sys_platform and python_version env markers in requirements files should respect --platform and --python-version

[chadrik]

Environment

• pip version: 18.0
• Python version: 2.7.7
• OS: centos 7

Description

pip download allows you to specify an explicit platform (--platform) and python version (--python-version) which overrides the current environment, however, the sys_platform and python_version markers in requirements files still use the current environment, making them useless in practice.

Expected behavior

sys_platform and python_version markers should be overridden by --platform and --python-version .

How to Reproduce

create a requirments.txt file:

future  ; python_version < '3.0'

Install from python2 for python3:

$ python2 -m pip download --python-version 36 --abi cp36m --platform linux_x86_64 -r requirements.txt --only-binary=:all:

the future package should be skipped because we're trying to download packages for python 3.6 and the marker specifies that it should only be installed for python_version < '3.0', but because the current python interpreter is 2.7, pip attempts to download it anyway.

Output

Collecting future (from -r requirements.txt (line 9))
  Could not find a version that satisfies the requirement future (from -r requirements.txt (line 9)) (from versions: )
No matching distribution found for future (from -r requirements.txt (line 9))

Expected Output

Ignoring future: markers 'python_version < "3.0"' don't match your environment

[cjerdonek]

I just posted PR #6591 ("Add a TargetPython class"), which should help some more with this.

[pradyunsg]

Consolidating this into #11664