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When a body of C extensions needs to be ported from python <3.8 to 3.8, one of the issues one might run into is improperly defined methods in a C extension, which results in SystemErrors stating:
>>> SystemError: bad call flags
This new behavior was added as part of Issue # 33012.
While the issues definitely need to be resolved in the C extensions, where to start is not completely clear. I had to put `printfs` in PyCFunction_NewEx and PyDescr_NewMethod to track down the issues, e.g.,
>>> printf("method name: %s\n", method->ml_name);
While this might be misleading for duplicate method definitions, it definitely helps narrow down the offending code.
Adding the method name to the SystemError would be a big step in the right direction in terms of making it easier to resolve these issues.
PS I realize that this might be masked by casting PyCFunction on methods or by not using gcc 8+, but I'd argue that C extensions need to have developer issues like this be clearer to the end-reader.

New changeset c7d2d69d95b263ee5f83511bc6fbe53acdc24ea3 by Victor Stinner in branch 'master':
bpo-39884: Add method name in "bad call flags" error (GH-18944)
https://github.com/python/cpython/commit/c7d2d69d95b263ee5f83511bc6fbe53acdc24ea3

New changeset 03ac090c5f8d5b281e56c5f5431c1754fd4efe5c by Victor Stinner in branch '3.8':
bpo-39884: Add method name in "bad call flags" error (GH-18944) (GH-18956)
https://github.com/python/cpython/commit/03ac090c5f8d5b281e56c5f5431c1754fd4efe5c

New changeset 6a12676b1910d52c85561bdf4f1e20aa13fc8f46 by Victor Stinner in branch '3.7':
bpo-39884: Add method name in "bad call flags" error (GH-18944) (GH-18957)
https://github.com/python/cpython/commit/6a12676b1910d52c85561bdf4f1e20aa13fc8f46

> While the issues definitely need to be resolved in the C extensions, where to start is not completely clear. I had to put `printfs` in PyCFunction_NewEx and PyDescr_NewMethod to track down the issues, e.g., (...)
I agree, I had the same issue :-) I fixed the issue in 3.7, 3.8 and master branches.
In 3.7, the modified functions are different and the error message was different but similar.
Thanks for your bug report ;-)