Debug build does not set `-DPy_DEBUG` with cmake

[SimonHeybrock]

TLDR

I think the cmake variable PYTHON_IS_DEBUG is not visible inside the scope of the function pybind11_add_module.

What

Trying to create a debug build, using the debug version of Python with cmake:

• -DCMAKE_BUILD_TYPE=Debug
• -DPYTHON_EXECUTABLE=/usr/lib/python3.6-dgb

Build goes fine, but importing the resulting module gives an Import Error about the undefined symbol PyModule_Create2.

From what I have read, this indicates that we are linking correctly against the debug version of Python, but the includes are wrong, since PyModule_Create2 has a different name in the debug version of Python.

Solution?

• pybind11/tools/pybind11Tools.cmake

  Lines 142 to 147 in 3735249

  	# Python debug libraries expose slightly different objects
  	# https://docs.python.org/3.6/c-api/intro.html#debugging-builds
  	# https://stackoverflow.com/questions/39161202/how-to-work-around-missing-pymodule-create2-in-amd64-win-python35-d-lib
  	if(PYTHON_IS_DEBUG)
  	target_compile_definitions(${target_name} PRIVATE Py_DEBUG)
  	endif()

  is not working as it should.
• I have verified that PYTHON_IS_DEBUG is set correctly in https://github.com/pybind/pybind11/blob/master/tools/FindPythonLibsNew.cmake#L115, and it is also correct in pybind11Tools.cmake (outside the function scope)...
• ... but not when the function is called.

I am having trouble understanding cmake's scoping rules. If I set PYTHON_IS_DEBUG in the cmake CACHE it works. I cannot tell whether this is the correct solution though?

Versions

• cmake 3.10.2
• pybind11 2.4.3
• python3.6-dbg
• Ubuntu