Document that "coding: utf-8" is supported in requirements.txt

[chrahunt]

As mentioned in #6401 (comment), requirements files support # -*- coding: utf-8 -*- declarations. Some details about this convention can be found here.

A sentence should be added to the Requirements File Format section of the docs saying that this is supported, for users that want their requirements files to always be interpreted as UTF-8 no matter what.

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[pradyunsg]

@chrahunt could you possibly link to the corresponding code?

I looked in https://github.com/pypa/pip/blob/master/src/pip/_internal/req/req_file.py and can't figure out where exactly the behavior you're pointing out is implemented.

[chrahunt]

Sure. It's done in utils.encoding.auto_decode, via:

1. req_file.parse_requirements calls download.get_file_content here
2. download.get_file_content calls utils.encoding.auto_decode here for files
3. utils.encoding.auto_decode checks for an encoding line here

[pradyunsg]

That's deeeep. And, there's a doc-fix worth making in download.get_file_content.

[ananyo2012]

Hey I am working on this in PyCon India 2019 Devsprint.

As per my understanding I am supposed to update documentation in Requirements File Format and download.get_file_content