Fields are assigned using the Field class, instantiated within a Message declaration.

Fields always have a type (either a primitive, a message, or an enum) and a number.

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)

For messages and enums, assign the message or enum class directly (as shown in the example above).


For messages declared in the same module, it is also possible to use a string with the message class’ name if the class is not yet declared, which allows for declaring messages out of order or with circular references.

Repeated fields

Some fields are actually repeated fields. In protocol buffers, repeated fields are generally equivalent to typed lists. In protocol buffers, these are declared using the repeated keyword:

message Album {
  repeated Song songs = 1;
  string publisher = 2;

Declare them in Python using the RepeatedField class:

class Album(proto.Message):
    songs = proto.RepeatedField(Song, number=1)
    publisher = proto.Field(proto.STRING, number=2)

Map fields

Similarly, some fields are map fields. In protocol buffers, map fields are equivalent to typed dictionaries, where the keys are either strings or integers, and the values can be any type. In protocol buffers, these use a special map syntax:

message Album {
  map<uint32, Song> track_list = 1;
  string publisher = 2;

Declare them in Python using the MapField class:

class Album(proto.Message):
    track_list = proto.MapField(proto.UINT32, Song, number=1)
    publisher = proto.Field(proto.STRING, number=2)

Oneofs (mutually-exclusive fields)

Protocol buffers allows certain fields to be declared as mutually exclusive. This is done by wrapping fields in a oneof syntax:

import "google/type/postal_address.proto";

message AlbumPurchase {
  Album album = 1;
  oneof delivery {
    google.type.PostalAddress postal_address = 2;
    string download_uri = 3;

When using this syntax, protocol buffers will enforce that only one of the given fields is set on the message, and setting a field within the oneof will clear any others.

Declare this in Python using the oneof keyword-argument, which takes a string (which should match for all fields within the oneof):

from google.type.postal_address import PostalAddress

class AlbumPurchase(proto.Message):
    album = proto.Field(Album, number=1)
    postal_address = proto.Field(PostalAddress, number=2, oneof='delivery')
    download_uri = proto.Field(proto.STRING, number=3, oneof='delivery')


oneof fields must be declared consecutively, otherwise the C implementation of protocol buffers will reject the message. They need not have consecutive field numbers, but they must be declared in consecutive order.