips all whitespace characters from both ends.</span>
<str> = <str>.strip(<span class="hljs-string">'<chars>'</span>) <span class="hljs-comment"># Strips passed characters. Also lstrip/rstrip().</span>
</code></pre></div>
<pre><code class="python language-python hljs"><list> = <str>.split() <span class="hljs-comment"># Splits on one or more whitespace characters.</span>
<list> = <str>.split(sep=<span class="hljs-keyword">None</span>, maxsplit=<span class="hljs-number">-1</span>) <span class="hljs-comment"># Splits on 'sep' str at most 'maxsplit' times.</span>
<list> = <str>.splitlines(keepends=<span class="hljs-keyword">False</span>) <span class="hljs-comment"># On [\n\r\f\v\x1c-\x1e\x85\u2028\u2029] and \r\n.</span>
<str> = <str>.join(<coll_of_strings>) <span class="hljs-comment"># Joins elements using string as a separator.</span>
</code></pre>
<pre><code class="python language-python hljs"><bool> = <sub_str> <span class="hljs-keyword">in</span> <str> <span class="hljs-comment"># Checks if string contains the substring.</span>
<bool> = <str>.startswith(<sub_str>) <span class="hljs-comment"># Pass tuple of strings for multiple options.</span>
<int> = <str>.find(<sub_str>) <span class="hljs-comment"># Returns start index of the first match or -1.</span>
<int> = <str>.index(<sub_str>) <span class="hljs-comment"># Same, but raises ValueError if there's no match.</span>
</code></pre>
<pre><code class="python language-python hljs"><str> = <str>.lower() <span class="hljs-comment"># Changes the case. Also upper/capitalize/title().</span>
<str> = <str>.replace(old, new [, count]) <span class="hljs-comment"># Replaces 'old' with 'new' at most 'count' times.</span>
<str> = <str>.translate(<table>) <span class="hljs-comment"># Use `str.maketrans(<dict>)` to generate table.</span>
</code></pre>
<pre><code class="python language-python hljs"><str> = chr(<int>) <span class="hljs-comment"># Converts int to Unicode character.</span>
<int> = ord(<str>) <span class="hljs-comment"># Converts Unicode character to int.</span>
</code></pre>
<ul>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'unicodedata.normalize("NFC", <str>)'</span></code> on strings like <code class="python hljs"><span class="hljs-string">'Motörhead'</span></code> before comparing them to other strings, because <code class="python hljs"><span class="hljs-string">'ö'</span></code> can be stored as one or two characters.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'NFC'</span></code> converts such characters to a single character, while <code class="python hljs"><span class="hljs-string">'NFD'</span></code> converts them to two.</strong></li>
</ul>
<div><h3 id="propertymethods">Property Methods</h3><pre><code class="python language-python hljs"><bool> = <str>.isdecimal() <span class="hljs-comment"># Checks for [0-9]. Also [०-९] and [٠-٩].</span>
<bool> = <str>.isdigit() <span class="hljs-comment"># Checks for [²³¹…] and isdecimal().</span>
<bool> = <str>.isnumeric() <span class="hljs-comment"># Checks for [¼½¾…], [零〇一…] and isdigit().</span>
<bool> = <str>.isalnum() <span class="hljs-comment"># Checks for [a-zA-Z…] and isnumeric().</span>
<bool> = <str>.isprintable() <span class="hljs-comment"># Checks for [ !#$%…] and isalnum().</span>
<bool> = <str>.isspace() <span class="hljs-comment"># Checks for [ \t\n\r\f\v\x1c-\x1f\x85\xa0…].</span>
</code></pre></div>
<div><h2 id="regex"><a href="#regex" name="regex">#</a>Regex</h2><p><strong>Functions for regular expression matching.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> re
<str> = re.sub(<span class="hljs-string">r'<regex>'</span>, new, text, count=<span class="hljs-number">0</span>) <span class="hljs-comment"># Substitutes all occurrences with 'new'.</span>
<list> = re.findall(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># Returns all occurrences as strings.</span>
<list> = re.split(<span class="hljs-string">r'<regex>'</span>, text, maxsplit=<span class="hljs-number">0</span>) <span class="hljs-comment"># Add brackets around regex to keep matches.</span>
<Match> = re.search(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># First occurrence of the pattern or None.</span>
<Match> = re.match(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># Searches only at the beginning of the text.</span>
<iter> = re.finditer(<span class="hljs-string">r'<regex>'</span>, text) <span class="hljs-comment"># Returns all occurrences as Match objects.</span>
</code></pre></div>
<ul>
<li><strong>Raw string literals do not interpret escape sequences, thus enabling us to use regex-specific escape sequences that cause SyntaxWarning in normal string literals (since 3.12).</strong></li>
<li><strong>Argument 'new' of re.sub() can be a function that accepts Match object and returns a str.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.IGNORECASE'</span></code> can be used with all functions.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.MULTILINE'</span></code> makes <code class="python hljs"><span class="hljs-string">'^'</span></code> and <code class="python hljs"><span class="hljs-string">'$'</span></code> match the start/end of each line.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.DOTALL'</span></code> makes <code class="python hljs"><span class="hljs-string">'.'</span></code> also accept the <code class="python hljs"><span class="hljs-string">'\n'</span></code>.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'re.compile(<regex>)'</span></code> returns a Pattern object with methods sub(), findall(), etc.</strong></li>
</ul>
<div><h3 id="matchobject">Match Object</h3><pre><code class="python language-python hljs"><str> = <Match>.group() <span class="hljs-comment"># Returns the whole match. Also group(0).</span>
<str> = <Match>.group(<span class="hljs-number">1</span>) <span class="hljs-comment"># Returns part inside the first brackets.</span>
<tuple> = <Match>.groups() <span class="hljs-comment"># Returns all bracketed parts.</span>
<int> = <Match>.start() <span class="hljs-comment"># Returns start index of the match.</span>
<int> = <Match>.end() <span class="hljs-comment"># Returns exclusive end index of the match.</span>
</code></pre></div>
<div><h3 id="specialsequences">Special Sequences</h3><pre><code class="python language-python hljs"><span class="hljs-string">'\d'</span> == <span class="hljs-string">'[0-9]'</span> <span class="hljs-comment"># Also [०-९…]. Matches a decimal character.</span>
<span class="hljs-string">'\w'</span> == <span class="hljs-string">'[a-zA-Z0-9_]'</span> <span class="hljs-comment"># Also [ª²³…]. Matches an alphanumeric or _.</span>
<span class="hljs-string">'\s'</span> == <span class="hljs-string">'[ \t\n\r\f\v]'</span> <span class="hljs-comment"># Also [\x1c-\x1f…]. Matches a whitespace.</span>
</code></pre></div>
<ul>
<li><strong>By default, decimal characters and alphanumerics from all alphabets are matched unless <code class="python hljs"><span class="hljs-string">'flags=re.ASCII'</span></code> is used. It restricts special sequence matches to the first 128 Unicode characters and also prevents <code class="python hljs"><span class="hljs-string">'\s'</span></code> from accepting <code class="python hljs"><span class="hljs-string">'\x1c'</span></code>, <code class="python hljs"><span class="hljs-string">'\x1d'</span></code>, <code class="python hljs"><span class="hljs-string">'\x1e'</span></code> and <code class="python hljs"><span class="hljs-string">'\x1f'</span></code> (non-printable characters that divide text into files, tables, rows and fields, respectively).</strong></li>
<li><strong>Use a capital letter for negation (all non-ASCII characters will be matched when used in combination with ASCII flag).</strong></li>
</ul>
<div><h2 id="format"><a href="#format" name="format">#</a>Format</h2><pre><code class="python hljs"><str> = <span class="hljs-string">f'<span class="hljs-subst">{<el_1>}</span>, <span class="hljs-subst">{<el_2>}</span>'</span> <span class="hljs-comment"># Curly brackets can also contain expressions.</span>
<str> = <span class="hljs-string">'{}, {}'</span>.format(<el_1>, <el_2>) <span class="hljs-comment"># Or: '{0}, {a}'.format(<el_1>, a=<el_2>)</span>
<str> = <span class="hljs-string">'%s, %s'</span> % (<el_1>, <el_2>) <span class="hljs-comment"># Redundant and inferior C-style formatting.</span>
</code></pre></div>
<div><h3 id="example">Example</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>Person = collections.namedtuple(<span class="hljs-string">'Person'</span>, <span class="hljs-string">'name height'</span>)
<span class="hljs-meta">>>> </span>person = Person(<span class="hljs-string">'Jean-Luc'</span>, <span class="hljs-number">187</span>)
<span class="hljs-meta">>>> </span><span class="hljs-string">f'<span class="hljs-subst">{person.name}</span> is <span class="hljs-subst">{person.height / <span class="hljs-number">100</span>}</span> meters tall.'</span>
<span class="hljs-string">'Jean-Luc is 1.87 meters tall.'</span>
</code></pre></div>
<div><h3 id="generaloptions">General Options</h3><pre><code class="python language-python hljs">{<el>:<<span class="hljs-number">10</span>} <span class="hljs-comment"># '<el> '</span>
{<el>:^<span class="hljs-number">10</span>} <span class="hljs-comment"># ' <el> '</span>
{<el>:><span class="hljs-number">10</span>} <span class="hljs-comment"># ' <el>'</span>
{<el>:.<<span class="hljs-number">10</span>} <span class="hljs-comment"># '<el>......'</span>
{<el>:<span class="hljs-number">0</span>} <span class="hljs-comment"># '<el>'</span>
</code></pre></div>
<ul>
<li><strong>Objects are rendered using <code class="python hljs"><span class="hljs-string">'format(<el>, "<options>")'</span></code>.</strong></li>
<li><strong>Options can be generated dynamically: <code class="python hljs"><span class="hljs-string">f'<span class="hljs-subst">{<el>:{<str/int>}</span>[…]}'</span></code>.</strong></li>
<li><strong>Adding <code class="python hljs"><span class="hljs-string">'='</span></code> to the expression prepends it to the output: <code class="python hljs"><span class="hljs-string">f'<span class="hljs-subst">{<span class="hljs-number">1</span>+<span class="hljs-number">1</span>=}</span>'</span></code> returns <code class="python hljs"><span class="hljs-string">'1+1=2'</span></code>.</strong></li>
<li><strong>Adding <code class="python hljs"><span class="hljs-string">'!r'</span></code> to the expression converts object to string by calling its <a href="#class">repr()</a> method.</strong></li>
</ul>
<div><h3 id="strings">Strings</h3><pre><code class="python language-python hljs">{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">10</span>} <span class="hljs-comment"># 'abcde '</span>
{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">10.3</span>} <span class="hljs-comment"># 'abc '</span>
{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">.3</span>} <span class="hljs-comment"># 'abc'</span>
{<span class="hljs-string">'abcde'</span>!r:<span class="hljs-number">10</span>} <span class="hljs-comment"># "'abcde' "</span>
</code></pre></div>
<div><h3 id="numbers-1">Numbers</h3><pre><code class="python language-python hljs">{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>} <span class="hljs-comment"># ' 123456'</span>
{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>,} <span class="hljs-comment"># ' 123,456'</span>
{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>_} <span class="hljs-comment"># ' 123_456'</span>
{<span class="hljs-number">123456</span>:+<span class="hljs-number">10</span>} <span class="hljs-comment"># ' +123456'</span>
{<span class="hljs-number">123456</span>:=+<span class="hljs-number">10</span>} <span class="hljs-comment"># '+ 123456'</span>
{<span class="hljs-number">123456</span>: } <span class="hljs-comment"># ' 123456'</span>
{<span class="hljs-number">-123456</span>: } <span class="hljs-comment"># '-123456'</span>
</code></pre></div>
<div><h3 id="floats">Floats</h3><pre><code class="python language-python hljs">{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>} <span class="hljs-comment"># ' 1.23'</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>f} <span class="hljs-comment"># ' 1.235'</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>e} <span class="hljs-comment"># ' 1.235e+00'</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>%} <span class="hljs-comment"># ' 123.456%'</span>
</code></pre></div>
<div><h4 id="comparisonofpresentationtypes">Comparison of presentation types:</h4><pre><code class="text language-text">┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┓
┃ │ {<float>} │ {<float>:f} │ {<float>:e} │ {<float>:%} ┃
┠──────────────┼────────────────┼────────────────┼────────────────┼────────────────┨
┃ 0.000056789 │ '5.6789e-05' │ '0.000057' │ '5.678900e-05' │ '0.005679%' ┃
┃ 0.00056789 │ '0.00056789' │ '0.000568' │ '5.678900e-04' │ '0.056789%' ┃
┃ 0.0056789 │ '0.0056789' │ '0.005679' │ '5.678900e-03' │ '0.567890%' ┃
┃ 0.056789 │ '0.056789' │ '0.056789' │ '5.678900e-02' │ '5.678900%' ┃
┃ 0.56789 │ '0.56789' │ '0.567890' │ '5.678900e-01' │ '56.789000%' ┃
┃ 5.6789 │ '5.6789' │ '5.678900' │ '5.678900e+00' │ '567.890000%' ┃
┃ 56.789 │ '56.789' │ '56.789000' │ '5.678900e+01' │ '5678.900000%' ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┛
┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┓
┃ │ {<float>:.2} │ {<float>:.2f} │ {<float>:.2e} │ {<float>:.2%} ┃
┠──────────────┼────────────────┼────────────────┼────────────────┼────────────────┨
┃ 0.000056789 │ '5.7e-05' │ '0.00' │ '5.68e-05' │ '0.01%' ┃
┃ 0.00056789 │ '0.00057' │ '0.00' │ '5.68e-04' │ '0.06%' ┃
┃ 0.0056789 │ '0.0057' │ '0.01' │ '5.68e-03' │ '0.57%' ┃
┃ 0.056789 │ '0.057' │ '0.06' │ '5.68e-02' │ '5.68%' ┃
┃ 0.56789 │ '0.57' │ '0.57' │ '5.68e-01' │ '56.79%' ┃
┃ 5.6789 │ '5.7' │ '5.68' │ '5.68e+00' │ '567.89%' ┃
┃ 56.789 │ '5.7e+01' │ '56.79' │ '5.68e+01' │ '5678.90%' ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┛
</code></pre></div>
<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'{<float>:g}'</span></code> is <code class="python hljs"><span class="hljs-string">'{<float>:.6}'</span></code> with stripped zeros, exponent starting at <code class="python hljs"><span class="hljs-string">'1e+06'</span></code>.</strong></li>
<li><strong>When both rounding up and rounding down are possible, the one that returns result with even last digit is chosen. That makes <code class="python hljs"><span class="hljs-string">'{6.5:.0f}'</span></code> a <code class="python hljs"><span class="hljs-string">'6'</span></code> and <code class="python hljs"><span class="hljs-string">'{7.5:.0f}'</span></code> an <code class="python hljs"><span class="hljs-string">'8'</span></code>.</strong></li>
<li><strong>This rule only effects numbers that can be represented exactly by a float (<code class="python hljs"><span class="hljs-number">.5</span></code>, <code class="python hljs"><span class="hljs-number">.25</span></code>, …).</strong></li>
</ul>
<div><h3 id="ints">Ints</h3><pre><code class="python language-python hljs">{<span class="hljs-number">90</span>:c} <span class="hljs-comment"># 'Z'. Unicode character with value 90.</span>
{<span class="hljs-number">90</span>:b} <span class="hljs-comment"># '1011010'. Binary representation of the int.</span>
{<span class="hljs-number">90</span>:X} <span class="hljs-comment"># '5A'. Hexadecimal with upper-case letters.</span>
</code></pre></div>
<div><h2 id="numbers"><a href="#numbers" name="numbers">#</a>Numbers</h2><pre><code class="python language-python hljs"><int> = int(<float/str/bool>) <span class="hljs-comment"># Or: math.trunc(<float>)</span>
<float> = float(<int/str/bool>) <span class="hljs-comment"># Or: <int/float>e±<int></span>
<complex> = complex(real=<span class="hljs-number">0</span>, imag=<span class="hljs-number">0</span>) <span class="hljs-comment"># Or: <int/float> ± <int/float>j</span>
<Fraction> = fractions.Fraction(<span class="hljs-number">0</span>, <span class="hljs-number">1</span>) <span class="hljs-comment"># Or: Fraction(numerator=0, denominator=1)</span>
<Decimal> = decimal.Decimal(<str/int>) <span class="hljs-comment"># Or: Decimal((sign, digits, exponent))</span>
</code></pre></div>
<ul>
<li><strong>Decimal numbers are stored exactly, unlike most floats where <code class="python hljs"><span class="hljs-string">'1.1 + 2.2 != 3.3'</span></code>.</strong></li>
<li><strong>Floats can be compared with: <code class="python hljs"><span class="hljs-string">'math.isclose(<float>, <float>)'</span></code>.</strong></li>
<li><strong>Precision of decimal operations is set with: <code class="python hljs"><span class="hljs-string">'decimal.getcontext().prec = <int>'</span></code>.</strong></li>
<li><strong>Bools can be used anywhere ints can, because bool is a subclass of int: <code class="python hljs"><span class="hljs-string">'True + 1 == 2'</span></code>.</strong></li>
</ul>
<div><h3 id="basicfunctions">Basic Functions</h3><pre><code class="python language-python hljs"><num> = pow(<num>, <num>) <span class="hljs-comment"># Or: <number> ** <number></span>
<num> = abs(<num>) <span class="hljs-comment"># <float> = abs(<complex>)</span>
<num> = round(<num> [, ±ndigits]) <span class="hljs-comment"># `round(126, -1) == 130`</span>
</code></pre></div>
<div><h3 id="math">Math</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> e, pi, inf, nan, isinf, isnan <span class="hljs-comment"># `<el> == nan` is always False.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> sin, cos, tan, asin, acos, atan <span class="hljs-comment"># Also: degrees, radians.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> log, log10, log2 <span class="hljs-comment"># Log can accept base as second arg.</span>
</code></pre></div>
<div><h3 id="statistics">Statistics</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> statistics <span class="hljs-keyword">import</span> mean, median, variance <span class="hljs-comment"># Also: stdev, quantiles, groupby.</span>
</code></pre></div>
<div><h3 id="random">Random</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> random <span class="hljs-keyword">import</span> random, randint, uniform <span class="hljs-comment"># Also: gauss, choice, shuffle, seed.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><float> = random() <span class="hljs-comment"># Returns a float inside [0, 1).</span>
<num> = randint/uniform(a, b) <span class="hljs-comment"># Returns an int/float inside [a, b].</span>
<float> = gauss(mean, stdev) <span class="hljs-comment"># Also triangular(low, high, mode).</span>
<el> = choice(<sequence>) <span class="hljs-comment"># Keeps it intact. Also sample(pop, k).</span>
shuffle(<list>) <span class="hljs-comment"># Shuffles the list in place.</span>
</code></pre>
<div><h3 id="hexadecimalnumbers">Hexadecimal Numbers</h3><pre><code class="python language-python hljs"><int> = ±<span class="hljs-number">0x</span><hex> <span class="hljs-comment"># Or: ±0b<bin></span>
<int> = int(<span class="hljs-string">'±<hex>'</span>, <span class="hljs-number">16</span>) <span class="hljs-comment"># Or: int('±<bin>', 2)</span>
<int> = int(<span class="hljs-string">'±0x<hex>'</span>, <span class="hljs-number">0</span>) <span class="hljs-comment"># Or: int('±0b<bin>', 0)</span>
<str> = hex(<int>) <span class="hljs-comment"># Returns '[-]0x<hex>'. Also bin().</span>
</code></pre></div>
<div><h3 id="bitwiseoperators">Bitwise Operators</h3><pre><code class="python language-python hljs"><int> = <int> & <int> <span class="hljs-comment"># And (0b1100 & 0b1010 == 0b1000).</span>
<int> = <int> | <int> <span class="hljs-comment"># Or (0b1100 | 0b1010 == 0b1110).</span>
<int> = <int> ^ <int> <span class="hljs-comment"># Xor (0b1100 ^ 0b1010 == 0b0110).</span>
<int> = <int> << n_bits <span class="hljs-comment"># Left shift. Use >> for right.</span>
<int> = ~<int> <span class="hljs-comment"># Not. Also -<int> - 1.</span>
</code></pre></div>
<div><h2 id="combinatorics"><a href="#combinatorics" name="combinatorics">#</a>Combinatorics</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> itertools <span class="hljs-keyword">as</span> it
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>list(it.product([<span class="hljs-number">0</span>, <span class="hljs-number">1</span>], repeat=<span class="hljs-number">3</span>))
[(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>), (<span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">1</span>), (<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">0</span>), (<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">1</span>),
(<span class="hljs-number">1</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>), (<span class="hljs-number">1</span>, <span class="hljs-number">0</span>, <span class="hljs-number">1</span>), (<span class="hljs-number">1</span>, <span class="hljs-number">1</span>, <span class="hljs-number">0</span>), (<span class="hljs-number">1</span>, <span class="hljs-number">1</span>, <span class="hljs-number">1</span>)]
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>list(it.product(<span class="hljs-string">'abc'</span>, <span class="hljs-string">'abc'</span>)) <span class="hljs-comment"># a b c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># a x x x</span>
(<span class="hljs-string">'b'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># b x x x</span>
(<span class="hljs-string">'c'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'c'</span>)] <span class="hljs-comment"># c x x x</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>list(it.permutations(<span class="hljs-string">'abc'</span>, <span class="hljs-number">2</span>)) <span class="hljs-comment"># a b c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># a . x x</span>
(<span class="hljs-string">'b'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># b x . x</span>
(<span class="hljs-string">'c'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'b'</span>)] <span class="hljs-comment"># c x x .</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>list(it.combinations(<span class="hljs-string">'abc'</span>, <span class="hljs-number">2</span>)) <span class="hljs-comment"># a b c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># a . x x</span>
(<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>), <span class="hljs-comment"># b . . x</span>
] <span class="hljs-comment"># c . . .</span>
</code></pre>
<div><h2 id="datetime"><a href="#datetime" name="datetime">#</a>Datetime</h2><p><strong>Provides 'date', 'time', 'datetime' and 'timedelta' classes. All are immutable and hashable.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install python-dateutil</span>
<span class="hljs-keyword">from</span> datetime <span class="hljs-keyword">import</span> date, time, datetime, timedelta, timezone
<span class="hljs-keyword">import</span> zoneinfo, dateutil.tz
</code></pre></div>
<pre><code class="python language-python apache hljs"><D> = date(year, month, day) <span class="hljs-comment"># Only accepts valid dates from 1 to 9999 AD.</span>
<T> = time(hour=<span class="hljs-number">0</span>, minute=<span class="hljs-number">0</span>, second=<span class="hljs-number">0</span>) <span class="hljs-comment"># Also: `microsecond=0, tzinfo=None, fold=0`.</span>
<DT> = datetime(year, month, day, hour=<span class="hljs-number">0</span>) <span class="hljs-comment"># Also: `minute=0, second=0, microsecond=0, …`.</span>
<TD> = timedelta(weeks=<span class="hljs-number">0</span>, days=<span class="hljs-number">0</span>, hours=<span class="hljs-number">0</span>) <span class="hljs-comment"># Also: `minutes=0, seconds=0, microseconds=0`.</span>
</code></pre>
<ul>
<li><strong>Aware times and datetimes have defined timezone, while naive don't. If object is naive, it is presumed to be in the system's timezone!</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'fold=1'</span></code> means the second pass in case of time jumping back for one hour.</strong></li>
<li><strong>Timedelta normalizes arguments to ±days, seconds (< 86 400) and microseconds (< 1M). Its str() method returns <code class="python hljs"><span class="hljs-string">'[±D, ]H:MM:SS[.…]'</span></code> and total_seconds() a float of all seconds.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'<D/DT>.weekday()'</span></code> to get the day of the week as an int, with Monday being 0.</strong></li>
</ul>
<div><h3 id="now">Now</h3><pre><code class="python language-python hljs"><D/DTn> = D/DT.today() <span class="hljs-comment"># Current local date or naive DT. Also DT.now().</span>
<DTa> = DT.now(<tzinfo>) <span class="hljs-comment"># Aware DT from current time in passed timezone.</span>
</code></pre></div>
<ul>
<li><strong>To extract time use <code class="python hljs"><span class="hljs-string">'<DTn>.time()'</span></code>, <code class="python hljs"><span class="hljs-string">'<DTa>.time()'</span></code> or <code class="python hljs"><span class="hljs-string">'<DTa>.timetz()'</span></code>.</strong></li>
</ul>
<div><h3 id="timezone">Timezone</h3><pre><code class="python language-python apache hljs"><tzinfo> = timezone.utc <span class="hljs-comment"># London without daylight saving time (DST).</span>
<tzinfo> = timezone(<timedelta>) <span class="hljs-comment"># Timezone with fixed offset from UTC.</span>
<tzinfo> = dateutil.tz.tzlocal() <span class="hljs-comment"># Local timezone with dynamic offset from UTC.</span>
<tzinfo> = zoneinfo.ZoneInfo(<span class="hljs-string">'<iana_key>'</span>) <span class="hljs-comment"># 'Continent/City_Name' zone with dynamic offset.</span>
<DTa> = <DT>.astimezone([<tzinfo>]) <span class="hljs-comment"># Converts DT to the passed or local fixed zone.</span>
<Ta/DTa> = <T/DT>.replace(tzinfo=<tzinfo>) <span class="hljs-comment"># Changes object's timezone without conversion.</span>
</code></pre></div>
<ul>
<li><strong>Timezones returned by tzlocal(), ZoneInfo() and implicit local timezone of naive objects have offsets that vary through time due to DST and historical changes of the base offset.</strong></li>
<li><strong>To get ZoneInfo() to work on Windows run <code class="python hljs"><span class="hljs-string">'> pip3 install tzdata'</span></code>.</strong></li>
</ul>
<div><h3 id="encode">Encode</h3><pre><code class="python language-python apache hljs"><D/T/DT> = D/T/DT.fromisoformat(<str>) <span class="hljs-comment"># Object from ISO string. Raises ValueError.</span>
<DT> = DT.strptime(<str>, <span class="hljs-string">'<format>'</span>) <span class="hljs-comment"># Datetime from custom string. See Format.</span>
<D/DTn> = D/DT.fromordinal(<int>) <span class="hljs-comment"># D/DT from days since the Gregorian NYE 1.</span>
<DTn> = DT.fromtimestamp(<float>) <span class="hljs-comment"># Local naive DT from seconds since the Epoch.</span>
<DTa> = DT.fromtimestamp(<float>, <tz>) <span class="hljs-comment"># Aware datetime from seconds since the Epoch.</span>
</code></pre></div>
<ul>
<li><strong>ISO strings come in following forms: <code class="python hljs"><span class="hljs-string">'YYYY-MM-DD'</span></code>, <code class="python hljs"><span class="hljs-string">'HH:MM:SS.mmmuuu[±HH:MM]'</span></code>, or both separated by an arbitrary character. All parts following the hours are optional.</strong></li>
<li><strong>Python uses the Unix Epoch: <code class="python hljs"><span class="hljs-string">'1970-01-01 00:00 UTC'</span></code>, <code class="python hljs"><span class="hljs-string">'1970-01-01 01:00 CET'</span></code>, …</strong></li>
</ul>
<div><h3 id="decode">Decode</h3><pre><code class="python language-python hljs"><str> = <D/T/DT>.isoformat(sep=<span class="hljs-string">'T'</span>) <span class="hljs-comment"># Also `timespec='auto/hours/minutes/seconds/…'`.</span>
<str> = <D/T/DT>.strftime(<span class="hljs-string">'<format>'</span>) <span class="hljs-comment"># Custom string representation of the object.</span>
<int> = <D/DT>.toordinal() <span class="hljs-comment"># Days since Gregorian NYE 1, ignoring time and tz.</span>
<float> = <DTn>.timestamp() <span class="hljs-comment"># Seconds since the Epoch, from local naive DT.</span>
<float> = <DTa>.timestamp() <span class="hljs-comment"># Seconds since the Epoch, from aware datetime.</span>
</code></pre></div>
<div><h3 id="format-1">Format</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>dt = datetime.strptime(<span class="hljs-string">'2025-08-14 23:39:00.00 +0200'</span>, <span class="hljs-string">'%Y-%m-%d %H:%M:%S.%f %z'</span>)
<span class="hljs-meta">>>> </span>dt.strftime(<span class="hljs-string">"%dth of %B '%y (%a), %I:%M %p %Z"</span>)
<span class="hljs-string">"14th of August '25 (Thu), 11:39 PM UTC+02:00"</span>
</code></pre></div>
<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'%z'</span></code> accepts <code class="python hljs"><span class="hljs-string">'±HH[:]MM'</span></code> and returns <code class="python hljs"><span class="hljs-string">'±HHMM'</span></code> or empty string if object is naive.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'%Z'</span></code> accepts <code class="python hljs"><span class="hljs-string">'UTC/GMT'</span></code> and local timezone's code and returns timezone's name, <code class="python hljs"><span class="hljs-string">'UTC[±HH:MM]'</span></code> if timezone is nameless, or an empty string if object is naive.</strong></li>
</ul>
<div><h3 id="arithmetics">Arithmetics</h3><pre><code class="python language-python apache hljs"><bool> = <D/T/DTn> > <D/T/DTn> <span class="hljs-comment"># Ignores time jumps (fold attribute). Also ==.</span>
<bool> = <DTa> > <DTa> <span class="hljs-comment"># Ignores time jumps if they share tzinfo object.</span>
<TD> = <D/DTn> - <D/DTn> <span class="hljs-comment"># Ignores jumps. Convert to UTC for actual delta.</span>
<TD> = <DTa> - <DTa> <span class="hljs-comment"># Ignores jumps if they share tzinfo object.</span>
<D/DT> = <D/DT> ± <TD> <span class="hljs-comment"># Returned datetime can fall into missing hour.</span>
<TD> = <TD> * <float> <span class="hljs-comment"># Also `<TD> = abs(<TD>)`, `<TD> = <TD> ± <TD>`.</span>
<float> = <TD> / <TD> <span class="hljs-comment"># Also `(<int>, <TD>) = divmod(<TD>, <TD>)`.</span>
</code></pre></div>
<div><h2 id="function"><a href="#function" name="function">#</a>Function</h2><p><strong>Independent block of code that returns a value when called.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <<span class="hljs-title">func_name</span>><span class="hljs-params">(<nondefault_args>)</span>:</span> ... <span class="hljs-comment"># E.g. `def func(x, y): ...`.</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <<span class="hljs-title">func_name</span>><span class="hljs-params">(<default_args>)</span>:</span> ... <span class="hljs-comment"># E.g. `def func(x=0, y=0): ...`.</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <<span class="hljs-title">func_name</span>><span class="hljs-params">(<nondefault_args>, <default_args>)</span>:</span> ... <span class="hljs-comment"># E.g. `def func(x, y=0): ...`.</span>
</code></pre></div>
<ul>
<li><strong>Function returns None if it doesn't encounter <code class="python hljs"><span class="hljs-string">'return <obj/exp>'</span></code> statement.</strong></li>
<li><strong>Run <code class="python hljs"><span class="hljs-string">'global <var_name>'</span></code> inside the function before assigning to global variable.</strong></li>
<li><strong>Default values are evaluated when function is first encountered in the scope. Any mutation of a mutable default value will persist between invocations!</strong></li>
</ul>
<div><h3 id="functioncall">Function Call</h3><pre><code class="python language-python hljs"><obj> = <function>(<positional_args>) <span class="hljs-comment"># E.g. `func(0, 0)`.</span>
<obj> = <function>(<keyword_args>) <span class="hljs-comment"># E.g. `func(x=0, y=0)`.</span>
<obj> = <function>(<positional_args>, <keyword_args>) <span class="hljs-comment"># E.g. `func(0, y=0)`.</span>
</code></pre></div>
<div><h2 id="splatoperator"><a href="#splatoperator" name="splatoperator">#</a>Splat Operator</h2><p><strong>Splat expands a collection into positional arguments, while splatty-splat expands a dictionary into keyword arguments.</strong></p><pre><code class="python language-python hljs">args, kwargs = (<span class="hljs-number">1</span>, <span class="hljs-number">2</span>), {<span class="hljs-string">'z'</span>: <span class="hljs-number">3</span>}
func(*args, **kwargs)
</code></pre></div>
<div><h4 id="isthesameas">Is the same as:</h4><pre><code class="python language-python hljs">func(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, z=<span class="hljs-number">3</span>)
</code></pre></div>
<div><h3 id="insidefunctiondefinition">Inside Function Definition</h3><p><strong>Splat combines zero or more positional arguments into a tuple, while splatty-splat combines zero or more keyword arguments into a dictionary.</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(*a)</span>:</span>
<span class="hljs-meta">... </span> <span class="hljs-keyword">return</span> sum(a)
<span class="hljs-meta">... </span>
<span class="hljs-meta">>>> </span>add(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
<span class="hljs-number">6</span>
</code></pre></div>
<div><h4 id="allowedcompositionsofargumentsinsidefunctiondefinitionandthewaystheycanbecalled">Allowed compositions of arguments inside function definition and the ways they can be called:</h4><pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━┓
┃ │ f(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>) │ f(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, z=<span class="hljs-number">3</span>) │ f(<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>, z=<span class="hljs-number">3</span>) │ f(x=<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>, z=<span class="hljs-number">3</span>) ┃
┠────────────────────┼────────────┼──────────────┼────────────────┼──────────────────┨
┃ <span class="hljs-title">f</span>(x, *args, **kw): │ ✓ │ ✓ │ ✓ │ ✓ ┃
┃ <span class="hljs-title">f</span>(*args, z, **kw): │ │ ✓ │ ✓ │ ✓ ┃
┃ <span class="hljs-title">f</span>(x, **kw): │ │ │ ✓ │ ✓ ┃
┃ <span class="hljs-title">f</span>(*, x, **kw): │ │ │ │ ✓ ┃
┗━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━┛
</code></pre></div>
<div><h3 id="otheruses">Other Uses</h3><pre><code class="python language-python hljs"><list> = [*<collection> [, ...]] <span class="hljs-comment"># Or: list(<coll>) [+ ...]</span>
<tuple> = (*<collection>, [...]) <span class="hljs-comment"># Or: tuple(<coll>) [+ ...]</span>
<set> = {*<collection> [, ...]} <span class="hljs-comment"># Or: set(<coll>) [| ...]</span>
<dict> = {**<dict> [, ...]} <span class="hljs-comment"># Or: <dict> | ...</span>
</code></pre></div>
<pre><code class="python language-python hljs">head, *body, tail = <collection> <span class="hljs-comment"># Head or tail can be omitted.</span>
</code></pre>
<div><h2 id="inline"><a href="#inline" name="inline">#</a>Inline</h2><div><h3 id="lambda">Lambda</h3><pre><code class="python language-python hljs"><func> = <span class="hljs-keyword">lambda</span>: <return_value> <span class="hljs-comment"># A single statement function.</span>
<func> = <span class="hljs-keyword">lambda</span> <arg_1>, <arg_2>: <return_value> <span class="hljs-comment"># Also allows default arguments.</span>
</code></pre></div></div>
<div><h3 id="comprehensions">Comprehensions</h3><pre><code class="python language-python hljs"><list> = [i+<span class="hljs-number">1</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)] <span class="hljs-comment"># Or: [1, 2, ..., 10]</span>
<iter> = (i <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>) <span class="hljs-keyword">if</span> i > <span class="hljs-number">5</span>) <span class="hljs-comment"># Or: iter([6, 7, 8, 9])</span>
<set> = {i+<span class="hljs-number">5</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)} <span class="hljs-comment"># Or: {5, 6, ..., 14}</span>
<dict> = {i: i*<span class="hljs-number">2</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)} <span class="hljs-comment"># Or: {0: 0, 1: 2, ..., 9: 18}</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[l+r <span class="hljs-keyword">for</span> l <span class="hljs-keyword">in</span> <span class="hljs-string">'abc'</span> <span class="hljs-keyword">for</span> r <span class="hljs-keyword">in</span> <span class="hljs-string">'abc'</span>] <span class="hljs-comment"># Inner loop is on the right side.</span>
[<span class="hljs-string">'aa'</span>, <span class="hljs-string">'ab'</span>, <span class="hljs-string">'ac'</span>, ..., <span class="hljs-string">'cc'</span>]
</code></pre>
<div><h3 id="mapfilterreduce">Map, Filter, Reduce</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> reduce
</code></pre></div>
<pre><code class="python language-python hljs"><iter> = map(<span class="hljs-keyword">lambda</span> x: x + <span class="hljs-number">1</span>, range(<span class="hljs-number">10</span>)) <span class="hljs-comment"># Or: iter([1, 2, ..., 10])</span>
<iter> = filter(<span class="hljs-keyword">lambda</span> x: x > <span class="hljs-number">5</span>, range(<span class="hljs-number">10</span>)) <span class="hljs-comment"># Or: iter([6, 7, 8, 9])</span>
<obj> = reduce(<span class="hljs-keyword">lambda</span> out, x: out + x, range(<span class="hljs-number">10</span>)) <span class="hljs-comment"># Or: 45</span>
</code></pre>
<div><h3 id="anyall">Any, All</h3><pre><code class="python language-python hljs"><bool> = any(<collection>) <span class="hljs-comment"># Is `bool(<el>)` True for any el?</span>
<bool> = all(<collection>) <span class="hljs-comment"># True for all? Also True if empty.</span>
</code></pre></div>
<div><h3 id="conditionalexpression">Conditional Expression</h3><pre><code class="python language-python hljs"><obj> = <exp> <span class="hljs-keyword">if</span> <condition> <span class="hljs-keyword">else</span> <exp> <span class="hljs-comment"># Only one expression is evaluated.</span>
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[i <span class="hljs-keyword">if</span> i <span class="hljs-keyword">else</span> <span class="hljs-string">'zero'</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> (<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)] <span class="hljs-comment"># `any([0, '', [], None]) == False`</span>
[<span class="hljs-string">'zero'</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>]
</code></pre>
<div><h3 id="andor">And, Or</h3><pre><code class="python language-python hljs"><obj> = <exp> <span class="hljs-keyword">and</span> <exp> [<span class="hljs-keyword">and</span> ...] <span class="hljs-comment"># Returns first false or last operand.</span>
<obj> = <exp> <span class="hljs-keyword">or</span> <exp> [<span class="hljs-keyword">or</span> ...] <span class="hljs-comment"># Returns first true or last operand.</span>
</code></pre></div>
<div><h3 id="walrusoperator">Walrus Operator</h3><pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>[i <span class="hljs-keyword">for</span> a <span class="hljs-keyword">in</span> <span class="hljs-string">'0123'</span> <span class="hljs-keyword">if</span> (i := int(a)) > <span class="hljs-number">0</span>] <span class="hljs-comment"># Assigns to variable mid-sentence.</span>
[<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>]
</code></pre></div>
<div><h3 id="namedtupleenumdataclass">Named Tuple, Enum, Dataclass</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> namedtuple
Point = namedtuple(<span class="hljs-string">'Point'</span>, <span class="hljs-string">'x y'</span>) <span class="hljs-comment"># Creates tuple's subclass.</span>
point = Point(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>) <span class="hljs-comment"># Returns its instance.</span>
<span class="hljs-keyword">from</span> enum <span class="hljs-keyword">import</span> Enum
Direction = Enum(<span class="hljs-string">'Direction'</span>, <span class="hljs-string">'N E S W'</span>) <span class="hljs-comment"># Creates Enum's subclass.</span>
direction = Direction.N <span class="hljs-comment"># Returns its member.</span>
<span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> make_dataclass
Player = make_dataclass(<span class="hljs-string">'Player'</span>, [<span class="hljs-string">'loc'</span>, <span class="hljs-string">'dir'</span>]) <span class="hljs-comment"># Creates a class.</span>
player = Player(point, direction) <span class="hljs-comment"># Returns its instance.</span>
</code></pre></div>
<div><h2 id="imports"><a href="#imports" name="imports">#</a>Imports</h2><p><strong>Mechanism that makes code in one file available to another file.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> <module> <span class="hljs-comment"># Imports a built-in or '<module>.py'.</span>
<span class="hljs-keyword">import</span> <package> <span class="hljs-comment"># Imports a built-in or '<package>/__init__.py'.</span>
<span class="hljs-keyword">import</span> <package>.<module> <span class="hljs-comment"># Imports a built-in or '<package>/<module>.py'.</span>
</code></pre></div>
<ul>
<li><strong>Package is a collection of modules, but it can also define its own objects.</strong></li>
<li><strong>On a filesystem this corresponds to a directory of Python files with an optional init script.</strong></li>
<li><strong>Running <code class="python hljs"><span class="hljs-string">'import <package>'</span></code> does not automatically provide access to the package's modules unless they are explicitly imported in its init script.</strong></li>
<li><strong>Directory of the file that is passed to python command serves as a root of local imports.</strong></li>
<li><strong>For relative imports use <code class="python hljs"><span class="hljs-string">'from .[…][<pkg/module>[.…]] import <obj>'</span></code>.</strong></li>
</ul>
<div><h2 id="closure"><a href="#closure" name="closure">#</a>Closure</h2><p><strong>We have/get a closure in Python when a nested function references a value of its enclosing function and then the enclosing function returns its nested function.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_multiplier</span><span class="hljs-params">(a)</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(b)</span>:</span>
<span class="hljs-keyword">return</span> a * b
<span class="hljs-keyword">return</span> out
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>multiply_by_3 = get_multiplier(<span class="hljs-number">3</span>)
<span class="hljs-meta">>>> </span>multiply_by_3(<span class="hljs-number">10</span>)
<span class="hljs-number">30</span>
</code></pre>
<ul>
<li><strong>Any value that is referenced from within multiple nested functions gets shared.</strong></li>
</ul>
<div><h3 id="partial">Partial</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> partial
<function> = partial(<function> [, <arg_1> [, ...]])
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">multiply</span><span class="hljs-params">(a, b)</span>:</span>
<span class="hljs-meta">... </span> <span class="hljs-keyword">return</span> a * b
<span class="hljs-meta">>>> </span>multiply_by_3 = partial(multiply, <span class="hljs-number">3</span>)
<span class="hljs-meta">>>> </span>multiply_by_3(<span class="hljs-number">10</span>)
<span class="hljs-number">30</span>
</code></pre>
<ul>
<li><strong>Partial is also useful in cases when a function needs to be passed as an argument because it enables us to set its arguments beforehand.</strong></li>
<li><strong>A few examples being: <code class="python hljs"><span class="hljs-string">'defaultdict(<func>)'</span></code>, <code class="python hljs"><span class="hljs-string">'iter(<func>, to_exc)'</span></code> and dataclass's <code class="python hljs"><span class="hljs-string">'field(default_factory=<func>)'</span></code>.</strong></li>
</ul>
<div><h3 id="nonlocal">Non-Local</h3><p><strong>If variable is being assigned to anywhere in the scope, it is regarded as a local variable, unless it is declared as a 'global' or a 'nonlocal'.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_counter</span><span class="hljs-params">()</span>:</span>
i = <span class="hljs-number">0</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">()</span>:</span>
<span class="hljs-keyword">nonlocal</span> i
i += <span class="hljs-number">1</span>
<span class="hljs-keyword">return</span> i
<span class="hljs-keyword">return</span> out
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>counter = get_counter()
<span class="hljs-meta">>>> </span>counter(), counter(), counter()
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
</code></pre>
<div class="pagebreak"></div><div><h2 id="decorator"><a href="#decorator" name="decorator">#</a>Decorator</h2><ul>
<li><strong>A decorator takes a function, adds some functionality and returns it.</strong></li>
<li><strong>It can be any <a href="#callable">callable</a>, but is usually implemented as a function that returns a <a href="#closure">closure</a>.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-meta">@decorator_name</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">function_that_gets_passed_to_decorator</span><span class="hljs-params">()</span>:</span>
...
</code></pre></div>
<div><h3 id="debuggerexample">Debugger Example</h3><p><strong>Decorator that prints function's name every time the function is called.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> wraps
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">debug</span><span class="hljs-params">(func)</span>:</span>
<span class="hljs-meta"> @wraps(func)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(*args, **kwargs)</span>:</span>
print(func.__name__)
<span class="hljs-keyword">return</span> func(*args, **kwargs)
<span class="hljs-keyword">return</span> out
<span class="hljs-meta">@debug</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(x, y)</span>:</span>
<span class="hljs-keyword">return</span> x + y
</code></pre></div>
<ul>
<li><strong>Wraps is a helper decorator that copies the metadata of the passed function (func) to the function it is wrapping (out). Without it, <code class="python hljs"><span class="hljs-string">'add.__name__'</span></code> would return <code class="python hljs"><span class="hljs-string">'out'</span></code>.</strong></li>
</ul>
<div><h3 id="cache">Cache</h3><p><strong>Decorator that caches function's return values. All function's arguments must be hashable.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> cache
<span class="hljs-meta">@cache</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">fib</span><span class="hljs-params">(n)</span>:</span>
<span class="hljs-keyword">return</span> n <span class="hljs-keyword">if</span> n < <span class="hljs-number">2</span> <span class="hljs-keyword">else</span> fib(n-<span class="hljs-number">2</span>) + fib(n-<span class="hljs-number">1</span>)</code></pre></div>
<ul>
<li><strong>Potential problem with cache is that it can grow indefinitely. To clear stored values run <code class="python hljs"><span class="hljs-string">'<func>.cache_clear()'</span></code>, or use <code class="python hljs"><span class="hljs-string">'@lru_cache(maxsize=<int>)'</span></code> decorator instead.</strong></li>
<li><strong>CPython interpreter limits recursion depth to 3000 by default. To increase it run <code class="python hljs"><span class="hljs-string">'sys.setrecursionlimit(<int>)'</span></code>.</strong></li>
</ul>
<div><h3 id="parametrizeddecorator">Parametrized Decorator</h3><p><strong>A decorator that accepts arguments and returns a normal decorator that accepts a function.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> wraps
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">debug</span><span class="hljs-params">(print_result=<span class="hljs-keyword">False</span>)</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">decorator</span><span class="hljs-params">(func)</span>:</span>
<span class="hljs-meta"> @wraps(func)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(*args, **kwargs)</span>:</span>
result = func(*args, **kwargs)
print(func.__name__, result <span class="hljs-keyword">if</span> print_result <span class="hljs-keyword">else</span> <span class="hljs-string">''</span>)
<span class="hljs-keyword">return</span> result
<span class="hljs-keyword">return</span> out
<span class="hljs-keyword">return</span> decorator
<span class="hljs-meta">@debug(print_result=True)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(x, y)</span>:</span>
<span class="hljs-keyword">return</span> x + y
</code></pre></div>
<ul>
<li><strong>Using only <code class="python hljs"><span class="hljs-string">'@debug'</span></code> to decorate the add() function would not work here, because debug would then receive the add() function as a 'print_result' argument. Decorators can however manually check if the argument they received is a function and act accordingly.</strong></li>
</ul>
<div><h2 id="class"><a href="#class" name="class">#</a>Class</h2><p><strong>A template for creating user-defined objects.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyClass</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
self.a = a
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__str__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> str(self.a)
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__repr__</span><span class="hljs-params">(self)</span>:</span>
class_name = self.__class__.__name__
<span class="hljs-keyword">return</span> <span class="hljs-string">f'<span class="hljs-subst">{class_name}</span>(<span class="hljs-subst">{self.a!r}</span>)'</span>
<span class="hljs-meta"> @classmethod</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_class_name</span><span class="hljs-params">(cls)</span>:</span>
<span class="hljs-keyword">return</span> cls.__name__
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>obj = MyClass(<span class="hljs-number">1</span>)
<span class="hljs-meta">>>> </span>obj.a, str(obj), repr(obj)
(<span class="hljs-number">1</span>, <span class="hljs-string">'1'</span>, <span class="hljs-string">'MyClass(1)'</span>)
</code></pre>
<ul>
<li><strong>Return value of str() should be readable and of repr() unambiguous.</strong></li>
<li><strong>If only repr() is defined, it will also be used for str().</strong></li>
<li><strong>Methods decorated with <code class="python hljs"><span class="hljs-string">'@staticmethod'</span></code> do not receive 'self' nor 'cls' as their first argument.</strong></li>
</ul>
<div><h4 id="expressionsthatcallthestrmethod">Expressions that call the str() method:</h4><pre><code class="python language-python hljs">print(<obj>)
<span class="hljs-string">f'<span class="hljs-subst">{<obj>}</span>'</span>
logging.warning(<obj>)
csv.writer(<file>).writerow([<obj>])
<span class="hljs-keyword">raise</span> Exception(<obj>)
</code></pre></div>
<div><h4 id="expressionsthatcallthereprmethod">Expressions that call the repr() method:</h4><pre><code class="python language-python hljs">print/str/repr([<obj>])
print/str/repr({<obj>: <obj>})
<span class="hljs-string">f'<span class="hljs-subst">{<obj>!r}</span>'</span>
Z = make_dataclass(<span class="hljs-string">'Z'</span>, [<span class="hljs-string">'a'</span>]); print/str/repr(Z(<obj>))
<span class="hljs-meta">>>> </span><obj>
</code></pre></div>
<div><h3 id="inheritance">Inheritance</h3><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Person</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, name)</span>:</span>
self.name = name
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Employee</span><span class="hljs-params">(Person)</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, name, staff_num)</span>:</span>
super().__init__(name)
self.staff_num = staff_num
</code></pre></div>
<div><h4 id="multipleinheritance">Multiple inheritance:</h4><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">A</span>:</span> <span class="hljs-keyword">pass</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">B</span>:</span> <span class="hljs-keyword">pass</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">C</span><span class="hljs-params">(A, B)</span>:</span> <span class="hljs-keyword">pass</span>
</code></pre></div>
<p><strong>MRO determines the order in which parent classes are traversed when searching for a method or an attribute:</strong></p>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>C.mro()
[<<span class="hljs-class"><span class="hljs-title">class</span> '<span class="hljs-title">C</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">A</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">B</span>'>, <<span class="hljs-title">class</span> '<span class="hljs-title">object</span>'>]
</span></code></pre>
<div><h3 id="typeannotations">Type Annotations</h3><ul>
<li><strong>They add type hints to variables, arguments and functions (<code class="python hljs"><span class="hljs-string">'def f() -> <type>:'</span></code>).</strong></li>
<li><strong>Hints are used by type checkers like <a href="https://pypi.org/project/mypy/">mypy</a>, data validation libraries such as <a href="https://pypi.org/project/pydantic/">Pydantic</a> and lately also by <a href="https://pypi.org/project/Cython/">Cython</a> compiler. However, they are not enforced by CPython interpreter.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> abc
<name>: <type> [| ...] [= <obj>]
<name>: list/set/abc.Iterable/abc.Sequence[<type>] [= <obj>]
<name>: dict/tuple[<type>, ...] [= <obj>]
</code></pre></div>
<div><h3 id="dataclass">Dataclass</h3><p><strong>Decorator that uses class variables to generate init(), repr() and eq() special methods.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> dataclass, field, make_dataclass
<span class="hljs-meta">@dataclass(order=False, frozen=False)</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <<span class="hljs-title">class_name</span>>:</span>
<attr_name>: <type>
<attr_name>: <type> = <default_value>
<attr_name>: list/dict/set = field(default_factory=list/dict/set)
</code></pre></div>
<ul>
<li><strong>Objects can be made <a href="#sortable">sortable</a> with <code class="python hljs"><span class="hljs-string">'order=True'</span></code> and immutable with <code class="python hljs"><span class="hljs-string">'frozen=True'</span></code>.</strong></li>
<li><strong>For object to be <a href="#hashable">hashable</a>, all attributes must be hashable and 'frozen' must be True.</strong></li>
<li><strong>Function field() is needed because <code class="python hljs"><span class="hljs-string">'<attr_name>: list = []'</span></code> would make a list that is shared among all instances. Its 'default_factory' argument can be any <a href="#callable">callable</a>.</strong></li>
<li><strong>For attributes of arbitrary type use <code class="python hljs"><span class="hljs-string">'typing.Any'</span></code>.</strong></li>
</ul>
<pre><code class="python language-python hljs">P = make_dataclass(<span class="hljs-string">'P'</span>, [<span class="hljs-string">'x'</span>, <span class="hljs-string">'y'</span>])
P = make_dataclass(<span class="hljs-string">'P'</span>, [(<span class="hljs-string">'x'</span>, float), (<span class="hljs-string">'y'</span>, float)])
P = make_dataclass(<span class="hljs-string">'P'</span>, [(<span class="hljs-string">'x'</span>, float, <span class="hljs-number">0</span>), (<span class="hljs-string">'y'</span>, float, <span class="hljs-number">0</span>)])
</code></pre>
<div><h3 id="property">Property</h3><p><strong>Pythonic way of implementing getters and setters.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Person</span>:</span>
<span class="hljs-meta"> @property</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">name</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> <span class="hljs-string">' '</span>.join(self._name)
<span class="hljs-meta"> @name.setter</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">name</span><span class="hljs-params">(self, value)</span>:</span>
self._name = value.split()
</code></pre></div>
<pre><code class="python language-python hljs"><span class="hljs-meta">>>> </span>person = Person()
<span class="hljs-meta">>>> </span>person.name = <span class="hljs-string">'\t Guido van Rossum \n'</span>
<span class="hljs-meta">>>> </span>person.name
<span class="hljs-string">'Guido van Rossum'</span>
</code></pre>
<div><h3 id="slots">Slots</h3><p><strong>Mechanism that restricts objects to attributes listed in 'slots', reduces their memory footprint.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyClassWithSlots</span>:</span>
__slots__ = [<span class="hljs-string">'a'</span>]
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self)</span>:</span>
self.a = <span class="hljs-number">1</span>
</code></pre></div>
<div><h3 id="copy">Copy</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> copy <span class="hljs-keyword">import</span> copy, deepcopy
<object> = copy/deepcopy(<object>)
</code></pre></div>
<div><h2 id="ducktypes"><a href="#ducktypes" name="ducktypes">#</a>Duck Types</h2><p><strong>A duck type is an implicit type that prescribes a set of special methods. Any object that has those methods defined is considered a member of that duck type.</strong></p><div><h3 id="comparable">Comparable</h3><ul>
<li><strong>If eq() method is not overridden, it returns <code class="python hljs"><span class="hljs-string">'id(self) == id(other)'</span></code>, which is the same as <code class="python hljs"><span class="hljs-string">'self is other'</span></code>.</strong></li>
<li><strong>That means all user-defined objects compare not equal by default.</strong></li>
<li><strong>Only the left side object has eq() method called, unless it returns NotImplemented, in which case the right object is consulted. False is returned if both return NotImplemented.</strong></li>
<li><strong>Ne() automatically works on any object that has eq() defined.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyComparable</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
self.a = a
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(other, type(self)):
<span class="hljs-keyword">return</span> self.a == other.a
<span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
</code></pre></div></div>
<div><h3 id="hashable">Hashable</h3><ul>
<li><strong>Hashable object needs both hash() and eq() methods and its hash value should never change.</strong></li>
<li><strong>Hashable objects that compare equal must have the same hash value, meaning default hash() that returns <code class="python hljs"><span class="hljs-string">'id(self)'</span></code> will not do.</strong></li>
<li><strong>That is why Python automatically makes classes unhashable if you only implement eq().</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyHashable</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
self._a = a
<span class="hljs-meta"> @property</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">a</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> self._a
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(other, type(self)):
<span class="hljs-keyword">return</span> self.a == other.a
<span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__hash__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> hash(self.a)
</code></pre></div>
<div><h3 id="sortable">Sortable</h3><ul>
<li><strong>With 'total_ordering' decorator, you only need to provide eq() and one of lt(), gt(), le() or ge() special methods and the rest will be automatically generated.</strong></li>
<li><strong>Functions sorted() and min() only require lt() method, while max() only requires gt(). However, it is best to define them all so that confusion doesn't arise in other contexts.</strong></li>
<li><strong>When two lists, strings or dataclasses are compared, their values get compared in order until a pair of unequal values is found. The comparison of this two values is then returned. The shorter sequence is considered smaller in case of all values being equal.</strong></li>
<li><strong>To sort collection of strings in proper alphabetical order pass <code class="python hljs"><span class="hljs-string">'key=locale.strxfrm'</span></code> to sorted() after running <code class="python hljs"><span class="hljs-string">'locale.setlocale(locale.LC_COLLATE, "en_US.UTF-8")'</span></code>.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> total_ordering
<span class="hljs-meta">@total_ordering</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MySortable</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
self.a = a
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(other, type(self)):
<span class="hljs-keyword">return</span> self.a == other.a
<span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__lt__</span><span class="hljs-params">(self, other)</span>:</span>
<span class="hljs-keyword">if</span> isinstance(other, type(self)):
<span class="hljs-keyword">return</span> self.a < other.a
<span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
</code></pre></div>
<div><h3 id="iterator-1">Iterator</h3><ul>
<li><strong>Any object that has methods next() and iter() is an iterator.</strong></li>
<li><strong>Next() should return next item or raise StopIteration exception.</strong></li>
<li><strong>Iter() should return 'self', i.e. unmodified object on which it was called.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Counter</span>:</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self)</span>:</span>
self.i = <span class="hljs-number">0</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__next__</span><span class="hljs-params">(self)</span>:</span>
self.i += <span class="hljs-number">1</span>
<span class="hljs-keyword">return</span> self.i
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__iter__</span><span class="hljs-params">(self)</span>:</span>
<span class="hljs-keyword">return</span> self