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

RSpec

The above approach works with RSpec as well. In addition to this you may use the 'use_fakefs' macro to turn FakeFS on and off in a given example group. See lib/spec_helpers for more details on it's usage.

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

Gemcutter

$ gem install fakefs

Rip

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

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