messages.rst

Messages

The fundamental building block in protocol buffers are messages. Messages are essentially permissive, strongly-typed structs (dictionaries), which have zero or more fields that may themselves contain primitives or other messages.

syntax = "proto3";

message Song {
  Composer composer = 1;
  string title = 2;
  string lyrics = 3;
  int32 year = 4;
}

message Composer {
  string given_name = 1;
  string family_name = 2;
}

The most common use case for protocol buffers is to write a .proto file, and then use the protocol buffer compiler to generate code for it.

Declaring messages

However, it is possible to declare messages directly. This is the equivalent message declaration in Python, using this library:

import proto

class Composer(proto.Message):
    given_name = proto.Field(proto.STRING, number=1)
    family_name = proto.Field(proto.STRING, number=2)

class Song(proto.Message):
    composer = proto.Field(Composer, number=1)
    title = proto.Field(proto.STRING, number=2)
    lyrics = proto.Field(proto.STRING, number=3)
    year = proto.Field(proto.INT32, number=4)

A few things to note:

• This library only handles proto3.
• The number is really a field ID. It is not a value of any kind.
• All fields are optional (as is always the case in proto3). The only general way to determine whether a field was explicitly set to its falsy value or not set all is to mark it optional.
• Because all fields are optional, it is the responsibility of application logic to determine whether a necessary field has been set.

Messages are fundamentally made up of :doc:`fields`. Most messages are nothing more than a name and their set of fields.

Usage

Instantiate messages using either keyword arguments or a :class:`dict` (and mix and matching is acceptable):

>>> song = Song(
...     composer={'given_name': 'Johann', 'family_name': 'Pachelbel'},
...     title='Canon in D',
...     year=1680,
... )
>>> song.composer.family_name
'Pachelbel'
>>> song.title
'Canon in D'

Enums

Enums are also supported:

import proto

class Genre(proto.Enum):
    GENRE_UNSPECIFIED = 0
    CLASSICAL = 1
    JAZZ = 2
    ROCK = 3

class Composer(proto.Message):
    given_name = proto.Field(proto.STRING, number=1)
    family_name = proto.Field(proto.STRING, number=2)

class Song(proto.Message):
    composer = proto.Field(Composer, number=1)
    title = proto.Field(proto.STRING, number=2)
    lyrics = proto.Field(proto.STRING, number=3)
    year = proto.Field(proto.INT32, number=4)
    genre = proto.Field(Genre, number=5)

All enums must begin with a 0 value, which is always the default in proto3 (and, as above, indistuiguishable from unset).

Enums utilize Python :class:`enum.IntEnum` under the hood:

>>> song = Song(
...     composer={'given_name': 'Johann', 'family_name': 'Pachelbel'},
...     title='Canon in D',
...     year=1680,
...     genre=Genre.CLASSICAL,
... )
>>> song.genre
<Genre.CLASSICAL: 1>
>>> song.genre.name
'CLASSICAL'
>>> song.genre.value
1

Additionally, it is possible to provide strings or plain integers:

>>> song.genre = 2
>>> song.genre
<Genre.JAZZ: 2>
>>> song.genre = 'CLASSICAL'
<Genre.CLASSICAL: 1>

Serialization

Serialization and deserialization is available through the :meth:`~.Message.serialize` and :meth:`~.Message.deserialize` class methods.

The :meth:`~.Message.serialize` method is available on the message classes only, and accepts an instance:

serialized_song = Song.serialize(song)

The :meth:`~.Message.deserialize` method accepts a :class:`bytes`, and returns an instance of the message:

song = Song.deserialize(serialized_song)

JSON serialization and deserialization are also available from message classes via the :meth:`~.Message.to_json` and :meth:`~.Message.from_json` methods.

json = Song.to_json(song)

new_song = Song.from_json(json)

Similarly, messages can be converted into dictionaries via the :meth:`~.Message.to_dict` helper method. There is no :meth:`~.Message.from_dict` method because the Message constructor already allows construction from mapping types.

song_dict = Song.to_dict(song)

new_song = Song(song_dict)