Managing Dependencies in Python vs Ruby

Ruby's Bundler tool is amazing.

Ruby’s Bundler tool is amazing.

With Python projects, the standard way of doing things is to set up a virtualenv and use pip to install packages from PyPI specified in a requirements.txt file. This way, each of your project’s dependencies are kept separate, installed in their own directories in isolated sandboxed environments.

This works pretty well. But sometimes, when I am debugging a third party package, I want to be able to get the source code from git and use it instead of the package from PyPI, so I can make changes, troubleshoot, experiment, etc. This is a pain in the butt. You have to remove the installed package and either 1) install manually (and repeatedly, as you work) from your cloned repository, or 2) add the repository directory to your Python library path somehow. Then you have to undo these changes to go back to using the PyPI package. Either way, it’s clunky and annoying.

Ruby’s bundler tool has a very different approach to dependencies. It, too, downloads appropriate versions of gems (which is what packages are called), which are listed in a Gemfile. But unlike pip, it can store multiple versions of a gem, and even let you specify that a gem lives in a github or local repository; moreover, it makes the right packages available each time you run your program! That is, each time you run the “bundle exec” wrapper to run Rails or anything else, it sets up a custom set of directories for Ruby’s library path that point ONLY to the versions you want, ignoring the others.

I did this today when trying to pin down the source of some deprecation warnings I was seeing after some gem upgrades. My Gemfile had these lines in it:

gem 'sunspot_rails', '~> 2.1.0'
gem 'sunspot_solr', '~> 2.1.0'

I cloned the sunspot repository containing those gems. Then I ran:

# bundle config local.sunspot_rails ~/sunspot
# bundle config local.sunspot_solr ~/sunspot

And changed the Gemfile lines:

gem 'sunspot_rails', :github => 'sunspot/sunspot_rails', :branch => 'master'
gem 'sunspot_solr', :github => 'sunspot/sunspot_solr', :branch => 'master'

Finally, I ran “bundler update”. That’s it! I could make changes to my cloned repository, restart Rails, and see the changes immediately.

When I was done messing around, I changed my Gemfile back, ran “bundler update” again, and I was back to using my original gems.

Being able to work so easily with third party code allowed me to quickly figure out where the deprecated calls were being made and file an issue with the sunspot project.

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