virtualenv 20.0.x changes break default layout install

[earthgecko]

Changes introduced in virtualenv 20.0.x appear to have invalidated the installation documentation.
When installing graphite-web in a virtualenv neither the export PYTHONPATH="/opt/graphite/lib/:/opt/graphite/webapp/" method, nor the method passing the pip --install-option works any longer (related to pypa/pip#7309).

Passing the --install-option

DEPRECATION: Location-changing options found in --install-option: ['--prefix', '--install-lib'] from command line. This configuration may cause unexpected behavior and is unsupported. pip 20.2 will remove support for this functionality. A possible replacement is using pip-level options like --user, --prefix, --root, and --target. You can find discussion regarding this at https://github.com/pypa/pip/issues/7309.

This results in graphite not being installed the in /opt/graphite/webapp/graphite but in /opt/graphite/lib/python3.7/site-packages/graphite

I have not been able to debug why the exported PYTHONPATH is not being honoured by pip (20.0.2) yet.

To Reproduce
Steps to reproduce the behaviour:

pip install virtualenv==20.0.10  # system pip virtualenv
cd /opt
# Using a virtualenv with Python 3.7.6 built from source, set python interpretator as appropriate
virtualenv --python=/opt/python_virtualenv/versions/3.7.6/bin/python3.7 graphite
cd /opt/graphite
source bin/activate
export PYTHONPATH="/opt/graphite/lib/:/opt/graphite/webapp/"
bin/pip3 install --no-binary=:all: https://github.com/graphite-project/graphite-web/tarball/1.1.6

Expected behaviour

Create /opt/graphite/webapp/graphite but creates /opt/graphite/lib/python3.7/site-packages/graphite.

Environment:

• OS flavor: CentOS 8.1.1911
• Graphite-web version: 1.1.6
• Python version: 3.7.6
• Setup type: pip (from sources)
• pip versions: system - 9.03, virtualenv - 20.0.2

Additional context

This affects carbon in the same way, carbon is not installed in /opt/graphite/lib/carbon but as /opt/graphite/lib/python3.7/site-packages/carbon.

I believe this may have been responsible for the issue reported in #2415 and @pszafer may find that when py3-virtualenv updated on Alpine the --install-options method may no longer work.

Workaround for users

For now a simple workaround for users is to use virtualenv 16.7.9 or 16.7.10 as virtualenv is still releasing 16.7.x along with 20.0.x

If you know what you are doing and you have virtualenv 20.0x installed with the system pip, then you can install 16.7.10

pip uninstall virtualenv
pip install virtualenv==16.7.10

Your installation will now work as per the docs.

[ploxiln]

this might be addressed by (long un-merged) #2409

[deniszh]

Btw, py3-virtualenv on Alpine is still 16.7.x

[piotr1212]

time to get rid of the prefix.