Hello there. I'm sure this has come up before, but here it is again: Python exports a different api in debug mode, depending on whether PYMALLOC_DEBUG and WITH_PYMALLOC are exported. This means that _d.pyd files that are used must have been compiled with a version of python using the same settings for these macros. It is unfortunate that the _PyObject_DebugMalloc() api is exposed to external applications using macros in objimpl.h I would suggest two things: 1) provide dummy or thunking versions of those in builds that don't have PYMALLOC_DEBUG impolemented, that thunk to PyObject_Malloc et al. (This is what we have done at CCP) 2) Remove the _PyObject_DebugMalloc() from the api. It really should be an implementation of in the exposed PyObject_Malloc() functions whether they use debug functionality at all. the _PyObject_DebugCheckAddress and _PyObject_DebugDumpAddress() can be left in place. But exposing this functionality in macros that external moduled compile in, is not good at all. The reason why this is annoying: Some external software comes with proprietary .pyd bindings. When developing my own application, with modified preprocessor definitions (e.g. to turn off PYMALLOC_DEBUG) we find that those externally provided libraries don't work. It takes a fair amount of detective work to find out why exactly linkage fails. The external API really shouldn't change depending on preprocessor definitions. Cheers, K