PEP 544: Couple final edits

pep-0544.txt

@@ -611,6 +611,32 @@ The self-types in protocols follow the corresponding specification
   c = Other()  # Also OK
 
 
+Callback protocols
+------------------
+
+Protocols can be used to define flexible callback types that are hard
+(or even impossible) to express using the ``Callable[...]`` syntax
+specified by PEP 484, such as variadic, overloaded, and complex generic
+callbacks. They can be defined as protocols with a ``__call__`` member::
+
+  from typing import Optional, List, Protocol
+
+  class Combiner(Protocol):
+      def __call__(self, *vals: bytes,
+                   maxlen: Optional[int] = None) -> List[bytes]: ...
+
+  def good_cb(*vals: bytes, maxlen: Optional[int] = None) -> List[bytes]:
+      ...
+  def bad_cb(*vals: bytes, maxitems: Optional[int]) -> List[bytes]:
+      ...
+
+  comb: Combiner = good_cb  # OK
+  comb = bad_cb  # Error! Argument 2 has incompatible type because of
+                 # different name and kind in the callback
+
+Callback protocols and ``Callable[...]`` types can be used interchangeably.
+
+
 Using Protocols
 ===============
 
@@ -930,7 +956,7 @@ effects on the core interpreter and standard library except in the
 ``typing`` module, and a minor update to ``collections.abc``:
 
 * Define class ``typing.Protocol`` similar to ``typing.Generic``.
-* Implement metaclass functionality to detect whether a class is
+* Implement functionality to detect whether a class is
   a protocol or not. Add a class attribute ``_is_protocol = True``
   if that is the case. Verify that a protocol class only has protocol
   base classes in the MRO (except for object).