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This module contains related classes and functions for serialization.

Plain serializers use a function to modify the output of serialization.

This is particularly helpful when you want to customize the serialization for annotated types. Consider an input of list, which will be serialized into a space-delimited string.

from typing import Annotated

from pydantic import BaseModel, PlainSerializer

CustomStr = Annotated[
    list, PlainSerializer(lambda x: ' '.join(x), return_type=str)
]

class StudentModel(BaseModel):
    courses: CustomStr

student = StudentModel(courses=['Math', 'Chemistry', 'English'])
print(student.model_dump())
#> {'courses': 'Math Chemistry English'}

The serializer function.

Type: core_schema.SerializerFunction

The return type for the function. If omitted it will be inferred from the type annotation.

Type: Any

Determines when this serializer should be used. Accepts a string with values 'always', 'unless-none', 'json', and 'json-unless-none'. Defaults to 'always'.

Type: WhenUsed

Wrap serializers receive the raw inputs along with a handler function that applies the standard serialization logic, and can modify the resulting value before returning it as the final output of serialization.

For example, here's a scenario in which a wrap serializer transforms timezones to UTC and utilizes the existing datetime serialization logic.

from datetime import datetime, timezone
from typing import Annotated, Any

from pydantic import BaseModel, WrapSerializer

class EventDatetime(BaseModel):
    start: datetime
    end: datetime

def convert_to_utc(value: Any, handler, info) -> dict[str, datetime]:
    # Note that `handler` can actually help serialize the `value` for
    # further custom serialization in case it's a subclass.
    partial_result = handler(value, info)
    if info.mode == 'json':
        return {
            k: datetime.fromisoformat(v).astimezone(timezone.utc)
            for k, v in partial_result.items()
        }
    return {k: v.astimezone(timezone.utc) for k, v in partial_result.items()}

UTCEventDatetime = Annotated[EventDatetime, WrapSerializer(convert_to_utc)]

class EventModel(BaseModel):
    event_datetime: UTCEventDatetime

dt = EventDatetime(
    start='2024-01-01T07:00:00-08:00', end='2024-01-03T20:00:00+06:00'
)
event = EventModel(event_datetime=dt)
print(event.model_dump())
'''
{
    'event_datetime': {
        'start': datetime.datetime(
            2024, 1, 1, 15, 0, tzinfo=datetime.timezone.utc
        ),
        'end': datetime.datetime(
            2024, 1, 3, 14, 0, tzinfo=datetime.timezone.utc
        ),
    }
}
'''

print(event.model_dump_json())
'''
{"event_datetime":{"start":"2024-01-01T15:00:00Z","end":"2024-01-03T14:00:00Z"}}
'''

The serializer function to be wrapped.

Type: core_schema.WrapSerializerFunction

The return type for the function. If omitted it will be inferred from the type annotation.

Type: Any

Determines when this serializer should be used. Accepts a string with values 'always', 'unless-none', 'json', and 'json-unless-none'. Defaults to 'always'.

Type: WhenUsed

Annotation used to mark a type as having duck-typing serialization behavior.

See usage documentation for more details.

def field_serializer(
    field: str,
    /,
    *fields: str,
    mode: Literal['wrap'],
    return_type: Any = ...,
    when_used: WhenUsed = ...,
    check_fields: bool | None = ...,
) -> Callable[[_FieldWrapSerializerT], _FieldWrapSerializerT]
def field_serializer(
    field: str,
    /,
    *fields: str,
    mode: Literal['plain'] = ...,
    return_type: Any = ...,
    when_used: WhenUsed = ...,
    check_fields: bool | None = ...,
) -> Callable[[_FieldPlainSerializerT], _FieldPlainSerializerT]

Decorator that enables custom field serialization.

In the below example, a field of type set is used to mitigate duplication. A field_serializer is used to serialize the data as a sorted list.

from pydantic import BaseModel, field_serializer

class StudentModel(BaseModel):
    name: str = 'Jane'
    courses: set[str]

    @field_serializer('courses', when_used='json')
    def serialize_courses_in_order(self, courses: set[str]):
        return sorted(courses)

student = StudentModel(courses={'Math', 'Chemistry', 'English'})
print(student.model_dump_json())
#> {"name":"Jane","courses":["Chemistry","English","Math"]}

See the usage documentation for more information.

Four signatures are supported for the decorated serializer:

• (self, value: Any, info: FieldSerializationInfo)
• (self, value: Any, nxt: SerializerFunctionWrapHandler, info: FieldSerializationInfo)
• (value: Any, info: SerializationInfo)
• (value: Any, nxt: SerializerFunctionWrapHandler, info: SerializationInfo)

Callable[[_FieldWrapSerializerT], _FieldWrapSerializerT] | Callable[[_FieldPlainSerializerT], _FieldPlainSerializerT]

*fields : str Default: ()

The field names the serializer should apply to.

mode : Literal['plain', 'wrap'] Default: 'plain'

The serialization mode.

• plain means the function will be called instead of the default serialization logic,
• wrap means the function will be called with an argument to optionally call the default serialization logic.

return_type : Any Default: PydanticUndefined

Optional return type for the function, if omitted it will be inferred from the type annotation.

when_used : WhenUsed Default: 'always'

Determines the serializer will be used for serialization.

check_fields : bool | None Default: None

Whether to check that the fields actually exist on the model.

• PydanticUserError --
• If the decorator is used without any arguments (at least one field name must be provided).
• If the provided field names are not strings.

def model_serializer(f: _ModelPlainSerializerT, /) -> _ModelPlainSerializerT
def model_serializer(
    *,
    mode: Literal['wrap'],
    when_used: WhenUsed = 'always',
    return_type: Any = ...,
) -> Callable[[_ModelWrapSerializerT], _ModelWrapSerializerT]
def model_serializer(
    *,
    mode: Literal['plain'] = ...,
    when_used: WhenUsed = 'always',
    return_type: Any = ...,
) -> Callable[[_ModelPlainSerializerT], _ModelPlainSerializerT]

Decorator that enables custom model serialization.

This is useful when a model need to be serialized in a customized manner, allowing for flexibility beyond just specific fields.

An example would be to serialize temperature to the same temperature scale, such as degrees Celsius.

from typing import Literal

from pydantic import BaseModel, model_serializer

class TemperatureModel(BaseModel):
    unit: Literal['C', 'F']
    value: int

    @model_serializer()
    def serialize_model(self):
        if self.unit == 'F':
            return {'unit': 'C', 'value': int((self.value - 32) / 1.8)}
        return {'unit': self.unit, 'value': self.value}

temperature = TemperatureModel(unit='F', value=212)
print(temperature.model_dump())
#> {'unit': 'C', 'value': 100}

Two signatures are supported for mode='plain', which is the default:

• (self)
• (self, info: SerializationInfo)

And two other signatures for mode='wrap':

• (self, nxt: SerializerFunctionWrapHandler)

• (self, nxt: SerializerFunctionWrapHandler, info: SerializationInfo)

  See the usage documentation for more information.

_ModelPlainSerializerT | Callable[[_ModelWrapSerializerT], _ModelWrapSerializerT] | Callable[[_ModelPlainSerializerT], _ModelPlainSerializerT] -- The decorator function.

f : _ModelPlainSerializerT | _ModelWrapSerializerT | None Default: None

The function to be decorated.

mode : Literal['plain', 'wrap'] Default: 'plain'

The serialization mode.

• 'plain' means the function will be called instead of the default serialization logic
• 'wrap' means the function will be called with an argument to optionally call the default serialization logic.

when_used : WhenUsed Default: 'always'

Determines when this serializer should be used.

return_type : Any Default: PydanticUndefined

The return type for the function. If omitted it will be inferred from the type annotation.

A field serializer method or function in plain mode.

Type: TypeAlias Default: 'core_schema.SerializerFunction | _Partial'

A field serializer method or function in wrap mode.

Type: TypeAlias Default: 'core_schema.WrapSerializerFunction | _Partial'

A field serializer method or function.

Type: TypeAlias Default: 'FieldPlainSerializer | FieldWrapSerializer'

A model serializer method with the info argument, in plain mode.

Type: TypeAlias Default: Callable[[Any, SerializationInfo[Any]], Any]

A model serializer method without the info argument, in plain mode.

Type: TypeAlias Default: Callable[[Any], Any]

A model serializer method in plain mode.

Type: TypeAlias Default: 'ModelPlainSerializerWithInfo | ModelPlainSerializerWithoutInfo'

A model serializer method with the info argument, in wrap mode.

Type: TypeAlias Default: Callable[[Any, SerializerFunctionWrapHandler, SerializationInfo[Any]], Any]

A model serializer method without the info argument, in wrap mode.

Type: TypeAlias Default: Callable[[Any, SerializerFunctionWrapHandler], Any]

A model serializer method in wrap mode.

Type: TypeAlias Default: 'ModelWrapSerializerWithInfo | ModelWrapSerializerWithoutInfo'