def __setitem__(self, extension_handle, value):
    """If extension_handle specifies a non-repeated, scalar extension
    field, sets the value of that field.
    """

    _VerifyExtensionHandle(self._extended_message, extension_handle)

    if (extension_handle.label == FieldDescriptor.LABEL_REPEATED or
        extension_handle.cpp_type == FieldDescriptor.CPPTYPE_MESSAGE):
      raise TypeError(
          'Cannot assign to extension "%s" because it is a repeated or '
          'composite type.' % extension_handle.full_name)

    # It's slightly wasteful to lookup the type checker each time,
    # but we expect this to be a vanishingly uncommon case anyway.
    type_checker = type_checkers.GetTypeChecker(extension_handle)
    # pylint: disable=protected-access
    self._extended_message._fields[extension_handle] = (
        type_checker.CheckValue(value))
    self._extended_message._Modified()
If extension_handle specifies a non-repeated, scalar extension
    field, sets the value of that field.
    
deps/protobuf/python/google/protobuf/internal/extension_dict.py