In a setup.py involving Cython, if install_requires, then how can from library import something?

Question

This doesn't make sense to me. How can I use the setup.py to install Cython and then also use the setup.py to compile a library proxy?

import sys, imp, os, glob
from setuptools import setup
from Cython.Build import cythonize # this isn't installed yet

setup(
    name='mylib',
    version='1.0',
    package_dir={'mylib': 'mylib', 'mylib.tests': 'tests'},
    packages=['mylib', 'mylib.tests'],
    ext_modules = cythonize("mylib_proxy.pyx"), #how can we call cythonize here?
    install_requires=['cython'],
    test_suite='tests',
)

Later: python setup.py build

Traceback (most recent call last):
  File "setup.py", line 3, in <module>
    from Cython.Build import cythonize
ImportError: No module named Cython.Build

It's because cython isn't installed yet.

What's odd is that a great many projects are written this way. A quick github search reveals as much: https://github.com/search?utf8=%E2%9C%93&q=install_requires+cython&type=Code

Answer 1

As I understand it, this is where PEP 518 comes in - also see some clarifications by one of its authors.

The idea is that you add yet another file to your Python project / package: pyproject.toml. It is supposed to contain information on build environment dependencies (among other stuff, long term). pip (or just any other package manager) could look into this file and before running setup.py (or any other build script) install the required build environment. A pyproject.toml could therefore look like this:

[build-system]
requires = ["setuptools", "wheel", "Cython"]

It is a fairly recent development and, as of yet (January 2019), it is not finalized / approved by the Python community, though (limited) support was added to pip in May 2017 / the 10.0 release.