code: def FindExtensionByName(self, full_name):
      
      full_name = _NormalizeFullyQualifiedName(full_name)
      message_name, _, extension_name = full_name.rpartition('.')
      try:
        # Most extensions are nested inside a message.
        scope = self.FindMessageTypeByName(message_name)
      except KeyError:
        # Some extensions are defined at file scope.
        scope = self.FindFileContainingSymbol(full_name)
      return scope.extensions_by_name[extension_name]
docs: Loads the named extension descriptor from the pool.
  
      Args:
        full_name: The full name of the extension descriptor to load.
  
      Returns:
        A FieldDescriptor, describing the named extension.