pip: pip download --platform --python-version does not work for some packages

Environment

• pip version: 18.0
• Python version: 2.7
• OS: centos 7

Description

When using pip to download wheels for it does not find them despite the fact that there are obvious matches available on PyPI

How to Reproduce

I tried all of the following commands <and many other permutations) to try to make this work

pip download --python-version 36 --abi cp36 --platform linux_x86_64  "hiredis==0.3.1" --only-binary=:all: --dest ./wheelhouse -v
pip download --python-version 36 --abi cp36 --platform manylinux1_x86_64  "hiredis==0.3.1" --only-binary=:all: --dest ./wheelhouse -v
pip download --python-version 36 --abi cp36 --platform linux_x86_64  --implementation py "hiredis==0.3.1" --only-binary=:all: --dest ./wheelhouse -v

...
    Skipping link https://files.pythonhosted.org/packages/0d/d1/f0346030e4e5fbd931c7d81acdb8c81ea57733f6d87bc15f3dc28a51eb30/hiredis-0.3.1-cp36-cp36m-manylinux1_x86_64.whl#sha256=736a30992ede64b79989869e1a068fec93405dac71d87a316f22ee72e75cb6a5 (from https://pypi.org/simple/hiredis/) (requires-python:>=2.6, !=3.0.*, !=3.1.*); it is not compatible with this Python
...
No matching distribution found for hiredis==0.3.1

The wheel it skips looks perfectly valid to me: hiredis-0.3.1-cp36-cp36m-manylinux1_x86_64.whl . I assume linux_x86_64 matches manylinux1_x86_64, and even if it doesn’t, I tried specifiying manylinux1_x86_64 just to be safe.

The same thing happens when trying to download wheels for python 2.7.

The documentation on how these work together is pretty sorely lacking (

• what is the valid list of platforms and what is the logic for how they match?
• the provided examples about macosx use the wrong specifier, AFAICT, using a dash instead of an underscore.
• the pep that is referred to is far too long to grok and it also seems out of date
• the verbose output about the skipped wheels above provides no information about how pip came to the conclusion that the wheel “is not compatible with this Python”. It would be nice if the note provided some more info about why the wheel is not considered compatible.

Along with the other issue I posted #5369 it feels like the intersection of what is required to be useful and what actually works is frustratingly small.