Env variables do not get substituted in extras_require

Environment

• pip version: 20.2.2
• Python version: 3.8
• OS: any

Description A PEP 508 URL with a ${ENVVAR} placeholder like git+https://${GITHUB_TOKEN}@github.com/foo/bar.git should have the placeholder replaced in every set of dependencies. It however seems to be only done in the installation requirements, not e.g. extra requirements.

Expected behavior PIP replaces the env var and says something like

Collecting testdep@ git+https://github.com/foo/bar.git
  Cloning https://****@github.com/foo/bar.git to /private/var/folders/yn/_1p23xlj3ls77fstj2n2cbp80000gp/T/pip-install-cn6g7m6h/testdep

How to Reproduce

#!/bin/bash
set -e

rm -rf test-venv
python3 -m venv test-venv
source test-venv/bin/activate

pip install -U pip
cat >setup.py <<EOF
from setuptools import setup
setup(
  name='test',
  extras_require=dict(doc=[
    'testdep@git+https://github.com/\${PROJ}.git',
  ]),
)
EOF
# example package without deps
export PROJ=flying-sheep/bcode
pip install .[doc] || true

deactivate

Output

Processing ~/test
Collecting testdep@ git+https://github.com/${PROJ}.git
  Cloning https://github.com/${PROJ}.git to /private/var/folders/yn/_1p23xlj3ls77fstj2n2cbp80000gp/T/pip-install-dlnwrdhp/testdep
ERROR: Command errored out with exit status 128: git clone -q 'https://github.com/${PROJ}.git' /private/var/folders/yn/_1p23xlj3ls77fstj2n2cbp80000gp/T/pip-install-dlnwrdhp/testdep Check the logs for full command output.

[sbidoul]

Environment variables are supported in pip requirement files.

However that does not mean they are nor should be supported in setup.py. ~If that would be supported it would be the a feature of the build backend (setuptools in this case).~

I am somewhat surprised it works in setup.py install_requires. If it does, could you try again with wheel installed in the virtualenv (test-env/bin/pip install wheel)?

To support your use case you could use a requirements or constraints file. Write extra_requires={'doc': ['testdep']} in your setup.py then:

$ echo "testdep@git+https://github.com/\${PROJ}.git" > requirements.txt
$ pip install -r requirements.txt .[doc]

[flying-sheep]

huh. well, nevermind then, thank you for indulging me!