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---

# Functional Serializers

This module contains related classes and functions for serialization.

## PlainSerializer

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.

```python
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'}
```

### Attributes

#### func

The serializer function.

**Type:** `core_schema.SerializerFunction`

#### return\_type

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

**Type:** [`Any`](https://docs.python.org/3/library/typing.html#typing.Any)

#### when\_used

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`

## WrapSerializer

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.

```python
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"}}
'''
```

### Attributes

#### func

The serializer function to be wrapped.

**Type:** `core_schema.WrapSerializerFunction`

#### return\_type

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

**Type:** [`Any`](https://docs.python.org/3/library/typing.html#typing.Any)

#### when\_used

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`

## SerializeAsAny

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

See [usage documentation](/docs/validation/latest/concepts/serialization#serializing-with-duck-typing) for more details.

## field\_serializer

```python
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.

```python
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](/docs/validation/latest/concepts/serialization#serializers) 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)`

### Returns

[`Callable`](https://docs.python.org/3/library/typing.html#typing.Callable)\[\[`_FieldWrapSerializerT`\], `_FieldWrapSerializerT`\] | [`Callable`](https://docs.python.org/3/library/typing.html#typing.Callable)\[\[`_FieldPlainSerializerT`\], `_FieldPlainSerializerT`\]

### Parameters

**`*fields`** : [`str`](https://docs.python.org/3/library/stdtypes.html#str) _Default:_ `()`

The field names the serializer should apply to.

**`mode`** : [`Literal`](https://docs.python.org/3/library/typing.html#typing.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`](https://docs.python.org/3/library/typing.html#typing.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`](https://docs.python.org/3/library/functions.html#bool) | [`None`](https://docs.python.org/3/library/constants.html#None) _Default:_ `None`

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

### Raises

-   `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.

## model\_serializer

```python
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.

```python
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](/docs/validation/latest/concepts/serialization#serializers) for more information.
    

### Returns

`_ModelPlainSerializerT` | [`Callable`](https://docs.python.org/3/library/typing.html#typing.Callable)\[\[`_ModelWrapSerializerT`\], `_ModelWrapSerializerT`\] | [`Callable`](https://docs.python.org/3/library/typing.html#typing.Callable)\[\[`_ModelPlainSerializerT`\], `_ModelPlainSerializerT`\] -- The decorator function.

### Parameters

**`f`** : `_ModelPlainSerializerT` | `_ModelWrapSerializerT` | [`None`](https://docs.python.org/3/library/constants.html#None) _Default:_ `None`

The function to be decorated.

**`mode`** : [`Literal`](https://docs.python.org/3/library/typing.html#typing.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`](https://docs.python.org/3/library/typing.html#typing.Any) _Default:_ `PydanticUndefined`

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

## FieldPlainSerializer

A field serializer method or function in `plain` mode.

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `'core_schema.SerializerFunction | _Partial'`

## FieldWrapSerializer

A field serializer method or function in `wrap` mode.

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `'core_schema.WrapSerializerFunction | _Partial'`

## FieldSerializer

A field serializer method or function.

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `'FieldPlainSerializer | FieldWrapSerializer'`

## ModelPlainSerializerWithInfo

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

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `Callable[[Any, SerializationInfo[Any]], Any]`

## ModelPlainSerializerWithoutInfo

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

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `Callable[[Any], Any]`

## ModelPlainSerializer

A model serializer method in `plain` mode.

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `'ModelPlainSerializerWithInfo | ModelPlainSerializerWithoutInfo'`

## ModelWrapSerializerWithInfo

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

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `Callable[[Any, SerializerFunctionWrapHandler, SerializationInfo[Any]], Any]`

## ModelWrapSerializerWithoutInfo

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

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `Callable[[Any, SerializerFunctionWrapHandler], Any]`

## ModelWrapSerializer

A model serializer method in `wrap` mode.

**Type:** [`TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) **Default:** `'ModelWrapSerializerWithInfo | ModelWrapSerializerWithoutInfo'`