code: def find_location(self, root, path, prefix=None):
          
          if prefix:
              prefix = '%s%s' % (prefix, os.sep)
              if not path.startswith(prefix):
                  return None
              path = path[len(prefix):]
          path = safe_join(root, path)
          if os.path.exists(path):
              return path
docs: Finds a requested media file in a location, returning the found
          absolute path (or ``None`` if no match).