Configure behavior for extra fields at runtime / validation time

[lesinigo]

Initial Checks

• I have searched Google & GitHub for similar requests and couldn't find anything
• I have read and followed the docs and still think this feature is missing

Description

I would like to change the extra configuration of a model dynamically at runtime / validation time.

Example of hypothetical implementation:

SomeModel(BaseModel):
    my_field: int
    model_config = ConfigDict(extra="allow")

data = SomeModel.model_validate(
    {"myfield": 1, "extra field": 2},
    extra="forbid"  # override model config and forbid extra fields just this time
)

Affected Components

• Compatibility between releases
• Data validation/parsing
• Data serialization - .model_dump() and .model_dump_json()
• JSON Schema
• Dataclasses
• Model Config
• Field Types - adding or changing a particular data type
• Function validation decorator
• Generic Models
• Other Model behaviour - model_construct(), pickling, private attributes, ORM mode
• Plugins and integration with other tools - mypy, FastAPI, python-devtools, Hypothesis, VS Code, PyCharm, etc.

[sydney-runkle]

I wonder if this falls in the V3 bucket, I think we might want to reconsider the relationship between config settings and runtime options...