Add yaml config env defaults parsing

src/dependency_injector/providers.pyx

@@ -91,10 +91,25 @@ else:
 
 if yaml:
     # TODO: use SafeLoader without env interpolation by default in version 5.*
-    yaml_env_marker_pattern = re.compile(r'\$\{([^}^{]+)\}')
+    yaml_env_marker_pattern = re.compile(
+        r'\${(?P<name>[^}^{:]+)(?P<separator>:?)(?P<default>.*?)}',
+    )
+
     def yaml_env_marker_constructor(_, node):
         """"Replace environment variable marker with its value."""
-        return os.path.expandvars(node.value)
+        node_value = node.value
+        for match in reversed(list(yaml_env_marker_pattern.finditer(node.value))):
+            has_default = match.group('separator') == ':'
+
+            value = os.getenv(match.group('name'))
+            if value is None:
+                if not has_default:
+                    continue
+                value = match.group('default')
+
+            span_min, span_max = match.span()
+            node_value = f'{node_value[:span_min]}{value}{node_value[span_max:]}'
+        return node_value
 
     yaml.add_implicit_resolver('!path', yaml_env_marker_pattern)
     yaml.add_constructor('!path', yaml_env_marker_constructor)

tests/unit/providers/test_configuration_py2_py3.py

@@ -882,6 +882,30 @@ class ConfigFromYamlWithEnvInterpolationTests(unittest.TestCase):
         self.assertEqual(self.config.section1.value1(), '${CONFIG_TEST_ENV}')
         self.assertEqual(self.config.section1.value2(), '${CONFIG_TEST_PATH}/path')
 
+    @unittest.skipIf(sys.version_info[:2] == (3, 4), 'PyYAML does not support Python 3.4')
+    def test_default_values(self):
+        os.environ['DEFINED'] = 'defined'
+        self.addCleanup(os.environ.pop, 'DEFINED')
+
+        with open(self.config_file, 'w') as config_file:
+            config_file.write(
+                'section:\n'
+                '  defined_with_default: ${DEFINED:default}\n'
+                '  undefined_with_default: ${UNDEFINED:default}\n'
+                '  complex: ${DEFINED}/path/${DEFINED:default}/${UNDEFINED}/${UNDEFINED:default}\n'
+            )
+
+        self.config.from_yaml(self.config_file)
+
+        self.assertEqual(
+            self.config.section(),
+            {
+                'defined_with_default': 'defined',
+                'undefined_with_default': 'default',
+                'complex': 'defined/path/defined/${UNDEFINED}/default',
+            },
+        )
+
     @unittest.skipIf(sys.version_info[:2] == (3, 4), 'PyYAML does not support Python 3.4')
     def test_option_env_variable_interpolation(self):
         self.config.option.from_yaml(self.config_file)