bpo-44806: Fix __init__ in subclasses of protocols (GH-27545)

Non-protocol subclasses of protocol ignore now the __init__ method
inherited from protocol base classes.
(cherry picked from commit 043cd60abed09edddc7185bcf7d039771acc734d)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/Lib/typing.py b/Lib/typing.py
index e492bd2..6f884e1 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1379,8 +1379,7 @@ def _is_callable_members_only(cls):
 
 
 def _no_init(self, *args, **kwargs):
-    if type(self)._is_protocol:
-        raise TypeError('Protocols cannot be instantiated')
+    raise TypeError('Protocols cannot be instantiated')
 
 def _caller(depth=1, default='__main__'):
     try:
@@ -1523,6 +1522,15 @@ class Protocol(Generic, metaclass=_ProtocolMeta):
 
         # We have nothing more to do for non-protocols...
         if not cls._is_protocol:
+            if cls.__init__ == _no_init:
+                for base in cls.__mro__:
+                    init = base.__dict__.get('__init__', _no_init)
+                    if init != _no_init:
+                        cls.__init__ = init
+                        break
+                else:
+                    # should not happen
+                    cls.__init__ = object.__init__
             return
 
         # ... otherwise check consistency of bases, and prohibit instantiation.