import collections


def get_app_url(config):
  """ Returns the application's URL, based on the given config. """
  return '%s://%s' % (config['PREFERRED_URL_SCHEME'], config['SERVER_HOSTNAME'])


def slash_join(*args):
  """
  Joins together strings and guarantees there is only one '/' in between the
  each string joined. Double slashes ('//') are assumed to be intentional and
  are not deduplicated.
  """
  def rmslash(path):
    path = path[1:] if path[0] == '/' else path
    path = path[:-1] if path[-1] == '/' else path
    return path

  args = [rmslash(path) for path in args]
  return '/'.join(args)


def canonicalize(json_obj):
  """ Returns a JSON object sorted by key. """
  if isinstance(json_obj, collections.MutableMapping):
    sorted_obj = sorted({key: canonicalize(val) for key, val in json_obj}.items())
    return collections.OrderedDict(sorted_obj)
  elif isinstance(json_obj, (list, tuple)):
    return [canonicalize(val) for val in json_obj]
  return json_obj