Q:

Add numpy.get_include() argument to setuptools without preinstalled numpy

I am currently developing a python package that uses cython and numpy and I want the package to be installable using the pip install command from a clean python installation. All dependencies should be installed automatically. I am using setuptools with the following setup.py:
import setuptools

my_c_lib_ext = setuptools.Extension(
    name="my_c_lib",
    sources=["my_c_lib/some_file.pyx"]
)

setuptools.setup(
    name="my_lib",
    version="0.0.1",
    author="Me",
    author_email="me@myself.com",
    description="Some python library",
    packages=["my_lib"],
    ext_modules=[my_c_lib_ext],
    setup_requires=["cython >= 0.29"],
    install_requires=["numpy >= 1.15"],
    classifiers=[
        "Programming Language :: Python :: 3",
        "Operating System :: OS Independent"
    ]
)

This has worked great so far. The pip install command downloads cython for the build and is able to build my package and install it together with numpy.
Now I want to improve the performance of my cython code, which leads to some changes in my setup.py. I need to add include_dirs=[numpy.get_include()] to either the call of setuptools.Extension(...) or setuptools.setup(...) which means that I also need to import numpy. (See http://docs.cython.org/en/latest/src/tutorial/numpy.html and Make distutils look for numpy header files in the correct place for rationals.)
This is bad. Now the user cannot call pip install from a clean environment, because import numpy will fail. The user needs to pip install numpy before installing my library. Even if I move "numpy >= 1.15" from install_requires to setup_requires the installation fails, because the import numpy is evaluated earlier.
Is there a way to evaluate the include_dirs at a later point of the installation, for example, after the dependencies from setup_requires or install_requires have been resolved? I really like to have all dependencies resolved automatically and I dont want the user to type multiple pip install commands.
The following snippet works, but it is not officially supported because it uses an undocumented (and private) method:
class NumpyExtension(setuptools.Extension):
    # setuptools calls this function after installing dependencies
    def _convert_pyx_sources_to_lang(self):
        import numpy
        self.include_dirs.append(numpy.get_include())
        super()._convert_pyx_sources_to_lang()

my_c_lib_ext = NumpyExtension(
    name="my_c_lib",
    sources=["my_c_lib/some_file.pyx"]
)

The article How to Bootstrap numpy installation in setup.py proposes using a cmdclass with custom build_ext class. Unfortunately, this breaks the build of the cython extension because cython also customizes build_ext.

A:

First question, when is numpy needed? It is needed during the setup (i.e. when build_ext-funcionality is called) and in the installation, when the module is used. That means numpy should be in setup_requires and in install_requires.
There are following alternatives to solve the issue for the setup:

using PEP 517/518 (which is more straight forward IMO)
using setup_requires-argument of setup and postponing  import of numpy until setup's requirements are satisfied (which is not the case at the start of setup.py's execution)

PEP 517/518-solution:
Put next to setup.py a pyproject.toml-file , with the following content:
[build-system]
requires = ["setuptools", "wheel", "Cython>=0.29", "numpy >= 1.15"]

which defines packages needed for building, and then install using pip install . in the folder with setup.py. A disadvantage of this method is that python setup.py install no longer works, as it is pip that reads pyproject.toml. However, I would use this approach whenever possible.

Posponing import
This approach is more complicated and somewhat hacky, but works also without pip.
First, let's take a look at unsuccessful tries so far:
pybind11-trick
@chrisb's "pybind11"-trick, which can be found here: With help of an indirection, one delays the call to import numpy until numpy is present during the setup-phase, i.e.:
class get_numpy_include(object):

    def __str__(self):
        import numpy
        return numpy.get_include()
...
my_c_lib_ext = setuptools.Extension(
    ...
    include_dirs=[get_numpy_include()]
)

Clever! The problem: it doesn't work with the Cython-compiler: somewhere down the line, Cython passes the get_numpy_include-object to os.path.join(...,...) which checks whether the argument is really a string, which it obviously isn't.
This could be fixed by inheriting from str, but the above shows the dangers of the approach in the long run - it doesn't use the designed mechanics, is brittle and may easily fail in the future.
the classical build_ext-solution
Which looks as following:
...
from setuptools.command.build_ext import build_ext as _build_ext

class build_ext(_build_ext):
    def finalize_options(self):
        _build_ext.finalize_options(self)
        # Prevent numpy from thinking it is still in its setup process:
        __builtins__.__NUMPY_SETUP__ = False
        import numpy
        self.include_dirs.append(numpy.get_include())

setupttools.setup(
    ...
    cmdclass={'build_ext':build_ext},
    ...
)

Yet also this solution doesn't work with cython-extensions, because pyx-files don't get recognized.
The real question is, how did pyx-files get recognized in the first place? The answer is this part of setuptools.command.build_ext:
...
try:
    # Attempt to use Cython for building extensions, if available
    from Cython.Distutils.build_ext import build_ext as _build_ext
    # Additionally, assert that the compiler module will load
    # also. Ref #1229.
    __import__('Cython.Compiler.Main')
except ImportError:
    _build_ext = _du_build_ext
...

That means setuptools tries to use the Cython's build_ext if possible, and because the import of the module is delayed until build_ext is called, it founds Cython present.
The situation is different when setuptools.command.build_ext is imported at the beginning of the setup.py - the Cython isn't yet present and a fall back without cython-functionality is used.
mixing up pybind11-trick and classical solution
So let's add an indirection, so we don't have to import setuptools.command.build_ext directly at the beginning of setup.py:
....
# factory function
def my_build_ext(pars):
     # import delayed:
     from setuptools.command.build_ext import build_ext as _build_ext#
 
     # include_dirs adjusted: 
     class build_ext(_build_ext):
         def finalize_options(self):
             _build_ext.finalize_options(self)
             # Prevent numpy from thinking it is still in its setup process:
             __builtins__.__NUMPY_SETUP__ = False
             import numpy
             self.include_dirs.append(numpy.get_include())
     
    #object returned:
    return build_ext(pars)
...
setuptools.setup(
    ...
    cmdclass={'build_ext' : my_build_ext},
    ...
)

A:

One (hacky) suggestion would be using the fact that extension.include_dirs is first requested in build_ext, which is called after the setup dependencies are downloaded.
class MyExt(setuptools.Extension):
    def __init__(self, *args, **kwargs):
        self.__include_dirs = []
        super().__init__(*args, **kwargs)

    @property
    def include_dirs(self):
        import numpy
        return self.__include_dirs + [numpy.get_include()]

    @include_dirs.setter
    def include_dirs(self, dirs):
        self.__include_dirs = dirs

my_c_lib_ext = MyExt(
    name="my_c_lib",
    sources=["my_c_lib/some_file.pyx"]
)

setup(
    ...,
    setup_requires=['cython', 'numpy'],
)

Update
Another (less, but I guess still pretty hacky) solution would be overriding build instead of build_ext, since we know that build_ext is a subcommand of build and will always be invoked by build on installation. This way, we don't have to touch build_ext and leave it to Cython. This will also work when invoking build_ext directly (e.g., via python setup.py build_ext to rebuild the extensions inplace while developing) because build_ext ensures all options of build are initialized, and by coincidence, Command.set_undefined_options first ensures the command has finalized (I know, distutils is a mess).
Of course, now we're misusing build - it runs code that belongs to build_ext finalization. However, I'd still probably go with this solution rather than with the first one, ensuring the relevant piece of code is properly documented.
import setuptools
from distutils.command.build import build as build_orig

class build(build_orig):

    def finalize_options(self):
        super().finalize_options()
        # I stole this line from ead's answer:
        __builtins__.__NUMPY_SETUP__ = False
        import numpy
        # or just modify my_c_lib_ext directly here, ext_modules should contain a reference anyway
        extension = next(m for m in self.distribution.ext_modules if m == my_c_lib_ext)
        extension.include_dirs.append(numpy.get_include())

my_c_lib_ext = setuptools.Extension(
    name="my_c_lib",
    sources=["my_c_lib/some_file.pyx"]
)

setuptools.setup(
    ...,
    ext_modules=[my_c_lib_ext],
    cmdclass={'build': build},
    ...
)

