Add examples for functional_serializers in API docs (#8546)

Co-authored-by: Sydney Runkle <54324534+sydney-runkle@users.noreply.github.com>

pydantic/functional_serializers.py

@@ -18,6 +18,28 @@
 class 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 List
+
+    from typing_extensions 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.
         return_type: The return type for the function. If omitted it will be inferred from the type annotation.
@@ -61,6 +83,59 @@ class 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 Any, Dict
+
+    from typing_extensions import Annotated
+
+    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 `helper` 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.
         return_type: The return type for the function. If omitted it will be inferred from the type annotation.
@@ -151,6 +226,26 @@ def field_serializer(
 ) -> Callable[[Any], Any]:
     """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 typing import Set
+
+    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(courses: Set[str]):
+            return sorted(courses)
+
+    student = StudentModel(courses={'Math', 'Chemistry', 'English'})
+    print(student.model_dump_json())
+    #> {"name":"Jane","courses":["Chemistry","English","Math"]}
+    ```
+
     See [Custom serializers](../concepts/serialization.md#custom-serializers) for more information.
 
     Four signatures are supported:
@@ -217,6 +312,30 @@ def model_serializer(
 ) -> Callable[[Any], Any]:
     """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}
+    ```
+
     See [Custom serializers](../concepts/serialization.md#custom-serializers) for more information.
 
     Args: