Make Poetry executable use python3 from path if >= 3.5

[frankier]

Currently SetupReader only works with Python versions >=3.5 -- but on some versions of Ubuntu and Debian, Poetry may run under Python 2, even when Python 3 is available.

On some distributions /usr/bin/python is still Python 2 by default and /usr/bin/python3 is Python 3. pyenv follows a similar convention, shimming both the python and python3 executables.

This patch changes the poetry executable to first use the python executable, if it is >= 3.5, then use the python3 executable, if it is >= 3.5, and otherwise fall back to whichever version is available under python.

[sdispater]

I think this is a duplicate of #1878 or at least close in spirit, so I am closing this to centralize discussions and reviews.

[jonapich]

@frankier can you expand a bit on your issue? Technically, poetry can prepare e.g.: 3.8 environments from a 2.7 runtime, so I'm not sure why SetupReader would suffer from poetry's interpreter version 😕

[frankier]

The code running SetupReader is run within the main Poetry process, not within the context of the environment. Whichever Python version ends up running $HOME/.poetry/bin/poetry is used. On some systems this will be Python 2.7.

[jonapich]

Thanks for the additional info!

The general case of favoring python3 over python2 will fix it for you, but I think your claim points to a project-specific problem.

I think you should be able to write this in your pyproject.toml file (maybe it works, I dunno):

[build-system]
requires = ["poetry>=0.12", "python>=3.5"]

[github-actions]

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