[Python-checkins] [3.9] bpo-45121: Fix RecursionError when calling Protocol.__init__ from a subclass' __init__ (GH-28206) (GH-28233)

ambv webhook-mailer at python.org
Wed Sep 8 11:02:01 EDT 2021

commit: 99506dcbbe9fb56ceabe55f0a4333e5981b72095
branch: 3.9
author: Ken Jin <28750310+Fidget-Spinner at users.noreply.github.com>
committer: ambv <lukasz at langa.pl>
date: 2021-09-08T17:01:51+02:00

[3.9] bpo-45121: Fix RecursionError when calling Protocol.__init__ from a subclass' __init__ (GH-28206) (GH-28233)

Co-authored-by: Yurii Karabas <1998uriyyo at gmail.com>

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 e05ac4539fb503..930f675c624ec4 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -1459,6 +1459,17 @@ class CustomProtocol(TestCase, Protocol):
         class CustomContextManager(typing.ContextManager, Protocol):
+    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):
     def test_basics(self):
diff --git a/Lib/typing.py b/Lib/typing.py
index 9010775c0de7e1..775f17822afc1a 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1085,6 +1085,11 @@ 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 00000000000000..19eb3314125167
--- /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.

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