[Python-checkins] bpo-45121: Fix RecursionError when calling Protocol.__init__ from a subclass' __init__ (GH-28206)

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https://github.com/python/cpython/commit/c11956a8bddd75f02ccc7b4da7e4d8123e1f3c5f
commit: c11956a8bddd75f02ccc7b4da7e4d8123e1f3c5f
branch: main
author: Yurii Karabas <1998uriyyo at gmail.com>
committer: Fidget-Spinner <28750310+Fidget-Spinner at users.noreply.github.com>
date: 2021-09-08T18:25:09+08:00
summary:

bpo-45121: Fix RecursionError when calling Protocol.__init__ from a subclass' __init__ (GH-28206)

files:
A Misc/NEWS.d/next/Core and Builtins/2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst
M Lib/test/test_typing.py
M Lib/typing.py

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index 847d583cdafb9..fa49b90886c30 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -1610,6 +1610,16 @@ class P(Protocol):
         with self.assertRaisesRegex(TypeError, "@runtime_checkable"):
             isinstance(1, P)
 
+    def test_super_call_init(self):
+        class P(Protocol):
+            x: int
+
+        class Foo(P):
+            def __init__(self):
+                super().__init__()
+
+        Foo()  # Previously triggered RecursionError
+
 
 class GenericTests(BaseTestCase):
 
diff --git a/Lib/typing.py b/Lib/typing.py
index 892f1b3506851..e29d699283dfe 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1406,6 +1406,11 @@ def _no_init_or_replace_init(self, *args, **kwargs):
     if cls._is_protocol:
         raise TypeError('Protocols cannot be instantiated')
 
+    # Already using a custom `__init__`. No need to calculate correct
+    # `__init__` to call. This can lead to RecursionError. See bpo-45121.
+    if cls.__init__ is not _no_init_or_replace_init:
+        return
+
     # 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__`
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst b/Misc/NEWS.d/next/Core and Builtins/2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst
new file mode 100644
index 0000000000000..19eb331412516
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst	
@@ -0,0 +1,2 @@
+Fix issue where ``Protocol.__init__`` raises ``RecursionError`` when it's
+called directly or via ``super()``. Patch provided by Yurii Karabas.