pytest running with another version of python

Bottom line: run

• python -m pytest, or
• py.test-<version> if your alternative Python and pytest are installed with system package manager, or
• if your alternative Python has been installed with pyenv, switch with pyenv to that version and make sure you have pytest installed for it. Then you can just run pytest.
  • since the pip executable is also among the ones being switched, you need to switch to the alternative Python before installing pytest for it, too.

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As I can see, /usr/bin/pytest (that belongs to the system package manager's python-pytest package) has a shebang !#/usr/bin/python since it corresponds to the system python's installation.

pyenv, as its README.md says, does not replace /usr/bin/python - because it indeed should not be replaced to avoid breaking system packages.

Instead, it adds its directory to PATH and inserts a launcher there (called "shim") which is what gets invoked when you type "python". As you probably guessed, this hack is ignored by a shebang like the above - as it should.

• Running python -m pytest will make whichever python that launches itself use the package from its installation.
• Alternatively, pytest for your other Python version may include versioned executables on the PATH named py.test-<version> (e.g. py.test-3 or py.test-3.6) depending on the way you installed it.
  • If it's from a system package manager's package for nonstandard python - like python36-pytest - this is virtually guaranteed.
  • I checked that if you install a version with pip, it only creates an unversioned executable (though you can create a versioned one yourself). Moreover, if you install the same package for a different Python version but with the same --prefix, it will overwrite the existing one's executable!
• pyenv's suggested way seems to be to install all python versions of interest and packages for them under ~/.pyenv/versions.
  • This is not applicable for the system's Python but the default /usr/local can be used for it.
  • Once you switch to an alternative Python version, it claims to create shims for all scripts (including pip!) that are currently installed for that version, so invoking those scripts without a path would run those shims.
    • So, if a package (and thus its script) is not installed for the alternative version but installed for system version, trying to run its executable would "fall through" to /usr/local with just the result you're seeing now.

I just wanted to run "black" for Python 2.7, but I have installed it in pyenv's Python 3.7.9. It worked correctly with the script /usr/local/bin/black27:

PYENV_VERSION=3.7.9 black -t py27 "$@"

It didn't work with:

pyenv shell 3.7.9
black -t py27 "$@"

nor

pyenv shell 3.7.9
pyenv exec black -t py27 "$@"

I found this related question. For them it worked with this:

python -m pytest tests/my_test.py

I hope it works