Option --no-use-pep517 not supported in requirement files

[con-f-use]

Environment

• pip version: 19.1
• Python version: 3.6.7, 2.7.15
• OS: any

Description
Specifying the option --no-use-pep517 in a requirements file results in

ERROR: Invalid requirement: --no-use-pep517 docopt
pip: error: no such option: --no-use-pep517

Expected behavior
Package is installed not using PEP 517

How to Reproduce

1. Have this requirements file:

--no-use-pep517 docopt

2. Run pip install -r requirements.txt (removing the option form the file and passing it directly works normal)

Output

pip install -r requirements.txt 
Usage: pip [options]

ERROR: Invalid requirement: --no-use-pep517 docopt
pip: error: no such option: --no-use-pep517

This Issue regards specifying --no-use-pep517 inside a single requirement file. It is related to #6433, where it fails when we have nested requirement files and specify the option outside of them, i.e. give it directly as an argument to the pip invocation.

[cjerdonek]

I just posted PR #6447 for this.

[pradyunsg]

This issue is a good starting point for anyone who wants to help out with pip's development -- it's simple and the process of fixing this should be a good introduction to pip's development workflow. See the PR above to understand what the desired fix could look like. :)

[brainwane]

@pradyunsg Not to derail this particular issue, but I think this is not a good first issue; it might be a good second or third issue, but a contributor who is new to Python packaging is going to have trouble understanding specifiers, PEP 517, and so on. Removing the tag.

[pradyunsg]

Sounds fair to me. We have an awaiting PR tag for things like this then -- stuff that's not a good first issue, but can be a good issue for more involved contributors to tackle. :)

[chrahunt]

Ideally we could move away from --no-use-pep517, since it will be incompatible with an increasing number of packages as time goes on. What is the current use case for this flag?

[uranusjr]

I believe there are still packages out there using pyproject.toml but not actually compatible with PEP 517. Instead of adding --no-use-pep517 we should probably hunt those packages down and fix them instead. Maybe it would help to add a message to --no-use-pep517 to prompt the user report the usage somewhere (e.g. in a tracking issue here)?