How to access Protocol Buffers custom option in python?

Question

I'm following instructions from Google Developers guide in order to create custom message option. I have used their example but I've received an error:

Traceback (most recent call last):
  File "test_my_opt.py", line 2, in <module>
    value = my_proto_file_pb2.MyMessage.DESCRIPTOR.GetOptions().Extensions[my_proto_file_pb2.my_option]
  File "(...)\google\protobuf\internal\python_message.py", line 1167, in __getitem__
    _VerifyExtensionHandle(self._extended_message, extension_handle)
  File "(...)\google\protobuf\internal\python_message.py", line 170, in _VerifyExtensionHandle
    message.DESCRIPTOR.full_name))
KeyError: 'Extension "my_option" extends message type "google.protobuf.MessageOptions", but this message is of type "google.protobuf.MessageOptions".'

I simply used following code:

import my_proto_file_pb2
value = my_proto_file_pb2.MyMessage.DESCRIPTOR.GetOptions().Extensions[my_proto_file_pb2.my_option]

And this proto file:

import "beans-protobuf/proto/src/descriptor.proto";

extend google.protobuf.MessageOptions {
  optional string my_option = 51234;
}

message MyMessage {
  option (my_option) = "Hello world!";
}

Everything like in guide... so how should I access this option without error?

Answer 1

import "beans-protobuf/proto/src/descriptor.proto";

I think this is the problem. The correct import statement for descriptor.proto is:

import "google/protobuf/descriptor.proto";

The path string is important because you need to be extending the original definitions of the descriptor types, not some copy of them. google/protobuf/descriptor.proto becomes the module google.protobuf.descriptor_pb2 in Python, and the Protobuf library expects that any custom options are extensions to the types in there. But you are actually extending beans-protobuf/proto/src/descriptor.proto, which becomes beans_protobuf.proto.src.descriptor_pb2 in Python, which is a completely different module! Hence, the protobuf library gets confused and doesn't think these extensions are applicable to protobuf descriptors.

I think if you just change the import statement, everything should work. When protobuf is correctly installed, google/protobuf/descriptor.proto should always work as an import -- there's no need to provide your own copy of the file.