Alternative: pip.req

[kousu]

It looks like this used to be a feature of pip itself: https://stackoverflow.com/questions/25192794/no-module-named-pip-req#49867265

You can still use it with

try: # for pip >= 10
    from pip._internal.req import parse_requirements
except ImportError: # for pip <= 9.0.3
    from pip.req import parse_requirements

[joshuacwnewton]

I tried the pip._internal version of parse_requirements, but the function required a PipSession object, which felt like a sign that I was doing something I shouldn't (re: accessing internal functions/methods).

I then poked around and found https://stackoverflow.com/a/59971236, which suggests pkg_resources.parse_requirements. This seems like it might be the proper way of accessing the "official" user-facing requirements parser for pip/setuptools?

There are some differences worth noting, though:

Handling of non-requirement specifications (e.g. --extra-url)

This line:

--extra-index-url https://download.pytorch.org/whl/cpu # append_to_freeze

Triggers the following traceback:

Traceback (most recent call last):
  File "/home/joshua/repos/spinalcordtoolbox/python/envs/venv_sct/lib/python3.8/site-packages/pkg_resources/_vendor/packaging/requirements.py", line 102, in __init__
    req = REQUIREMENT.parseString(requirement_string)
  File "/home/joshua/repos/spinalcordtoolbox/python/envs/venv_sct/lib/python3.8/site-packages/pkg_resources/_vendor/pyparsing/core.py", line 1141, in parse_string
    raise exc.with_traceback(None)
pkg_resources._vendor.pyparsing.exceptions.ParseException: Expected W:(0-9A-Za-z), found '-'  (at char 0), (line:1, col:1)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/joshua/repos/spinalcordtoolbox/python/envs/venv_sct/lib/python3.8/site-packages/pkg_resources/_vendor/packaging/requirements.py", line 104, in __init__
    raise InvalidRequirement(
pkg_resources.extern.packaging.requirements.InvalidRequirement: Parse error at "'--extra-'": Expected W:(0-9A-Za-z)

While requirements-parser (correctly) skips over this line just fine.

Requirement objects

The signature of the Requirement object returned by pkg_resources:

Is subtly different than that returned by requirements-parser:

Apart from that, they seem pretty comparable?