Classvars (#455)

* Support specialized ClassVars, e. g. ClassVar[int] now works in a model, fix #435

* Use apostrophes and pass the test also on python 3.6, where ClassVar[int] is a ClassVar, not a _GenericAlias. Note that the == operator gives False in this case, so we must really compare the classes directly.

* Another attempt at the ClassVar check.

* Ran make format.

* Make mypy happy.

* add HISTORY

HISTORY.rst

@@ -9,6 +9,7 @@ v0.23 (unreleased)
 * improve README.rst to include essential information about the package, #446 by @pilosus
 * ``IntEnum`` support, #444 by @potykion
 * fix ``ForwardRef`` collection bug, #450 by @tigerwings
+* Support specialized ``ClassVars``, #455 by @tyrylu
 
 v0.22 (2019-03-29)
 ....................

pydantic/main.py

@@ -11,7 +11,6 @@ from typing import (
     TYPE_CHECKING,
     Any,
     Callable,
-    ClassVar,
     Dict,
     Generator,
     List,
@@ -38,6 +37,7 @@ from .utils import (
     AnyType,
     ForwardRef,
     change_exception,
+    is_classvar,
     resolve_annotations,
     truncate,
     validate_field_name,
@@ -167,7 +167,7 @@ class MetaModel(ABCMeta):
         class_vars = set()
         # annotation only fields need to come first in fields
         for ann_name, ann_type in annotations.items():
-            if ann_type == ClassVar:
+            if is_classvar(ann_type):
                 class_vars.add(ann_name)
             elif not ann_name.startswith('_') and ann_name not in namespace:
                 validate_field_name(bases, ann_name)

pydantic/utils.py

@@ -7,7 +7,7 @@ from functools import lru_cache
 from importlib import import_module
 from textwrap import dedent
 from typing import _eval_type  # type: ignore
-from typing import TYPE_CHECKING, Any, Dict, Generator, List, Optional, Pattern, Tuple, Type, Union
+from typing import TYPE_CHECKING, Any, ClassVar, Dict, Generator, List, Optional, Pattern, Tuple, Type, Union
 
 from . import errors
 
@@ -268,3 +268,11 @@ def resolve_annotations(raw_annotations: Dict[str, AnyType], module_name: Option
 
 def is_callable_type(type_: AnyType) -> bool:
     return type_ is Callable or getattr(type_, '__origin__', None) is Callable
+
+
+def _check_classvar(v: AnyType) -> bool:
+    return type(v) == type(ClassVar) and (sys.version_info < (3, 7) or getattr(v, '_name', None) == 'ClassVar')
+
+
+def is_classvar(ann_type: AnyType) -> bool:
+    return _check_classvar(ann_type) or _check_classvar(getattr(ann_type, '__origin__', None))

tests/test_main.py

@@ -516,7 +516,7 @@ def test_value_field_name_shadows_attribute():
 def test_class_var():
     class MyModel(BaseModel):
         a: ClassVar
-        b: ClassVar = 1
+        b: ClassVar[int] = 1
         c: int = 2
 
     assert list(MyModel.__fields__.keys()) == ['c']