bpo-45081: Fix __init__ method generation when inheriting from Protocol

Lib/test/test_dataclasses.py

@@ -10,7 +10,7 @@
 import builtins
 import unittest
 from unittest.mock import Mock
-from typing import ClassVar, Any, List, Union, Tuple, Dict, Generic, TypeVar, Optional
+from typing import ClassVar, Any, List, Union, Tuple, Dict, Generic, TypeVar, Optional, Protocol
 from typing import get_type_hints
 from collections import deque, OrderedDict, namedtuple
 from functools import total_ordering
@@ -2150,6 +2150,26 @@ def __init__(self, x):
  self.x = 2 * x
  self.assertEqual(C(5).x, 10)
 
+ def test_inherit_from_protocol(self):
+ # Dataclasses inheriting from protocol should preserve their own `__init__`.
+ # See bpo-45081.
+
+ class P(Protocol):
+ a: int
+
+ @dataclass
+ class C(P):
+ a: int
+
+ self.assertEqual(C(5).a, 5)
+
+ @dataclass
+ class D(P):
+ def __init__(self, a):
+ self.a = a * 2
+
+ self.assertEqual(D(5).a, 10)
+
 
 class TestRepr(unittest.TestCase):
  def test_repr(self):

Lib/typing.py

@@ -1400,8 +1400,29 @@ def _is_callable_members_only(cls):
  return all(callable(getattr(cls, attr, None)) for attr in _get_protocol_attrs(cls))
 
 
-def _no_init(self, *args, **kwargs):
- raise TypeError('Protocols cannot be instantiated')
+def _no_init_or_replace_init(self, *args, **kwargs):
+ cls = type(self)
+
+ if cls._is_protocol:
+ raise TypeError('Protocols cannot be instantiated')
+
+ # Initially, `__init__` of a protocol subclass is set to `_no_init_or_replace_init`.
+ # The first instantiation of the subclass will call `_no_init_or_replace_init` which
+ # searches for a proper new `__init__` in the MRO. The new `__init__`
+ # replaces the subclass' old `__init__` (ie `_no_init_or_replace_init`). Subsequent
+ # instantiation of the protocol subclass will thus use the new
+ # `__init__` and no longer call `_no_init_or_replace_init`.
+ for base in cls.__mro__:
+ init = base.__dict__.get('__init__', _no_init_or_replace_init)
+ if init is not _no_init_or_replace_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 +1562,6 @@ 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.
@@ -1560,7 +1572,7 @@ def _proto_hook(other):
  issubclass(base, Generic) and base._is_protocol):
  raise TypeError('Protocols can only inherit from other'
  ' protocols, got %r' % base)
- cls.__init__ = _no_init
+ cls.__init__ = _no_init_or_replace_init
 
 
 class _AnnotatedAlias(_GenericAlias, _root=True):

Misc/NEWS.d/next/Library/2021-09-02-12-42-25.bpo-45081.tOjJ1k.rst

@@ -0,0 +1,2 @@
+Fix issue when dataclasses that inherit from ``typing.Protocol`` subclasses
+have wrong ``__init__``. Patch provided by Yurii Karabas.