In windows, requirements file parser collapses absolute reference to requirements file

Description

In Windows, running pip install -r against the following requirements file containing an absolute reference to other requirements files fails.

# In power-shell
$ echo pandas > requirements.txt
$ echo "-r $(pwd)\requirements.txt" > other-requirements.txt
$ pip install -r "$(pwd)\other-requirements.txt"

Could not open requirements file: [Errno 2] No such file or directory: 'D:\\absolute\\path\\to\\absolutepathtorequirements.txt'

Expected behavior

pip install should succeed

pip version

21.1.3

Python version

3.7.9

OS

Windows

How to Reproduce

echo pandas > requirements.txt
echo "-r $(pwd)\requirements.txt" > other_requirements.txt
cat requirements.txt
cat other_requirements.txt
pip install -r "$(pwd)\other_equirements.txt"

See this github actions log: https://github.com/harupy/windows-playground/pull/1/checks?check_run_id=2979532159

Output

pandas
-r D:\a\windows-playground\windows-playground\requirements.txt
ERROR: Could not open requirements file: [Errno 2] No such file or directory: 'D:\\a\\windows-playground\\windows-playground\\awindows-playgroundwindows-playgroundrequirements.txt'

Code of Conduct

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[uranusjr]

I think it’s more because of we need quotes (to handle special names, spaces, etc.), and once quoting is a thing you need a way to escape quotes, and everything cascades from there. The root of the issue is requirements.txt handles options by writing them like command line options, and cross-shell command line parsing is literally impossible. So there is a design decision to make, either we restrict the allowed quoting/escaping syntax, restrict what characters can appear in a path, or do platform-dependant parsing. All of them have significant setbacks.

[pfmoore]

As I said, I don’t like this solution because it makes Windows paths second-class. Why do we even need backslash-escaping in a requirements file?

Declaring forward slashes as the only supported option is easy and convenient, and may well be sufficient in practice, but that doesn’t mean it’s correct…