Lazly override __init__ method of a Protocol subclasses

Lib/dataclasses.py

@@ -1015,11 +1015,6 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
         # Does this class have a post-init function?
         has_post_init = hasattr(cls, _POST_INIT_NAME)
 
-        # typing.Protocol can override __init__ method to object.__init__
-        inherits_from_protocol = any(getattr(c, '_is_protocol', False) for c in cls.__bases__)
-        if inherits_from_protocol and cls.__dict__.get('__init__') is object.__init__:
-            del cls.__init__
-
         _set_new_attribute(cls, '__init__',
                            _init_fn(all_init_fields,
                                     std_init_fields,

Lib/test/test_dataclasses.py

@@ -2151,6 +2151,8 @@ def __init__(self, x):
         self.assertEqual(C(5).x, 10)
 
     def test_inherit_from_protocol(self):
+        # See bpo-45081.
+
         class P(Protocol):
             a: int
 

Lib/typing.py

@@ -1401,7 +1401,25 @@ def _is_callable_members_only(cls):
 
 
 def _no_init(self, *args, **kwargs):
-    raise TypeError('Protocols cannot be instantiated')
+    cls = type(self)
+
+    if cls._is_protocol:
+        raise TypeError('Protocols cannot be instantiated')
+
+    # set correct __init__ method on a first initialization
+    # so all further initialization will call it directly
+    # see bpo-45081
+    for base in cls.__mro__:
+        init = base.__dict__.get('__init__', _no_init)
+        if init is not _no_init:
+            cls.__init__ = init
+            break
+    else:
+        # should not happen
+        cls.__init__ = object.__init__
+
+    cls.__init__(self, *args, **kwargs)
+
 
 def _caller(depth=1, default='__main__'):
     try:
@@ -1541,15 +1559,6 @@ def _proto_hook(other):
 
         # 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.