FakeFS

Mocha is great. But when your library is all about manipulating the filesystem, you really want to test the behavior and not the implementation.

If you're mocking and stubbing every call to FileUtils or File, you're tightly coupling your tests with the implementation.

def test_creates_directory
  FileUtils.expects(:mkdir).with("directory").once
  Library.add "directory"
end

The above test will break if we decide to use mkdir_p in our code. Refactoring code shouldn't necessitate refactoring tests.

With FakeFS:

def test_creates_directory
  Library.add "directory"
  assert File.directory?("directory")
end

Woot.

Usage

require 'fakefs'

# That's it.

Don't Fake the FS Immediately

require 'fakefs/safe'

FakeFS.activate!
# your code
FakeFS.deactivate!

# or
FakeFS do
  # your code
end

Rails

If you are using fakefs in a rails project with bundler, you'll probably want to specify the following in your Gemfile:

gem "fakefs", :require => "fakefs/safe"

RSpec

The above approach works with RSpec as well. In addition you may include FakeFS::SpecHelpers to turn FakeFS on and off in a given example group:

require 'fakefs/spec_helpers'

describe "my spec" do
  include FakeFS::SpecHelpers
end

See lib/fakefs/spec_helpers.rb for more info.

Integrating with other filesystem libraries

Third-party libraries may add methods to filesystem-related classes. FakeFS doesn't support these methods out of the box, but you can define fake versions yourself on the equivalent FakeFS classes. For example, FileMagic adds File#content_type. A fake version can be provided as follows:

module FakeFS
  class File
    def content_type
      'fake/file'
    end
  end
end

How is this different than MockFS?

FakeFS provides a test suite and works with symlinks. It's also strictly a test-time dependency: your actual library does not need to use or know about FakeFS.

Caveats

FakeFS internally uses the Pathname and FileUtils constants. If you use these in your app, be certain you're properly requiring them and not counting on FakeFS' own require.

Speed?

http://gist.github.com/156091

Installation

RubyGems

$ gem install fakefs

Rip

$ rip install git://github.com/defunkt/fakefs.git

Contributing

Once you've made your great commits:

1. Fork FakeFS
2. Create a topic branch - git checkout -b my_branch
3. Push to your branch - git push origin my_branch
4. Open a Pull Request
5. That's it!

Meta

• Code: git clone git://github.com/defunkt/fakefs.git
• Home: http://github.com/defunkt/fakefs
• Docs: http://defunkt.github.com/fakefs
• Bugs: http://github.com/defunkt/fakefs/issues
• List: http://groups.google.com/group/fakefs
• Test: http://travisci.org/#!/defunkt/fakefs
• Gems: http://rubygems.org/gems/fakefs

Releasing

1. Update version in lib/fakefs/version.rb
2. Commit it
3. rake publish