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If __eq__() raise an exception, _PyDict_GetItem_KnownHash() currently returns NULL and pass through the exception. To me, it looks like the correct behaviour.
With your patch, it looks like the _PyDict_GetItem_KnownHash() clears the __eq__() exception: return NULL with no exception set, as if the key is simply missing.
PyDict_GetItem() ignores *almost* all exceptions, but for bad reasons:
/* Note that, for historical reasons, PyDict_GetItem() suppresses all errors
 * that may occur (originally dicts supported only string keys, and exceptions
 * weren't possible). (...) */
I would prefer to not ignore __eq__ exceptions, but pass them through.
To be clear: this is a behaviour change compared to Python 3.5 which works as PyDict_GetItem(), ignore *all* exceptions:
 ep = (mp->ma_keys->dk_lookup)(mp, key, hash, &value_addr);
 if (ep == NULL) {
 PyErr_Clear();
 return NULL;
 }
I consider that it's ok to change _PyDict_GetItem_KnownHash() behaviour because this function is private and only used 4 times in 250k lines of C code.
Would you be interested to write a different patch to pass through the exception?
Note: It seems like _count_elements() (Modules/_collectionsmodule.c) doesn't handle correctly such error.