Add NewType() to PEP 484.

pep-0484.txt

@@ -1173,6 +1173,68 @@ checker should blindly believe the programmer.  Also, casts can be used
 in expressions, while type comments only apply to assignments.
 
 
+NewType helper function
+-----------------------
+
+There are also situations where a programmer might want to avoid logical
+errors by creating simple classes. For example::
+
+  class UserId(int):
+      pass
+
+  get_by_user_id(user_id: UserId):
+      ...
+
+However, this approach introduces a runtime overhead. To avoid this,
+``typing.py`` provides a helper function ``NewType`` that creates
+simple unique types with almost zero runtime overhead. For a static type
+checker ``Derived = NewType('Derived', Base)`` is roughly equivalent
+to a definition::
+
+class Derived(Base):
+    def __init__(self, _x: Base) -> None:
+        ...
+
+While at runtime, ``NewType('Derived', Base)`` returns a dummy function
+that simply returns its argument. Type checkers require explicit casts
+from ``int`` where ``UserId`` is expected, while implicitly casting
+from ``UserId`` where ``int`` is expected. Examples::
+
+        UserId = NewType('UserId', int)
+
+        def name_by_id(user_id: UserId) -> str:
+            ...
+
+        UserId('user')          # Fails type check
+
+        name_by_id(42)          # Fails type check
+        name_by_id(UserId(42))  # OK
+
+        num = UserId(5) + 1     # type: int
+
+``NewType`` accepts only one argument that shoud be a proper class,
+i.e., not a type construct like ``Union``, etc. The function returned
+by ``NewType`` accepts only one argument; this is equivalent to supporting
+only one constructor accepting an instance of the base class (see above).
+Example::
+
+  class PacketId:
+      def __init__(self, major: int, minor: int) -> None:
+          self._major = major
+          self._minor = minor
+
+  TcpPacketId = NewType('TcpPacketId', PacketId)
+
+  packet = PacketId(100, 100)
+  tcp_packet = TcpPacketId(packet)  # OK
+
+  tcp_packet = TcpPacketId(127, 0)  # Fails in type checker and at runtime
+
+Both ``isinstance`` and ``issubclass``, as well as subclassing will fail
+for ``NewType('Derived', Base)`` since function objects don't support
+these operations.
+
+
 Stub Files
 ==========
 
@@ -1562,6 +1624,9 @@ Convenience definitions:
   This is useful to declare the types of the fields of a named tuple
   type.
 
+* NewType, used to create unique types with little runtime overhead
+  ``UserId = NewType('UserId', int)``
+
 * cast(), described earlier
 
 * @no_type_check, a decorator to disable type checking per class or