FakeFS

Mocking calls to FileUtils or File means tightly coupling tests with the implementation.

it "creates a directory" do
  FileUtils.expects(:mkdir).with("directory").once
  Library.add "directory"
end

The above test will break if mkdir_p is used instead. Refactoring code should not necessitate refactoring tests.

A better approach is to use a temp directory if you are working with relative directories.

require 'tmpdir'

it "creates a directory" do
  Dir.mktmpdir do |dir|
    Dir.chdir dir do
      Library.add "directory"
      assert File.directory?("directory")
    end
  end
end

But if you are working with absolute directories or do not want to use temporary directories, use FakeFS instead:

it "creates a directory" do
  FakeFS do
    Library.add "directory"
    assert File.directory?("directory")
  end
end

Installation

gem install fakefs

Usage

To fake out the FS:

require 'fakefs'

Temporarily faking the FS

require 'fakefs/safe'

FakeFS.activate!
# your code
FakeFS.deactivate!

# or
FakeFS do
  # your code
end

Rails

In rails projects, add this to your Gemfile:

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

RSpec

Include FakeFS::SpecHelpers to turn FakeFS on and off in an 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:

FakeFS::File.class_eval do
  def content_type
    'fake/file'
  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.

As of v0.5.0, FakeFS's current working directory (i.e. Dir.pwd) is independent of the real working directory. Previously if the real working directory were, for example, /Users/donovan/Desktop, then FakeFS would use that as the fake working directory too, even though it most likely didn't exist. This caused all kinds of subtle bugs. Now the default working directory is the only thing that is guaranteed to exist, namely the root (i.e. /). This may be important when upgrading from v0.4.x to v0.5.x, especially if you depend on the real working directory while using FakeFS.

Speed?

http://gist.github.com/156091

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: https://github.com/fakefs/fakefs
• Docs: http://www.rubydoc.info/github/defunkt/fakefs
• Bugs: https://github.com/fakefs/fakefs/issues
• Test: https://travis-ci.org/#!/defunkt/fakefs
• Gems: https://rubygems.org/gems/fakefs

Releasing

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