> I'm not sure if it should be used to install libraries in $HOME/.local/lib64 rather than $HOME/.local/lib. Previously, Fedora already used $HOME/.local/lib and $HOME/.local/lib64 is not in the sys.path.

This was also briefly discussed in bpo-1294959, but did not go through since “changing posix_user should have no impact on end users”.

> Does the site module add $HOME/.local/lib64 to sys.path if it exists?

It does not, only lib is checked right now.

https://github.com/python/cpython/blob/c7ea1e3dcea6fbc9842463ce2b785b43501b1eaa/Lib/site.py#L288-L298

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There are two possible solutions from what I can tell. We could just make posix_user match posix_prefix and always respect sys.platlibdir. This could be confusing to existing Python 3.9 users however since many of them already pip-installed things into ~/.local/lib and this would make their user-site packages split in two locations. The other would be to restore the pre-3.9 behaviour in sysconfig to use lib instead of depending on sys.platlibdir. I don’t know who uses sysconfig right now and can’t say what would break, but for pip this would be less disruptive since it currently installs things into ~/.local/ib (provided by distutils).