bin+lib python-launcher

The Python launcher for UNIX

12 releases (breaking)

✓ Uses Rust 2018 edition

0.10.0 Apr 18, 2020
0.9.0 Mar 8, 2020
0.7.2 Mar 29, 2019
0.5.0 Nov 25, 2018
0.4.0 Jun 26, 2018

#18 in #python

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MIT license

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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 all Python users use when executing a Python interpreter. By having a version-agnostic command 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 which has existed as the preferred command on Windows for some time.

See the top section of py --help for instructions.

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.

py -3.6 (specific version)

  1. Search PATH for python3.6

py -3 (loose/major version)

  1. Use the version found in the PY_PYTHON3 environment variable if defined and not the empty string (e.g. PY_PYTHON3=3.6)
  2. Search PATH for all instances of python3.Y
  3. Find the executable with largest Y that earliest on PATH

py (any/unknown version)

  1. Use ${VIRTUAL_ENV}/bin/python immediately if available
  2. If the first argument is a file path ...
    1. Check for a shebang
    2. If executable starts with /usr/bin/python, /usr/local/bin/python, /usr/bin/env python or python, proceed based on the version found (bare python is considered the equivalent of not specifying a Python version)
  3. Use the version found in the PY_PYTHON environment variable if defined (e.g. PY_PYTHON=3 or PY_PYTHON=3.6)
  4. Search PATH for all instances of pythonX.Y
  5. Find the executable with the largest X.Y earliest on PATH



NOTE: I am using this project to learn Rust, so please don't be offended if I choose to implement something myself instead of accepting a pull request that you submit. (Pull requests to do something I have already implemented in a more idiomatic fashion are very much appreciated, though.)

PEP 397: Python launcher for Windows (documentation; source)

Everything in bold is required to hit MVP.


  1. Provide a python_launcher extension module
    • It will make the pipenv developers happy
    • Might need a rename to pylauncher or pyfinder to follow Python practices if it isn't too much trouble)
  2. Windows support
    • PATH
    • Windows Store (should be covered by PATH search, but need to make sure)
    • Registry
    • Bitness
  3. Configuration files (key thing to remember is should not get to the point that you're using this to alias specific interpreters, just making it easier to specify constraints on what kind of interpreter you need and then letting the launcher pick for you)
    • Customized commands?
    • Want a better format like TOML?
    • Want a way to override/specify things, e.g. wanting a framework build on macOS?
      • Aliasing? E.g. 2.7-framework for /System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python?
      • Just provide a way to specify a specific interpreter for a specific version? E.g. 2.7 for /System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python
      • What about implementations that don't install to e.g. python3.7 like pypy3?
        • Need more than just being able to alias pypy3 to its Python version?
    • How should the config file search work?
  4. Replacement for .venv/bin/python (while keeping the python name)?
    • VENV_REDIRECT variant in launcher.c
    • Might need switch off CLI additions -- i.e. -h, --list, and version specifier support -- in this situation to make this work
    • Read ../pyvenv.cfg and its home key to determine where to look for the Python executable
      • What if home has multiple Python executables installed? Might need to add an executable key to give full path to the creating interpreter.
    • Acts as a heavyweight "symlink" to the Python executable for the virtual environment
    • Speeds up environment creation by not having to copy over entire Python installation on Windows (e.g. .pyd files)
    • See/edit the site module to gain ability to specify virtual environment location (while maintaining the invariant on how to detect virtual environments as outlined in the venv module docs)
  5. Automatically detect .venv/pyvenv.cfg and use that (basically an implicit setting of $VIRTUAL_ENV)?
    • Also detect venv directories as well?
    • Is it worth the overhead/perf hit to search all subdirectories for a pyvenv.cfg and only activate if a single one is found?
    • Is it worth making the directory that is searched for configurable?
    • Have --venv do the search for a pyvenv.cfg in subdirectories and use that (or maybe search all subdirectories and perform a normal version resolution to use the newest one; not sure if it's worth the hassle of supporting version specification as well)?
  6. Use OsString/OsStr everywhere (versus now which is wherever it's easy w/ path::Path)?
    • Widest compatibility for people where they have undecodable paths (which is hopefully a very small minority)
    • Massive pain to make work (e.g. cannot easily convert to a CString)


  1. Make sure all potential panic! points are rare enough to be acceptable
  2. Have --list somehow denote an activated virtual environment?
    • Python Launcher doesn't denote or list anything
    • Seems useful since that would take precedence if the user didn't specify a version
  3. Man page?
  4. PYLAUNCH_DEBUG? (Rust logging info)
  5. Distribute binaries


  1. Flesh out documentation
    1. CLI documentation
    2. Flowchart of how the interpreter is selected?
    3. API documentation
  2. Get ~100% unit test coverage
  3. Consider dropping nix for a straight libc dependency (to potentially make Debian packaging easier)
    • Otherwise update nix to the latest version

Output from py --list on Windows:

Installed Pythons found by C:\WINDOWS\py.exe Launcher for Windows
 -3.8-64 *


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