The Python Launcher for Unix

An implementation of the py command for Unix-based platforms (with some potential experimentation for good measure 😉)

The goal is to have py become the cross-platform command that Python users typically use to launch an interpreter. By having a command that is version-agnostic command when it comes to Python, it side-steps the "what should the python command point to?" debate by clearly specifying that upfront (i.e. the newest version of Python that can be found). This also unifies the suggested command to document for launching Python on both Windows as Unix as py has existed as the preferred command on Windows since 2012 with the release of Python 3.3.

A non-goal of this project is to become the way to launch the Python interpreter all the time. If you know the exact interpreter you want to launch then you should launch it directly; same goes for when you have requirements on the type of interpreter you want (e.g. 32-bit, framework build on macOS, etc.). The Python Launcher should be viewed as a tool of convenience, not necessity.

For instructions on how to use the Python Launcher, see the top section of py --help.

Installation

You can either install from crates.io or from source. Both approaches require you install the Rust toolchain. You can use rustup to accomplish this or whatever your OS suggests.

If you want to install from crates.io, run:

shell cargo install python-launcher

If you want to install from source, run:

shell cargo install --path .

Search order

Please note that while searching, the search for a Python version can become more specific. This leads to a switch in the search algorithm to the one most appropriate to the specificity of the version.

You can always run the Python Launcher with PYLAUNCH_DEBUG set to some value to have it output logging details of how it is performing its search.

py -3.6 (specific version)

  1. Search PATH for python3.6

py -3 (loose/major version)

  1. Check for the PY_PYTHON3 environment variable, and if defined and not the empty string then use it as the specific version (e.g. PY_PYTHON3=3.6)
  2. Search PATH for all instances of python3.*
  3. Find the executable with the newest version number that comes earliest on PATH

py (any version)

  1. Use ${VIRTUAL_ENV}/bin/python immediately if available
  2. Use .venv/bin/python if available in the current working directory or any of its parent directories
  3. If the first argument is a file path ...
    1. Check for a shebang
    2. If shebang path starts with /usr/bin/python, /usr/local/bin/python, /usr/bin/env python or python, proceed based on the version found on that path (bare python is considered the equivalent of not specifying a Python version)
  4. Check for the PY_PYTHON environment variable, and if defined then use it as the loose or specific version (e.g. PY_PYTHON=3 or PY_PYTHON=3.6)
  5. Search PATH for all instances of python*.*
  6. Find the executable with the newest version that is earliest on PATH

TODO

Issues to finish to reach MVP

Appendix