Added Type[C] implementation to typing.py.

Author: gvanrossum

Lib/test/test_typing.py

@@ -15,6 +15,7 @@
 from typing import cast
 from typing import get_type_hints
 from typing import no_type_check, no_type_check_decorator
+from typing import Type
 from typing import NamedTuple
 from typing import IO, TextIO, BinaryIO
 from typing import Pattern, Match
@@ -1373,6 +1374,33 @@ def manager():
         self.assertNotIsInstance(42, typing.ContextManager)
 
 
+class TypeTests(BaseTestCase):
+
+    def test_type_basic(self):
+
+        class User: pass
+        class BasicUser(User): pass
+        class ProUser(User): pass
+
+        def new_user(user_class: Type[User]) -> User:
+            return user_class()
+
+        joe = new_user(BasicUser)
+
+    def test_type_typevar(self):
+
+        class User: pass
+        class BasicUser(User): pass
+        class ProUser(User): pass
+
+        U = TypeVar('U', bound=User)
+
+        def new_user(user_class: Type[U]) -> U:
+            return user_class()
+
+        joe = new_user(BasicUser)
+
+
 class NamedTupleTests(BaseTestCase):
 
     def test_basics(self):

Lib/typing.py

@@ -19,9 +19,10 @@
     'Callable',
     'Generic',
     'Optional',
+    'Tuple',
+    'Type',
     'TypeVar',
     'Union',
-    'Tuple',
 
     # ABCs (from collections.abc).
     'AbstractSet',  # collections.abc.Set.
@@ -447,6 +448,7 @@ def __subclasscheck__(self, cls):
 
 
 # Some unconstrained type variables.  These are used by the container types.
+# (These are not for export.)
 T = TypeVar('T')  # Any type.
 KT = TypeVar('KT')  # Key type.
 VT = TypeVar('VT')  # Value type.
@@ -456,6 +458,7 @@ def __subclasscheck__(self, cls):
 T_contra = TypeVar('T_contra', contravariant=True)  # Ditto contravariant.
 
 # A useful type variable with constraints.  This represents string types.
+# (This one *is* for export!)
 AnyStr = TypeVar('AnyStr', bytes, str)
 
 
@@ -1572,6 +1575,35 @@ def __new__(cls, *args, **kwds):
         return super().__new__(cls, *args, **kwds)
 
 
+# Internal type variable used for Type[].
+CT = TypeVar('CT', covariant=True, bound=type)
+
+
+class Type(type, Generic[CT], extra=type):
+    """A generic type usable to annotate class objects.
+
+    For example, suppose we have the following classes::
+
+      class User: ...  # Abstract base for User classes
+      class BasicUser(User): ...
+      class ProUser(User): ...
+      class TeamUser(User): ...
+
+    And a function that takes a class argument that's a subclass of
+    User and returns an instance of the corresponding class::
+
+      U = TypeVar('U', bound=User)
+      def new_user(user_class: Type[U]) -> U:
+          user = user_class()
+          # (Here we could write the user object to a database)
+          return user
+
+      joe = new_user(BasicUser)
+
+    At this point the type checker knows that joe has type BasicUser.
+    """
+
+
 def NamedTuple(typename, fields):
     """Typed version of namedtuple.