Why doesn't setup_requires work properly for numpy?

Question

I wanted to create a setup.py file that automatically resolves a build-time dependency to numpy (for compiling extensions). My first guess was to use setu

Answer 1

Figured out, that a proper initialization of the numpy module is prevented by a check for __NUMPY_SETUP__ inside numpy/__init__.py:

if __NUMPY_SETUP__:
    import sys as _sys
    _sys.stderr.write('Running from numpy source directory.\n')
    del _sys
else:
    # import subodules etc. (main branch)

This global state is not reset by setuptools after the installation. The following works:

...
def run(self):
    __builtins__.__NUMPY_SETUP__ = False
    import numpy
    ...