Parsing a .proto file without creating the descriptor

Question

I understand that the normal way of using protobuf is to create the .proto and then compile it into the relevant class - Java, Python, etc. I have a requirement which might need to parse the .proto file in Python code. Has anyone tried creating own parser for the .proto file? Will it be recommended to always compile the class instead of directly parsing the .proto?

Answer 1

It probably won't help you directly, but yes, I've written my own parser (live demo, parser source). This code is C# hence why it probably won't help, but it clearly is possible. I started that branch 9 days ago, and now it is basically feature-complete including parser, generator, and an interactive web-site with syntax-error highlighting - so it isn't necessarily a huge amount of work.

However! You may find it easier just to shell execute "protoc" (available on maven). If you use the -oFILE / --descriptor_set_out=FILE switch (same thing, alternative syntax), then it parses the input .proto file and writes a file that is a serialized FileDescriptorSet from descriptor.proto. This means you can use your regular tools to generate code in your chosen language for descriptor.proto, then deserialize the file as a FileDescriptorSet instance. Once you've done that: you can just walk the object model to see the files, messages, enums, fields, etc. IIRC some protobuf implementations support working entirely from a descriptor (which is what protoc emits), without the codegen step.