[FR] Environment variables substitution for dynamic dependencies

[sondalex]

What's the problem this feature will solve?

Setuptools supports dynamically specifying dependencies through pyproject.toml using the following syntax:

[tool.setuptools.dynamic]
dependencies = { file = ["requirements.txt"] }

The requirements.txt format, as described in the pip documentation, includes a proposal for environment variable substitution. However, this functionality is not currently implemented in setuptools.

Describe the solution you'd like

Introduce support in setuptools for substituting environment variables when parsing dependencies from a requirements.txt file. This would enable use cases like the following:

1. Define a private dependency in requirements.txt:

   mypackage@https://${PAT}@<pypi-feed>/<path>
   

2. Specify dynamic dependencies in pyproject.toml:

   [project]
   # Other project metadata
   dynamic = ["dependencies"]
   
   [tool.setuptools.dynamic]
   dependencies = { file = ["requirements.txt"] }

3. Set the necessary environment variables before installation:

   export PAT=...
   export PYTHON_KEYRING_BACKEND=keyring.backends.null.Keyring # To avoid interactive prompts

4. Run the installation:

   pip install .

This feature would be especially beneficial for non-interactive sessions, such as CI/CD pipelines.

Alternative Solutions

No response

Additional context

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