pip: Native namespace package does not install properly when specifying a target directory (-t option)

Description

I cannot make native namespace packages to install properly when specifying a target directory (-t option).

This is important to me as I need to use the the target option to install some packages for an AWS Lambda.

This looks like a very old issue #1924. However I cannot find, after extensive search, any clarification anywhere and the case was closed without apparent resolution.

To confirm I tried it with the pypa samples from pypa/sample-namespace-packages and failed to install both native packages (pkg_a & pkg_b) with an error stating that the directory already exists.

Please note I tried this both on Windows and Linux and the other packaging methods ‘pkgutil’ and ‘pkg_resources’ had the same exact problem.

Thank You

Expected behavior

Both packages pkg_a & pkg_b should install in the same namespace directory.

pip version

21.1.3

Python version

3.8.5

OS

Windows/Linux

How to Reproduce

git clone https://github.com/pypa/sample-namespace-packages

cd sample-namespace-packages/native mkdir temp

pip install ./pkg_a -t temp pip install ./pkg_b -t temp

Output

The second install fails with a message like:

WARNING: Target directory .../sample-namespace-packages/native/temp/example_pkg already exists. Specify --upgrade to force replacement.

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