How pipx chooses which Python to install a package with

pipx is a tool for installing standalone Python packages in isolated environments. It’s useful for managing Python CLI tools that you want available globally and not confined to one virtualenv. Things like:

• aws-mfa-v2 — temporary AWS sessions using MFA
• llm — CLI access to Large Language Models
• yamllint — YAML linting

Syntax for installing a package is:

pipx install $package

Package installation problem?

After switching to a new laptop, llm wouldn’t install:

> pipx install llm
Fatal error from pip prevented installation. Full pip output in file:
    ~.local/pipx/logs/cmd_2023-11-17_11.07.16_pip_errors.log

pip failed to build package:
    aiohttp

Some possibly relevant errors from pip install:
    error: subprocess-exited-with-error
    aiohttp/_websocket.c:3042:55: error: no member named 'ob_digit' in 'struct _longobject'
    aiohttp/_websocket.c:3097:55: error: no member named 'ob_digit' in 'struct _longobject'
    aiohttp/_websocket.c:3238:55: error: no member named 'ob_digit' in 'struct _longobject'
    aiohttp/_websocket.c:3293:55: error: no member named 'ob_digit' in 'struct _longobject'
    aiohttp/_websocket.c:3744:47: error: no member named 'ob_digit' in 'struct _longobject'
    error: command '/usr/bin/clang' failed with exit code 1

Error installing llm.

A little digging revealed the latest version of aiohttp didn’t support Python 3.12, hence the compilation error. So pipx must be using Python 3.12 to install packages.

How does pipx choose which Python to use?

From the docs:

The default python executable used to install a package is typically the python used to execute pipx and can be overridden by setting the environment variable PIPX_DEFAULT_PYTHON.

I installed pipx from Homebrew and the relevant Python can be seen in the package info:

> brew info pipx | rg "Required.*python"
Required: python-argcomplete, python-packaging, python@3.12

As suspected: python@3.12.

How to install packages with a different Python version?

pipx install supports a --python flag that can be used to specify a different Python. I’m using pyenv to marshall local Python versions; you can determine the path to a given version like so:

> cd /tmp
> pyenv local 3.11.6
> PYTHON_PATH=$(pyenv which python)

then llm can be installed:

pipx install --python $PYTHON_PATH llm