fix(cmake): unset stale PYTHON_MODULE_DEBUG_POSTFIX and correct USE_PYTHON_INCLUDE_DIR variable - #6086
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henryiii merged 1 commit intoJul 23, 2026
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…YTHON_INCLUDE_DIR variable
When the Python executable changes between CMake runs,
PYTHON_MODULE_DEBUG_POSTFIX was not being unset alongside PYTHON_IS_DEBUG
and PYTHON_MODULE_EXTENSION, allowing a stale postfix to survive interpreter
changes.
In the USE_PYTHON_INCLUDE_DIR block, the elseif branch checked
PYTHON_INCLUDE_DIR (singular) but expanded PYTHON_INCLUDE_DIRS (plural),
always silently resolving to an empty string. Correct it to expand
${PYTHON_INCLUDE_DIR}. Also add a Python3_INCLUDE_DIRS branch so the
option works when pybind11NewTools resolves Python via the Python3
find-package component.
Assisted-by: ClaudeCode:claude-fable-5
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Part of #6084
Summary
Two small CMake correctness fixes:
Fix 1 — stale
PYTHON_MODULE_DEBUG_POSTFIXon interpreter change (tools/pybind11NewTools.cmake): When the Python executable changes between CMake runs,PYTHON_IS_DEBUGandPYTHON_MODULE_EXTENSIONare unset from the cache to force recomputation, butPYTHON_MODULE_DEBUG_POSTFIXwas not. The subsequentif(NOT DEFINED PYTHON_MODULE_DEBUG_POSTFIX)guard then skips recomputation and the stale postfix from the previous interpreter survives. The fix addsunset(PYTHON_MODULE_DEBUG_POSTFIX CACHE)alongside the other two unsets.Fix 2 — variable mismatch in
USE_PYTHON_INCLUDE_DIR(CMakeLists.txt): Theelseifbranch checkedPYTHON_INCLUDE_DIR(singular, the old FindPythonLibs variable) but then expanded${PYTHON_INCLUDE_DIRS}(plural), which is always empty in that path, silently installing headers to a wrong location. The fix uses the variable that was actually checked (${PYTHON_INCLUDE_DIR}). APython3_INCLUDE_DIRSbranch is also added soUSE_PYTHON_INCLUDE_DIRworks when pybind11NewTools resolves Python via thePython3component (the_Pythonvariable isPython3in that case, so the existingPython_INCLUDE_DIRSbranch never matches).Suggested changelog entry:
PYTHON_MODULE_DEBUG_POSTFIXand correctUSE_PYTHON_INCLUDE_DIRvariable.