bpo-44807: Forbid to define __init__ method in Protocol classes

Lib/test/test_typing.py

@@ -1610,6 +1610,14 @@ class P(Protocol):
         with self.assertRaisesRegex(TypeError, "@runtime_checkable"):
             isinstance(1, P)
 
+    def test_protocol_init_is_forbidden(self): # see bpo-44807
+        with self.assertRaisesRegex(TypeError, 'Protocols can not have __init__ method'):
+            class P(Protocol):
+                x: int
+
+                def __init__(self, x: int):
+                    self.x = x
+
     def test_super_call_init(self):
         class P(Protocol):
             x: int

Lib/typing.py

@@ -1577,6 +1577,10 @@ def _proto_hook(other):
                     issubclass(base, Generic) and base._is_protocol):
                 raise TypeError('Protocols can only inherit from other'
                                 ' protocols, got %r' % base)
+
+        if '__init__' in cls.__dict__:
+            raise TypeError('Protocols can not have __init__ method')
+
         cls.__init__ = _no_init_or_replace_init
 
 

Misc/NEWS.d/next/Library/2021-08-02-12-58-14.bpo-44807.UvHEKh.rst

@@ -0,0 +1,2 @@
+Forbid to define ``__init__`` method in ``Protocol`` classes. Patch provided
+by Yurii Karabas.