pyem 0.4.4

Install

I recommend using pipx:

pipx install pyem --spec="pyem[compat]"

The “compat” extra also installs virtualenv to support old Python versions without the builtin venv module. You can drop it if you don’t need this. (You can always pipx inject virtualenv back if you need to.)

In Action

Add a virtual environment:

$ pyem venv add python3.7  # Based on a command.

$ pyem venv add 3.6  # Based on interpreter found by the Python launcher.

$ pyem venv add /usr/local/bin/pypy3  # Based on an executable.

The second variant relies on the Python launcher to locate an interpreter. This tool should be installed by default if you use the officlal installer on Windows (and do not explicitly choose not to install it). For other platforms, Python launcher for UNIX by Brett Cannon can be used as an alternative.

List managed virtual environments:

$ pyem venv list
  Quintuplet
=============================================
  cpython-3.6-darwin-x86_64-f14a3513
  cpython-3.7-darwin-x86_64-dbe83ac5
* pypy-3.6-darwin-x86_64-dc1298a1

Set active virtual environment:

$ pyem venv set 3.7
Switched to cpython-3.7-darwin-x86_64-dbe83ac5

$ pyem venv set 3.6
Error: name '3.6' is ambiguous; choose from:
  cpython-3.6-darwin-x86_64-f14a3513
  pypy-3.6-darwin-x86_64-dc1298a1

$ pyem venv set cpython-3.6
Switched to cpython-3.6-darwin-x86_64-f14a3513

Run a command inside a virtual environment:

$ pyem poetry run python -c "import sys; print(sys.executable)"
/tmp/exampleproject/.venvs/bin/python

$ pyem --spec=pypy-3.6 pipenv run pypy3 -c "import sys; print(sys.executable)"
/tmp/exampleproject/.venvs/bin/pypy3

How does this work?

PyEM sets environment variables VIRTUAL_ENV and PATH, and hand off control to subprocess for the command specified. This is enough to trick a lot of Python project tools, including the Python launcher, Poetry, and Pipenv. Python interpreters with venv support (e.g. CPython 3.3 or later) should also integrate seamlessly.

Flit is more difficult to trick since it does not automatically inspect environment variables like other tools do. Use this workaround instead (requires the Python launcher):

$ pyem flit install --python=py

Starting from Flit 2.1, you can also set the environment variable FLIT_INSTALL_PYTHON=py for the same effect. This is a good default even when you’re not using PyEM IMO; it makes more sense than installing into Flit’s environment.