[Distutils] setup_requires: the obvious option(?)

[Antony Lee]

Hi all,

The `setup_requires` option to `setup()` is well-known to suffer from
multiple issues.  Most importantly, as it is a keyword argument to
`setup()`, it appears too late for modules that may need to be imported for
the build to occur (e.g., Cython, for which support must explicitly
provided by setuptools itself rather than by letting Cython hook into it);
additionally, there are various contorsions that people go to to avoid some
`setup_requires` when not building the package (such as checking the value
of `sys.argv`).  `setup_requires` also uses `easy_install` rather than
`pip`, but I do not see why this could not be fixed; let's focus on the
first issue instead.

If `setup_requires` appears too late to be useful, the obvious(?) option is
to move it earlier: provide a function, say, `setuptools.setup_requires()`,
that should be called *before* `setup()` itself, e.g.:

    from setuptools import setup, setup_requires
    setup_requires("numpy", needed_for=["build_ext"])
    try:
        import numpy as np
    except ImportError:
        np = None
    setup(..., include_dirs=[np.get_include()] if np else [])

When `setup.py` is invoked, either directly or by pip, upon the call to
`setup_requires()`, if `sys.argv[0]` is in the `needed_for` kwarg, and at
least one requirement is missing, `setup_requires()` calls asks pip to
install the required packages (similarly to `
https://bitbucket.org/dholth/setup-requires`) in a temporary directory, and
the whole Python process replaces itself (in the `os.execv()` sense) by a
new call to `python setup.py` with this temporary directory prepended to
the PYTHONPATH.  In this new process, the arguments to `setup_requires()`
are now available and we can proceed to the rest of `setup.py`.

I feel like this idea is natural enough that someone must already have come
up with it... but I may be missing something :-)

Best,
Antony
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