Python Launcher for POSIX

This projects implements a py executable that launches Python executables, akin to the py.exe launcher for Windows as first outlined by PEP 397, and various PEPs subsequently.

Rationale

The idea is simple: Instead of running a Python command directly (and rely on the name to know what version it is), a py command is always used. The Python version you want is specified by an option, so that

• py -2 runs Python 2.
• py -3 runs Python 3.
• py -3.5 runs Python 3.5.

and so on. This avoids relying on the PATH environment variable, which can be difficult to keep track of if you have a lot of different development environments on a machine.

Finding a Python

Unlike Windows (from which this utility drew inspiration from), POSIX systems does not have a registry to declare what is installed. This tool, therefore, uses some heuristics to determine find Pythons.

Python installations are categorised into two kinds: managed, and executable.

Managed Pythons

A managed Python is installed by a version manager, e.g. pyenv. These managers put all Python installations inside a directory (e.g. $PYENV_ROOT/versions). Each subdirectory inside is named after the version, and is the root prefix of that Python.

This tool looks for an environment variable, PY_MANAGED_DIR, to know where to look for managed Pythons. If you use pyenv, for example, you can set it like this:

export PY_MANAGED_DIR=$PYENV_ROOT/versions

So that installations in the directory can be found. You can also set multiple values seperated by : if you use multiple tools:

export PY_MANAGED_DIR=$PYENV_ROOT/versions:$ASDF_DATA_DIR/installs/python

The following managers are known to work at the current time:

• pyenv
• asdf
• Pythonz

Only stable CPython installations are supported.

Pythons in PATH

The PATH environment variable is also inspected to find Pythons. This uses the customary pythonX.Y naming convention to tell what version an executable is. The accuracy of the names is not checked. python3.5 will be identified as Python 3.5 (of unknown patch version), and python3 as Python 3 (of unknown minor and patch version).

Choosing a Python

A Python is chosen from the above findings with the following criteria:

1. The higher a version is, the better.
2. The more specified a version is, the better.
3. Managed over PATH.
4. The order of PY_MANAGED_DIR and PATH is respected.

Note that rule 2. means the result may not match the order in PATH. For py -3, for example, a python3.4 will be preferred over python3, even if the latter is specified earlier. I personally think this is fine (and, to be honest, this is much easier to implement), but am open to changing it if someone makes a good argument.

Also, conforming to PEP 486, if py is invoked (no version specifications) inside a virtual environment, the virtual environment’s Python is always used.

Future Works

• Support configuration by py.ini and environment variables, as specified in PEP 397.