A fish function to automatically activate a venv called ".env" in the current git root

cd.fish

#!/bin/env fish

function cd -d "change directory, and activate virtualenvs, if available"
    # first and foremost, change directory
    builtin cd $argv

    # find a parent git directory
    if git rev-parse --show-toplevel >/dev/null ^/dev/null
        set gitdir (realpath (git rev-parse --show-toplevel))
    else
        set gitdir ""
    end

    # if that directory contains a virtualenv in a ".env" directory, activate it
    if test \( -z "$VIRTUAL_ENV" -o "$VIRTUAL_ENV" != "$gitdir/.env" \) -a -f "$gitdir/.env/bin/activate.fish"
        source $gitdir/.env/bin/activate.fish
    end

    # deactivate an active virtualenv if not int a git directory with an ".env"
    if test -n "$VIRTUAL_ENV" -a "$VIRTUAL_ENV" != "$gitdir/.env"
        deactivate
    end
end

More robust version that work with current fish syntax: https://gist.github.com/tommyip/cf9099fa6053e30247e5d0318de2fb9e