Generating Tornado style static urls in Sass

config.rb

require 'digest/md5'

module Sass::Script::Functions

  # Returns a url for static media (e.g. an icon) consistent with those
  # generated by the static_url method in Tornado templates. The method
  # takes an md5 hash of the given file and attaches the first five chars to
  # the 'v' GET param in the url (i.e. it appends "?v=<hash_slice>").
  # Assumes you use a somewhat conventional Tornado layout:
  #     myapp/myapp       -  your code
  #     myapp/static      -  your static media
  #     myapp/sass        -  your base sass dir (probably has config.rb file)
  #     myapp/sass/sass   -  your scss dir
  # Modify the http_path and local_path vars as needed
  # Example SCSS:
  # .icon {
  #   background: transparent url(static_url("img/my_awesome_icon.png")) no-repeat;
  # }
  #
  # Generated CSS (the ?v=xxxxx hash will be different for your file obviously):
  # .icon {
  #   background: transparent url("/static/img/my_awesome_icon.png?v=65309") no-repeat;
  # }
  def static_url(path_relative)
    assert_type path_relative, :String
    path_relative = unquote(path_relative) # Remove the quotes that Sass adds

    http_path = "/static/#{path_relative}" # URL of the media file
    local_path = "../static/#{path_relative}" # Path relative to Sass

    # Make the 5 char hash a la Tornado
    hash = Digest::MD5.hexdigest(IO.read(local_path))[0..4]
    # Form the new url and return
    Sass::Script::String.new("#{http_path}?v=#{hash}")
  end
  declare :static_url, :args => [:path_relative]
end