I'm building my own install of Python, then using pip to install some packages into it. I want to use the pre-built binary wheels for packages like Cryptography.
On GNU/Linux is just works: it grabs the manylinux1 wheel and everything works just fine.
On MacOS, it refuses to download any of the binary wheels for most versions. I've added lots of -v options to pip, but all it says is:
$ mypython -s -u -m pip install -v --only-binary 'cryptography' 'cryptography==2.6.1'
...
Skipping link https://files.pythonhosted.org/packages/.../cryptography-2.6.1-cp27-cp27m-macosx_10_6_intel.whl#sha256=... (from https://pypi.org/simple/cryptography/) (requires-python:>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*); it is not compatible with this Python
...
Could not find a version that satisfies the requirement cryptography==2.6.1 (from versions: 1.0.1, 1.0.2, 1.1, 1.1.1, 1.1.2, 1.2, 1.2.1, 1.2.2, 1.3.1, 1.3.2, 1.3.3, 1.3.4, 1.4, 1.5, 1.5.1, 1.5.2, 1.5.3, 1.6, 1.7, 1.7.1, 1.8, 1.8.1, 1.8.2)
I've tried to understand why those particular versions are OK but not the latest, and the only thing I can see is that those versions have a specific x86_64 wheel package, while the later ones only have the fat binary intel wheel packages. My Python has a package definition of:
>>> import distutils.util
>>> distutils.util.get_platform()
'macosx-10.12-x86_64'
So I wondered if that was it. I modified my Python build to use MACOSX_DEPLOYMENT_TARGET=10.6 and added --enable-universalsdk=/ --with-universal-archs=intel to the configure line, and now my Python reports this:
>>> import distutils.util
>>> distutils.util.get_platform()
'macosx-10.6-intel'
However, I still get exactly the same messages from pip when I try to install.
So I'm wondering, is there any way I can get pip to be more informative and tell me exactly what about these binary packages it doesn't like, that is causing it to say "is not compatible" and skip them?
Thanks to a hint from @wim I found this command:
$ mypython
...
>>> from setuptools.pep425tags import get_supported
>>> for t in get_supported(): print(str(t))
...
This shows the full list of tuples used to match supported packages. Using this information I was able to compare it to the PyPI downloads and discover that I had built my MacOS Python with UCS4 support (which is common on Linux) where relatively few PyPI packages provide wide unicode binary wheels for MacOS.
I have no particular reason to prefer UCS4 so I switched my MacOS builds to UCS2 and it worked!
Hopefully this information will prove helpful to someone else.