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convert f strings to concat #32

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22 changes: 16 additions & 6 deletions py_backwards/transformers/formatted_values.py
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Original file line number Diff line number Diff line change
@@ -1,3 +1,5 @@
import functools

from typed_ast import ast3 as ast
from ..const import TARGET_ALL
from .base import BaseNodeTransformer
Expand All @@ -7,7 +9,7 @@ class FormattedValuesTransformer(BaseNodeTransformer):
"""Compiles:
f"hello {x}"
To
''.join(['hello ', '{}'.format(x)])
'hello ' + '{}'.format(x)

"""
target = TARGET_ALL
Expand All @@ -28,9 +30,17 @@ def visit_FormattedValue(self, node: ast.FormattedValue) -> ast.Call:

def visit_JoinedStr(self, node: ast.JoinedStr) -> ast.Call:
self._tree_changed = True
NONE = ast.Str(s='')

init = ast.BinOp(left=NONE, right=NONE, op=ast.Add())

def to_binop(acc, next):
left, right, op = acc.left, acc.right, acc.op
if left is NONE:
return ast.BinOp(left=right, right=next, op=op)
return ast.BinOp(left=acc, right=next, op=op)

value = functools.reduce(to_binop, node.values, init)
concat_expr = ast.Expr(value=value)

join_call = ast.Call(func=ast.Attribute(value=ast.Str(s=''),
attr='join'),
args=[ast.List(elts=node.values)],
keywords=[])
return self.generic_visit(join_call) # type: ignore
return self.generic_visit(concat_expr) # type: ignore
5 changes: 3 additions & 2 deletions tests/transformers/test_formatted_values.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4,9 +4,10 @@

@pytest.mark.parametrize('before, after', [
("f'hi'", "'hi'"),
("f'hi {x}'", "''.join(['hi ', '{}'.format(x)])"),
("f'hi {x}'", "'hi ' + '{}'.format(x)"),
("d = f'{a} {b}!'", "d = '{}'.format(a) + ' ' + '{}'.format(b) + '!'"),
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@graingert graingert Jun 5, 2017

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can't work out why this becomes:

d = 
'{}'.format(a) + ' ' + '{}'.format(b) + '!'

("f'hi {x.upper()} {y:1}'",
"''.join(['hi ', '{}'.format(x.upper()), ' ', '{:1}'.format(y)])")])
"'hi ' + '{}'.format(x.upper()) + ' ' + '{:1}'.format(y)")])
def test_transform(transform, ast, before, after):
code = transform(FormattedValuesTransformer, before)
assert ast(code) == ast(after)
Expand Down