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

1 files changed, 5 insertions, 0 deletions

diff --git a/Lib/typing.py b/Lib/typing.py
index 892f1b3..e29d699 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__`