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gh-125788: Improve ctypes error message for mismatched callback prototypes by aribaskagan · Pull Request #161873 · python/cpython · GitHub
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gh-125788: Improve ctypes error message for mismatched callback prototypes - #161873

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gh-125788: Improve ctypes error message for mismatched callback prototypes#161873
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aribaskagan:gh-125788-ctypes-callback-error

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@aribaskagan aribaskagan commented Nov 12, 2026 •
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When a CFUNCTYPE/WINFUNCTYPE callback is passed where a different callback prototype is expected, ctypes raised a confusing error because both prototype classes share the name CFunctionType:

ctypes.ArgumentError: argument 4: TypeError: expected CFunctionType instance instead of CFunctionType

This change makes CDataType_from_param_impl() report the expected and actual signatures when both sides are callback prototypes with argtypes set:

ctypes.ArgumentError: argument 4: TypeError: expected CFunctionType instance with signature (restype=<class 'ctypes.c_int'>, argtypes=(<class 'ctypes.c_void_p'>, <class 'ctypes.c_void_p'>)) instead of CFunctionType instance with signature (restype=<class 'ctypes.c_long'>, argtypes=(<class 'ctypes.c_void_p'>, <class 'ctypes.c_void_p'>))

Call behavior is unchanged; other mismatches keep the existing wording. Added test_mismatched_callback_prototype based on the qsort reproducer from the issue, plus a NEWS entry. ./python -m test test_ctypes passes on a Linux debug build.

AI assistance was used in preparing this change; I reviewed and tested it.

@aribaskagan
pythongh-125788: Improve ctypes error message for mismatched callback…
fd17c9a 
… prototypes

CFUNCTYPE and WINFUNCTYPE prototypes are distinct classes that share one
type name, so a mismatch was reported as "expected CFunctionType instance
instead of CFunctionType".  Include both signatures in the TypeError.

Co-authored-by: Kağan Arıbaş <aribaskagan@gmail.com>
Copilot AI balanced review requested due to automatic review settings November 12, 2026 14:20
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@bedevere-app bedevere-app Bot added the type-feature A feature request or enhancement label Nov 12, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Nov 12, 2026
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Codex Review Summary: ✅ Completed

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📝 Code Review ✅ Completed 2026-11-12T14:24:58.355484Z f6c92af PR opened
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cursor Bot force-pushed the gh-125788-ctypes-callback-error branch from f6c92af to fd17c9a Compare November 12, 2026 17:02
@ZeroIntensity ZeroIntensity added the AI-needs-explanation AI-assisted work that needs further explanation and follow up from the contributor label Nov 12, 2026
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