# 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)

Note

Elements must be appended individually for repeated fields of struct.Value.

    class Row(proto.Message):
        values = proto.RepeatedField(proto.MESSAGE, number=1, message=struct.Value,)

    >>> row = Row()
    >>> values = [struct_pb2.Value(string_value="hello")]
    >>> for v in values:
    >>>    row.values.append(v)

Direct assignment will result in an error.

    class Row(proto.Message):
        values = proto.RepeatedField(proto.MESSAGE, number=1, message=struct.Value,)

    >>> row = Row()
    >>> row.values = [struct_pb2.Value(string_value="hello")]
    Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/local/google/home/busunkim/github/python-automl/.nox/unit-3-8/lib/python3.8/site-packages/proto/message.py", line 543, in __setattr__
        self._pb.MergeFrom(self._meta.pb(**{key: pb_value}))
    TypeError: Value must be iterable

## 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.

Warning

If a message is constructed with multiple variants of a single `oneof` passed to its constructor, the last keyword/value pair passed will be the final value set.

This is consistent with PEP-468, which specifies the order that keyword args are seen by called functions, and with the regular protocol buffers runtime, which exhibits the same behavior.

Example:

    import proto

    class Song(proto.Message):
        name = proto.Field(proto.STRING, number=1, oneof="identifier")
        database_id = proto.Field(proto.STRING, number=2, oneof="identifier")

    s = Song(name="Canon in D minor", database_id="b5a37aad3")
    assert "database_id" in s and "name" not in s

    s = Song(database_id="e6aa708c7e", name="Little Fugue")
    assert "name" in s and "database_id" not in s

To query which `oneof` is present in a given message, use `proto.Message._pb("oneof")`.

Example:

    import proto

    class Song(proto.Message):
        name = proto.Field(proto.STRING, number=1, oneof="identifier")
        database_id = proto.Field(proto.STRING, number=2, oneof="identifier")

    s = Song(name="Canon in D minor")
    assert s._pb.WhichOneof("identifier") == "name"

    s = Song(database_id="e6aa708c7e")
    assert s._pb.WhichOneof("identifier") == "database_id"

## Optional fields

All fields in protocol buffers are optional, but it is often necessary to check for field presence. Sometimes legitimate values for fields can be falsy, so checking for truthiness is not sufficient. Proto3 v3.12.0 added the `optional` keyword to field descriptions, which enables a mechanism for checking field presence.

In proto plus, fields can be marked as optional by passing `optional=True` in the constructor. The message class then gains a field of the same name that can be used to detect whether the field is present in message instances.

    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)
        performer = proto.Field(proto.STRING, number=5, optional=True)

     >>> s = Song(
     ...     composer={'given_name': 'Johann', 'family_name': 'Pachelbel'},
     ...     title='Canon in D',
     ...     year=1680,
     ...     genre=Genre.CLASSICAL,
     ... )
     >>> Song.performer in s
     False
     >>> s.performer = 'Brahms'
     >>> Song.performer in s
     True
     >>> del s.performer
     >>> Song.performer in s
     False
     >>> s.performer = ""    # The mysterious, unnamed composer
     >>> Song.performer in s
     True

Under the hood, fields marked as optional are implemented via a synthetic one-variant `oneof`. See the protocolbuffers documentation for more information.