Hello everyone!

Working a lot with Pydantic models I got the idea that it would be nice to be able to conditionally exclude fields during serialization.
The need to exclude a field may depend on the value of the field or, for example, on the serialization context. In this case, the exclusion rules could be controlled from within the serializer handler.

Here's a simple toy example of what I mean:

class Model(BaseModel):
    foo: str | None = None

    @field_serializer('foo')
    def _serialize_foo(cls, v):
        if v == 'spam':
            return NotSerialized  # a special unique constant to mark the field as excluded
        return v

Also when using the serialization context:

class Model(BaseModel):
    foo: int | None = None

    @field_serializer('foo')
    def _serialize_foo(cls, v, info):
        if info.context and info.context.get('foo_rule') == 'spam':
            return NotSerialized  # a special unique constant to mark the field as excluded
        return v

This ability to control field exclusion rules directly from the serializer would be a useful more flexible addition to the existing exclusion mechanisms in Pydantic: Field(exclude=True), model_dump(exclude={}).

Perhaps it violates the architectural principles of the project or it already exists in some form and I just don't know about it.

What do you thing?
Thanks.