# TODO: remove when dropping support for Pydantic < v2.12.3
def asdict(field_info: FieldInfo) -> dict[str, Any]:
    attributes = {}
    for attr in _Attrs:
        value = getattr(field_info, attr, Undefined)
        if value is not Undefined:
            attributes[attr] = value
    return {
        "annotation": field_info.annotation,
        "metadata": field_info.metadata,
        "attributes": attributes,
    }

from pydantic import BaseModel, ConfigDict
from pydantic.fields import FieldInfo

from .utils import needs_py310


def test_model_field_default_required():
    from fastapi._compat import v2

    # For coverage
    field_info = FieldInfo(annotation=str)
    field = v2.ModelField(name="foo", field_info=field_info)
    assert field.default is Undefined


def test_complex():
    app = FastAPI()

            field_info = params.Body(annotation=use_annotation, default=default_value)
        else:
            field_info = params.Query(annotation=use_annotation, default=default_value)

    field = None
    # It's a field_info, not a dependency
    if field_info is not None:
        # Handle field_info.in_
        if is_path_param:
            assert isinstance(field_info, params.Path), (

            default_convert_underscores = getattr(
                first_field.field_info, "convert_underscores", True
            )
    for param_type, param_group in parameter_groups:
        for param in param_group:
            field_info = param.field_info
            # field_info = cast(Param, field_info)
            if not getattr(field_info, "include_in_schema", True):
                continue

            f" Please update the response model {type_!r}."
        )
    class_validators = class_validators or {}

    field_info = field_info or FieldInfo(annotation=type_, default=default, alias=alias)
    kwargs = {"mode": mode, "name": name, "field_info": field_info}
    try:
        return v2.ModelField(**kwargs)  # type: ignore[return-value,arg-type]
    except PydanticSchemaGenerationError:

    assert dependant.call is not None, "dependant.call must be a function"
    is_coroutine = dependant.is_coroutine_callable
    is_body_form = body_field and isinstance(body_field.field_info, params.Form)
    if isinstance(response_class, DefaultPlaceholder):
        actual_response_class: type[Response] = response_class.value
    else:
        actual_response_class = response_class

En realidad, `Query`, `Path` y otros que verás a continuación crean objetos de subclases de una clase común `Param`, que es a su vez una subclase de la clase `FieldInfo` de Pydantic.

Y `Field` de Pydantic también regresa una instance de `FieldInfo`.

`Body` también devuelve objetos de una subclase de `FieldInfo` directamente. Y hay otros que verás más adelante que son subclases de la clase `Body`.

from pydantic import AliasChoices, AliasPath
from pydantic.fields import FieldInfo
from typing_extensions import Literal, deprecated

from ._compat import (
    Undefined,
)

_Unset: Any = Undefined


class ParamTypes(Enum):
    query = "query"
    header = "header"
    path = "path"
    cookie = "cookie"


class Param(FieldInfo):  # type: ignore[misc]
    in_: ParamTypes

    def __init__(