Add option for raising an error on missing ENV variable

README.md

@@ -154,9 +154,26 @@ test1:
 ```
 will raise a `ValueError` because `data1:  !TEST ${ENV_TAG2}` there is no default value for `ENV_TAG2` in this line.
 
---- 
+---
+#### Raise an exception on missing environment variable without default
+
+Often, a proper default value for an environment variable doesn't exist and
+an exception should be thrown when the environment variable is not set.
+
+If you want config parsing to fail if the environment variable is missing 
+and no *default value* is specified, you can set the `raise_if_missing`
+flag to `True`.
+
+For example:
 
+```yaml
+test1:
+    data1: !TEST ${ENV_TAG2}
+```
+will raise a `ValueError` when `ENV_TAG2` is missing because no default is set
+explicitly. 
 
+---
 #### Using a different loader:
 
 The default yaml loader is `yaml.SafeLoader`. If you need to work with serialized Python objects, 

src/pyaml_env/parse_config.py

@@ -11,7 +11,8 @@ def parse_config(
         default_value='N/A',
         raise_if_na=False,
         loader=yaml.SafeLoader,
-        encoding='utf-8'
+        encoding='utf-8',
+        raise_if_missing=False
 ):
     """
         Load yaml configuration from path or from the contents of a file (data)
@@ -39,6 +40,8 @@ def parse_config(
         yaml.SafeLoader
         :param str encoding: the encoding of the data if a path is specified,
         defaults to utf-8
+        :param bool raise_if_missing: raise a ValueError when an env variable
+        is not set and no default value is given
         :return: the dict configuration
         :rtype: dict[str, T]
         """
@@ -86,10 +89,15 @@ def constructor_env_variables(loader, node):
                             _, curr_default_value = each.split(default_sep, 1)
                             found = True
                             break
-                    if not found and raise_if_na:
-                        raise ValueError(
-                            f'Could not find default value for {env_var_name}'
-                        )
+                    if not found:
+                        if raise_if_na:
+                            raise ValueError(f'Could not find default value '
+                                             f'for {env_var_name}')
+                        if raise_if_missing and env_var_name not in os.environ:
+                            raise ValueError(f'Could not find env variable '
+                                             f'{env_var_name} and no default '
+                                             f'is set')
+
                 full_value = full_value.replace(
                     f'${{{env_var_name_with_default}}}',
                     os.environ.get(env_var_name, curr_default_value)

tests/pyaml_env_tests/test_parse_config.py

@@ -582,6 +582,18 @@ def test_parse_config_default_sep_blank_value_error(self):
             expected_config
         )
 
+    def test_parse_config_raise_if_missing_raised(self):
+        test_data = 'data0: !TAG ${ENV_TAG1}'
+        self.assertRaises(ValueError, parse_config, data=test_data, tag='!TAG',
+                          raise_if_missing=True)
+
+    def test_parse_config_raise_if_missing_not_raised(self):
+        default_value = 'default_value'
+        test_data = f'data0: !TAG ${{ENV_TAG1:{default_value}}}'
+        config = parse_config(data=test_data, tag='!TAG',
+                              raise_if_missing=True)
+        self.assertEqual(config['data0'], default_value)
+
     def test_default_loader(self):
         os.environ[self.env_var1] = 'it works!'
         os.environ[self.env_var2] = 'this works too!'