Building a Python-C-Extension on Windows with a debug Python installation

Question

If I build CPython from source on Windows I encounter problems when I want to pip install a package that contains a C-Extension. It seems like the error happens while linking the libraries.

For example when installing cython (but it also crashes with the same error on other C extension packages):

LINK : fatal error LNK1104: cannot open file 'python38.lib'

error: command 'C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.23.28105\bin\HostX86\x86\link.exe' failed with exit status 1104

The reason why it cannot open the "python38.lib" is because the ".lib" file in debug mode is called "python38_d.lib".

A minimal reproducible example would be (on the command-line) based on the Quick Reference of the CPython developer guide:

git clone --branch v3.8.0 https://github.com/python/cpython.git
cd cpython
git checkout v3.8.0
.\PCbuild\build.bat -e -d
.\PCbuild\win32\python_d.exe -m ensurepip
.\PCbuild\win32\python_d.exe -m pip install pip --upgrade -vv
.\PCbuild\win32\python_d.exe -m pip install setuptools --upgrade -vv
.\PCbuild\win32\python_d.exe -m pip install cython -vv

The resulting distutils.sysconfig.get_config_vars() is:

{'BINDIR': '...\\cpython\\PCbuild\\win32',
 'BINLIBDEST': ...\\cpython\\Lib',
 'EXE': '.exe',
 'EXT_SUFFIX': '_d.cp38-win32.pyd',
 'INCLUDEPY': '...\\cpython\\include;...\\cpython\\PC',
 'LIBDEST': '...\\cpython\\Lib',
 'SO': '_d.cp38-win32.pyd',
 'VERSION': '38',
 'exec_prefix': '...\\cpython',
 'prefix': '...\\cpython',
 'srcdir': '...\\cpython'}

Is there something I'm missing? Is building C-Extensions on Python-debug builds on Windows simply not supported? If it is supported: how would I do it?

Answer 1

This code is a little bit hacky, but works for me on MSVC19, allowing to debug application without building debug python libraries.

#ifdef _DEBUG
#define _DEBUG_WAS_DEFINED
#undef _DEBUG
#endif

#include "Python.h"

#ifdef _DEBUG_WAS_DEFINED
#define _DEBUG
#undef _DEBUG_WAS_DEFINED
#endif