short Intro: here is a method for using virtualenv without virtualenvwrapper using a simple shell function.

If you’re like me you like to keep your system clean; not installing packages globally, using the builtin install mechanisms, using homebrew to install things to /usr/local knowing you can always blow that directory away and start fresh if you want. With Python on OS X and Linux I don’t want to pollute the system library directories when installing new packages. For me the minor reason is that I don’t like things installed to /Library/Python/2.7/site-packages and the major reason is that inevitably some Python packages install scripts to /usr/bin. And while pip can do a good job of uninstalling pacakges it doesn’t always work and the standard easy_install doesn’t have a remove option! Yes, WTF?

So I rely on virtualenv to keep my system Python directory clean and to allow me different package setups for different projects. I first started using virtualenv I immediately started using virtualenvwrapper. Eventually I started to wonder why my shell was taking so long to startup. So I realized that while virtualenvwrapper is great and full-featured, I had enough experience with the shell and Python that I didn’t need all of its features.

Installing virtualenv

Unfortunately no matter how clean you want to keep your system’s Python package directory, you need to at least install virtualenv. I’ll just leave this here:

easy_install virtualenv

Creating a default environment

Since I never want to pollute the system package directory I create a default environment that automatically loads when I start my shell. I play nice and install new virtualenvs to the ~/.virtualenvs directory which is the same directory that virtualenvwrapper uses.

virtualenv ~/.virtualenvs/default

The main reason I use a directory in ~ is that I sync my development files via Dropbox and thus cannot share libraries between machines; some packages actually compile code that is system dependent.

Activating the default environment

To activate the default environment manually you can just run,

. ~/.virtualenvs/default/bin/activate

but this gets tiresome and is not flexible enough. So in .zshrc.bashrc for bashers–just put,

activate() {
    if [ $# -le 0 ]; then
        set -- default
    . ${VIRTUALENV_BASE}/$1/bin/activate

The activate function has an optional argument that defaults to default and activates the specified virtualenv. Adjust VIRTUALENV_BASE if you despise the location and create virtualenvs in that directory.

Creating additional environments

When I want to create a new environment, say burger, I just do,

virtualenv ~/.virtualenvs/burger

Asteroid Crash

I am well aware that at some point if I were to add more functionality to this approach I would reach virtualenvwrapper level and at that point I’d be incurring as much overhead. But I don’t use all that functionality and I know how to traverse a tree. I want to live close to the metal because when something breaks I have not added another layer of functionality. To me this is simple.

More Information

Please read Better Package Management on how to actually manage dependencies properly using the virtualenvs you create.

Updates 2013-05-10

I had originally started using this approach a while back and couldn’t remember the timing differences. Here is a rough timing test for shel startup time

  • Using the approach here:

    zsh -i -c exit  0.16s user 0.27s system 76% cpu 0.570 total
    zsh -i -c exit  0.16s user 0.19s system 101% cpu 0.338 total
    zsh -i -c exit  0.16s user 0.19s system 101% cpu 0.343 total
    zsh -i -c exit  0.16s user 0.19s system 100% cpu 0.345 total
    zsh -i -c exit  0.15s user 0.18s system 101% cpu 0.329 total
    zsh -i -c exit  0.15s user 0.18s system 100% cpu 0.334 total
  • With virtualenvwrapper but no environment activated

    zsh -i -c exit  0.37s user 0.31s system 98% cpu 0.690 total
    zsh -i -c exit  0.35s user 0.31s system 96% cpu 0.694 total
    zsh -i -c exit  0.36s user 0.31s system 99% cpu 0.676 total
    zsh -i -c exit  0.36s user 0.30s system 99% cpu 0.672 total
    zsh -i -c exit  0.38s user 0.31s system 99% cpu 0.693 total
    zsh -i -c exit  0.37s user 0.33s system 95% cpu 0.730 total
  • With virtualenvwrapper and activating an environment

    zsh -i -c exit  0.81s user 0.64s system 96% cpu 1.507 total
    zsh -i -c exit  0.81s user 0.59s system 99% cpu 1.417 total
    zsh -i -c exit  0.81s user 0.62s system 98% cpu 1.459 total
    zsh -i -c exit  0.97s user 0.69s system 98% cpu 1.698 total
    zsh -i -c exit  0.81s user 0.63s system 95% cpu 1.498 total
    zsh -i -c exit  0.84s user 0.62s system 98% cpu 1.489 total

If you don’t want an environment loaded at startup you can source as mentioned here but this intended for people who will manually use workon to activate an environment once in the shell.