Pip fails if the python config file contains a percentage sign in the comments

Description

Hi,

With the following setup.cfg file that contains the percentage sign in the comment section as

$ cat setup.cfg
[flake8]
ignore =
        WPS323      #  percentage sign '%'

any attempt to use pip to install a package fails with InterpolationSyntaxError. Removing the percentage sign solves the issue, to me seems to be bug as according to the documentation1

Blank lines are ignored, as are comments, which run from a ‘#’ character until the end of the line.

my config file should be just fine 1

Expected behavior

Expected to not see the assertion and get the package installed as:

root@d3eef6b3d7d7:/tmp# cat setup.cfg
[flake8]
ignore =
        WPS323  # percentage char

root@d3eef6b3d7d7:/tmp# pip install numpy
Collecting numpy
  Downloading numpy-1.21.1-cp39-cp39-manylinux_2_12_x86_64.manylinux2010_x86_64.whl (15.8 MB)
     |████████████████████████████████| 15.8 MB 11.9 MB/s
Installing collected packages: numpy
Successfully installed numpy-1.21.1
WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv

pip version

21.2.3

Python version

3.9

OS

ubuntu16.04

How to Reproduce

1. Create a setup.cfg file with a percentage sign in the comments
2. In the same directory where the setup.cfg is, try to install any package via pip (i.e. pip install --user numpy)

Output

ERROR: Exception:
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/cli/base_command.py", line 173, in _main
    status = self.run(options, args)
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/cli/req_command.py", line 203, in wrapper
    return func(self, options, args)
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/commands/install.py", line 239, in run
    options.use_user_site = decide_user_install(
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/commands/install.py", line 652, in decide_user_install
    if site_packages_writable(root=root_path, isolated=isolated_mode):
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/commands/install.py", line 598, in site_packages_writable
    get_lib_location_guesses(root=root, isolated=isolated))
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/commands/install.py", line 584, in get_lib_location_guesses
    scheme = get_scheme(
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/locations/__init__.py", line 159, in get_scheme
    old = _distutils.get_scheme(
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/locations/_distutils.py", line 130, in get_scheme
    scheme = distutils_scheme(dist_name, user, home, root, isolated, prefix)
  File "/usr/local/lib/python3.9/site-packages/pip/_internal/locations/_distutils.py", line 46, in distutils_scheme
    d.parse_config_files()
  File "/usr/local/lib/python3.9/distutils/dist.py", line 413, in parse_config_files
    val = parser.get(section,opt)
  File "/usr/local/lib/python3.9/configparser.py", line 799, in get
    return self._interpolation.before_get(self, section, option, value,
  File "/usr/local/lib/python3.9/configparser.py", line 395, in before_get
    self._interpolate_some(parser, option, L, value, section, defaults, 1)
  File "/usr/local/lib/python3.9/configparser.py", line 442, in _interpolate_some
    raise InterpolationSyntaxError(
configparser.InterpolationSyntaxError: '%' must be followed by '%' or '(', found: "%' in string formatting"

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[DiddiLeija]

I created https://bugs.python.org/issue44892 to ask the CPython developers about this.

[DiddiLeija]

@berthin I can open an issue in bugs.python.org, if you want 😄.