Fix extra behaviour for multiple inheritance/mix-ins (pydantic#394)

* Fix `extra` behaviour for multiple inheritance/mix-ins

* Update HISTORY

HISTORY.rst

@@ -10,6 +10,7 @@ v0.20.0 (unreleased)
   however the variadic key word argument ("``**kwargs``") **must** be called ``kwargs``, #388 by @samuelcolvin
 * Adds ``skip_defaults`` argument to ``BaseModel.dict()`` to allow skipping of fields that were not
   explicitly set, #389 by @dgasmith
+* Fix ``extra`` behaviour for multiple inheritance/mix-ins, #394 by @YaraslauZhylko
 
 v0.19.0 (2019-02-04)
 ....................

pydantic/main.py

@@ -120,7 +120,7 @@ def set_extra(config: BaseConfig, cls_name: str) -> None:
             warnings.warn(
                 f'{cls_name}: "allow_extra" is deprecated and replaced by "extra", see {EXTRA_LINK}', DeprecationWarning
             )
-    else:
+    elif not isinstance(config.extra, Extra):
         try:
             config.extra = Extra(config.extra)
         except ValueError:

tests/test_edge_cases.py

@@ -793,3 +793,58 @@ class Model(BaseModel):
 
             class Config:
                 extra = 'foo'
+
+
+def test_multiple_inheritance_config():
+    class Parent(BaseModel):
+        class Config:
+            allow_mutation = False
+            extra = Extra.forbid
+
+    class Mixin(BaseModel):
+        class Config:
+            use_enum_values = True
+
+    class Child(Mixin, Parent):
+        class Config:
+            allow_population_by_alias = True
+
+    assert BaseModel.__config__.allow_mutation is True
+    assert BaseModel.__config__.allow_population_by_alias is False
+    assert BaseModel.__config__.extra is Extra.ignore
+    assert BaseModel.__config__.use_enum_values is False
+
+    assert Parent.__config__.allow_mutation is False
+    assert Parent.__config__.allow_population_by_alias is False
+    assert Parent.__config__.extra is Extra.forbid
+    assert Parent.__config__.use_enum_values is False
+
+    assert Mixin.__config__.allow_mutation is True
+    assert Mixin.__config__.allow_population_by_alias is False
+    assert Mixin.__config__.extra is Extra.ignore
+    assert Mixin.__config__.use_enum_values is True
+
+    assert Child.__config__.allow_mutation is False
+    assert Child.__config__.allow_population_by_alias is True
+    assert Child.__config__.extra is Extra.forbid
+    assert Child.__config__.use_enum_values is True
+
+
+def test_multiple_inheritance_config_legacy_extra():
+    with pytest.warns(DeprecationWarning, match='Parent: "ignore_extra" and "allow_extra" are deprecated and'):
+
+        class Parent(BaseModel):
+            class Config:
+                allow_extra = False
+                ignore_extra = False
+
+        class Mixin(BaseModel):
+            pass
+
+        class Child(Mixin, Parent):
+            pass
+
+    assert BaseModel.__config__.extra is Extra.ignore
+    assert Parent.__config__.extra is Extra.forbid
+    assert Mixin.__config__.extra is Extra.ignore
+    assert Child.__config__.extra is Extra.forbid