code: def _full_path(self, path_info):
          
          full_path = self.root + path_info
          if path.exists(full_path):
              return full_path
          else:
              for magic in self.magics:
                  if path.exists(magic.new_path(full_path)):
                      return magic.new_path(full_path)
              else:
                  return full_path
docs: Return the full path from which to read.