virtualenv-pyenv

A virtualenv Python discovery plugin for pyenv–installed interpreters

Installation

pip install virtualenv-pyenv

Usage

The Python discovery mechanism can be specified by:

• the CLI option --discovery:

  virtualenv --discovery pyenv -p 3.10 testenv

• the environment variable VIRTUALENV_DISCOVERY:

  export VIRTUALENV_DISCOVERY=pyenv
  virtualenv -p 3.10 testenv

• the config option discovery:

  [virtualenv]
  discovery = pyenv

  virtualenv -p 3.10 testenv

• the virtualenvwrapper environment variable VIRTUALENVWRAPPER_VIRTUALENV_ARGS:

  export VIRTUALENVWRAPPER_VIRTUALENV_ARGS='--discovery=pyenv'
  mkvirtualenv -p 3.10 testenv

Operation Mode

The plugin supports several operation modes. The operation mode affects various aspects of the discovery process.

mode	specifier	path specifier	fallback to builtin
compat	optional	allowed	no
fallback	optional	allowed	yes
strict	required	error	no

The mode is specified as a part of the discovery method name: pyenv-{mode}, e.g.,

virtualenv --discovery=pyenv-fallback -p 3.10 testenv

or

export VIRTUALENV_DISCOVERY=pyenv-strict
virtualenv -p 3.10 testenv

If no mode is specified, the compat mode is used, that is, --discovery=pyenv is the same as --discovery=pyenv-compat.

The compat mode is recommended for most cases. It mimics closely the builtin discovery plugin to maximize compatibility with existing tools (e.g., build, tox):

• if no specifier is provided, the plugin falls back to sys.executable, even if it is not installed by pyenv;
• if a path specifier (-p /path/to/bin/python) is provided, the path is used, even if it is not installed by pyenv;
• otherwise (-p 3.7, -p py37), only pyenv–installed interpreters are used.

The fallback mode is the same as compat, but in addition falls back to the builtin plugin if no interpreter was found. If multiple specifiers are provided, all of them are tried first before falling back to the builtin plugin as a last resort.

The strict mode, as its name suggests, ensures that only pyenv–installed interpreters are used:

• a specifier is required, sys.executable is never used as a fallback, even if it is installed by pyenv (may be relaxed in the future);
• a path specifier is not allowed, even if the path points to a pyenv–installed interpreter (may be relaxed in the future);
• no fallback to the builtin plugin.

Python Specifier Format

The plugin supports two specifier formats informally called “pyenv-style” and “virtualenv-style”.

The version part of both specifier formats can contain one (major), two (major.minor), or three (major.minor.patch) components. When a one–component version is specified, the latest installed final minor release is selected, ignoring pre–/dev–release. When a two–component version is specified, the latest installed final patch release is selected, ignoring pre–/dev–releases. When a three–component version is specified, the exact final release is selected, ignoring pre–/dev–releases. The pre–/dev–release version is selected only if it is explicitly requested.

installed	requested	selected
3.9.5; 3.9.17, 3.10.0	3	3.10.0
3.9.5; 3.9.17, 3.10.0	3.9	3.9.17
3.9.5; 3.9.17, 3.10.0	3.9.5	3.9.5
3.9.5; 3.9.17, 3.10.0	3.9.0	—
3.12-dev; 3.12.0b3	3.12	—
3.12-dev; 3.12.0b3	3.12.0	—
3.12-dev; 3.12.0b3	3.12-dev	3.12-dev
3.12-dev; 3.12.0b3	3.12.0b3	3.12.0b3

pyenv–style

The same format as used by pyenv (pyenv install --list).

• a final version with 1 version component: 3
• a final version with 2 version components: 3.11
• a final version with 3 version components: 3.11.2
• a pre–release version: 3.13.0a4, 3.12.0b3, 3.11.0rc1
• a dev version: 3.13-dev

virtualenv–style

The same format as used by virtualenv (docs). A subset of this format is used by tox (docs).

• a relative or absolute path: /path/to/bin/python (it can be any Python interpreter, not only installed by pyenv)
• a final version with 1 version component: py3, python3, cpython3, python3-32, py3-64
• a final version with 2 version components: 311, py311, py3.11, python311, cpython3.11, python3.11-32, py311-64
• a final version with 3 version components: py3.11.2, python3.11.2, cpython3.11.2, python3.11.2-32, py3.11.2-64

Limitations

• Only CPython is supported at the moment.
• The architecture part (-32/-64) of a specifier is ignored. For example, all of the following specifiers match any installed CPython 3.8.1 regardless of the architecture: python3.8.1, python3.8.1-32, python3.8.1-64.

Internals

virtualenv-pyenv does not rely on pyenv to discover Python interpreters, that is, it never calls any pyenv command and does not require pyenv to be in PATH. Instead, the plugin uses pyenv-inspect library, which, in turn, inspects $PYENV_ROOT/versions directory contents.