./.venv/bin/activate isn't magic

In Python, it’s recommended to use virtualenvs and run .venv/bin/activate prior to running your program or project dependencies.

So what’s in .venv/bin/activate, it must be some magic sys.path stuff right?

That’s what I always assumed, so I’d either activate the virtualenv, or use poetry run which is roughly equivalent.

Let’s take a look at the .venv/bin/activate source:

# This file must be used with "source bin/activate" *from bash*
# you cannot run it directly

deactivate () {
    # reset old environment variables
    if [ -n "${_OLD_VIRTUAL_PATH:-}" ] ; then
        PATH="${_OLD_VIRTUAL_PATH:-}"
        export PATH
        unset _OLD_VIRTUAL_PATH
    fi
    if [ -n "${_OLD_VIRTUAL_PYTHONHOME:-}" ] ; then
        PYTHONHOME="${_OLD_VIRTUAL_PYTHONHOME:-}"
        export PYTHONHOME
        unset _OLD_VIRTUAL_PYTHONHOME
    fi

    # This should detect bash and zsh, which have a hash command that must
    # be called to get it to forget past commands.  Without forgetting
    # past commands the $PATH changes we made may not be respected
    if [ -n "${BASH:-}" -o -n "${ZSH_VERSION:-}" ] ; then
        hash -r
    fi

    if [ -n "${_OLD_VIRTUAL_PS1:-}" ] ; then
        PS1="${_OLD_VIRTUAL_PS1:-}"
        export PS1
        unset _OLD_VIRTUAL_PS1
    fi

    unset VIRTUAL_ENV
    if [ ! "$1" = "nondestructive" ] ; then
    # Self destruct!
        unset -f deactivate
    fi
}

# unset irrelevant variables
deactivate nondestructive

VIRTUAL_ENV="/private/var/folders/6_/l5dqqh5x2f927hbmrsrnw_840000gp/T/tmp.De1HsaCn/.venv"
export VIRTUAL_ENV

_OLD_VIRTUAL_PATH="$PATH"
PATH="$VIRTUAL_ENV/bin:$PATH"
export PATH

# unset PYTHONHOME if set
# this will fail if PYTHONHOME is set to the empty string (which is bad anyway)
# could use `if (set -u; : $PYTHONHOME) ;` in bash
if [ -n "${PYTHONHOME:-}" ] ; then
    _OLD_VIRTUAL_PYTHONHOME="${PYTHONHOME:-}"
    unset PYTHONHOME
fi

if [ -z "${VIRTUAL_ENV_DISABLE_PROMPT:-}" ] ; then
    _OLD_VIRTUAL_PS1="${PS1:-}"
    if [ "x(.venv) " != x ] ; then
	PS1="(.venv) ${PS1:-}"
    else
    if [ "`basename \"$VIRTUAL_ENV\"`" = "__" ] ; then
        # special case for Aspen magic directories
        # see http://www.zetadev.com/software/aspen/
        PS1="[`basename \`dirname \"$VIRTUAL_ENV\"\``] $PS1"
    else
        PS1="(`basename \"$VIRTUAL_ENV\"`)$PS1"
    fi
    fi
    export PS1
fi

# This should detect bash and zsh, which have a hash command that must
# be called to get it to forget past commands.  Without forgetting
# past commands the $PATH changes we made may not be respected
if [ -n "${BASH:-}" -o -n "${ZSH_VERSION:-}" ] ; then
    hash -r
fi

Kind of lot there, let’s remove all the deactivation and shell prompt stuff to make the functionality more clear.

VIRTUAL_ENV="/private/var/folders/6_/l5dqqh5x2f927hbmrsrnw_840000gp/T/tmp.De1HsaCn/.venv"
export VIRTUAL_ENV

PATH="$VIRTUAL_ENV/bin:$PATH"
export PATH

So all activate does is add the .venv/bin directory to the shell’s path, which means we can call python, black, mypy, gunicorn, etc. directly rather than with a prefix .venv/.

If you don’t mind prefixing your tools with .venv/, you can skip .venv/bin/activate.

Of course, node seems to have this all figured out where it will automatically check in node_modules for dependencies when executing a program.

PEP 582 – Python local packages directory from May 2018 outlines similar functionality to node for python with a node_modules equivalent called __pypackages__. However, it’s still in the draft phase so we’re stuck with virtualenvs for the foreseeable future.

RTFM

I never actually read the docs on virtualenvs until writing this post, and they outline that activating a virtualenv is entirely optional.

You don’t specifically need to activate an environment; activation just prepends the virtual environment’s binary directory to your path, so that “python” invokes the virtual environment’s Python interpreter and you can run installed scripts without having to use their full path. However, all scripts installed in a virtual environment should be runnable without activating it, and run with the virtual environment’s Python automatically.