Installation of Python 3.8 and Python 3.9 side by side breaks pip version.

[maxim-lobanov]

Bug report

What you were trying to do (and why)

We are generating MacOS VMs for CI. We install python@3.8 formula. Then we install some other tools via brew and those tools bring python@3.9 as dependency to machine.

As a result, we have inconsistent python3 and pip3 on machine:

• python3 from python@3.8 formula
• pip3 from python@3.9 formula

It causes issues because If you install any packages via pip3 install, they won't be available via python3.

See related issues:

• pip3 and python3 seem to use different versions actions/runner-images#1818
• MacOS 10.5 20201011.1 pip package manager actions/runner-images#1819

Step-by-step reproduction instructions

Repro steps:

1. Take clean MacOS (10.14, 10.15 or 11.0)
2. Install brew
3. run brew install python@3.8
4. run brew install python@3.9
5. Run python3 --version and pip3 --version

What happened (include command output)

Actually, we have the following output:

Mac-1972:~ runner$ python3 --version
Python 3.8.6
Mac-1972:~ runner$ pip3 --version
pip 20.2.3 from /usr/local/lib/python3.9/site-packages/pip (python 3.9)
Mac-1972:~ runner$ ls -la /usr/local/bin/python3
lrwxr-xr-x  1 runner  admin  38 Oct 15 01:01 /usr/local/bin/python3 -> ../Cellar/python@3.8/3.8.6/bin/python3
Mac-1972:~ runner$ ls -la /usr/local/bin/pip3
lrwxr-xr-x  1 runner  admin  35 Oct 15 01:02 /usr/local/bin/pip3 -> ../Cellar/python@3.9/3.9.0/bin/pip3

It means that Python3 is used from python@3.8 formula. Pip3 is used from python@3.9 formula. Those versions are not compatible.
If you install any packages via pip3 install, they won't be available via python3

What you expected to happen

I am expecting that python3 version and pip3 will be consistent.

Possible workaround

We have found a workaround for this issue but it definitely looks like an issue with python formula that should be fixed:

brew install python@3.8
brew install python@3.9
brew unlink python@3.9
brew unlink python@3.8
brew link python@3.8

[ghost]

Please take a look at this issue. I think it's related.

[SMillerDev]

It's not

[farvour]

Also had this issue. A "keg-only" formula should never overwrite these links. This feels like a bug in the 3.9 formula...

[SMillerDev]

It is, there's a pull request that fixes it in the timeline here. We just need someone to write a check to make sure it doesn't happen again with the next version.

[fxcoudert]

This is now fixed. Anyone who is affected should be able to restore the expected state of the symlinks with brew reinstall python@3.8

[AFAgarap]

I'm using zsh, so I had to run echo 'export PATH="/usr/local/opt/python@3.8/bin:$PATH"' >> ~/.zshrc.