assigning distinct Field's to each element in pydantic Union

Question

How do I assign a different pydantic.Field() to each type within a union? In the example below, the Field(pattern=...) should only apply to the str.

class Foo(BaseModel):
    bar: str | FooDetail = Field(pattern=r'[bar]+') # but this should only apply to str

class FooDetail(BaseModel):
    baz: int
    fob: float

Answer 1

You can use Annotated to specify both the type and the Field, and then still create a union from that with FooDetail:

from typing_extensions import Annotated

class Foo(BaseModel):
    bar: Annotated[str, Field(pattern=r'[bar]+')] | FooDetail

class FooDetail(BaseModel):
    baz: int
    fob: float