Pre-commit should install correct python version

What? Pre-commit should install the correct version(s) of python required for running hooks.

Why? #869 shows language_version as the correct way to specify a dependency for a hook. However this isn’t acted on by pre-commit at the install stage, so at run time the hook doesn’t have that python version available. This violates the purpose of the library:

Some of the best linters are written in languages that you do not use in your project or have installed on your machine. For example scss-lint is a linter for SCSS written in Ruby. If you’re writing a project in node you should be able to use scss-lint as a pre-commit hook without adding a Gemfile to your project or understanding how to get scss-lint installed.

Notes:

• ruby.py contains an _install_ruby function, but the equivalent appears missing in python.py
• could potentially use pyenv to handle the installs

[asottile]

also pyenv doesn’t even work without also installing another toolchain or two. For example it requires an http downloader globally installed, and requires libssl-dev / gcc / etc. to be system installed on linuxes.

There aren’t great binary distribution methods for python and building from source is a real struggle. Distributions themselves are much better at providing a working binary

[ned2]

Chiming in a bit late to the party as I just ran into this issue with pyenv.

The error message I got after using python3.7 as the value for language_version was this:

pyenv: python3.7: command not found

The `python3.7' command exists in these Python versions:
  3.7.0
  3.7.2

It seems as though pyenv has not not registered python3.7 as a command for my virtualenv (which is based on 3.7.2). Strangely, when I activate the plain 3.7.2 installation, it is registered.

At any rate, python3 is a registered command in my virtualenv, so it looks like the value of python3 forlanguage_version makes everyone happy.