Fields¶
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).
Note
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')
Warning
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.