在FastAPI中如何使用pydantic的BaseModel验证Form请求

在使用FastAPI的时候，如果我们想使用Form表单提交数据这个时候使用示例如下：

from fastapi import FastAPI, Form

app = FastAPI()


@app.post("/login/")
async def login(username: str = Form(...), password: str = Form(...)):
    return {"username": username}

但是这种方式当参数过多，或者更高级的验证的时候就会显得复杂，能不能使用pydantic的BaseModel验证Form请求呢？

多方查找终于在 stackoverflow 找到了这个问答：fastapi form data with pydantic model

这个文档再FastAPI的官方github上引出了一个issue：multipart/form-data: Unable to parse complex types in a request form

从issue中我选择了一个最优的解决方案：

def as_form(cls: Type[BaseModel]) -> Type[BaseModel]:
    """
    Adds an `as_form` class method to decorated models. The `as_form` class
    method can be used with `FastAPI` endpoints.

    Args:
        cls: The model class to decorate.

    Returns:
        The decorated class.

    """

    def make_form_parameter(field: ModelField) -> Any:
        """
        Converts a field from a `Pydantic` model to the appropriate `FastAPI`
        parameter type.

        Args:
            field: The field to convert.

        Returns:
            Either the result of `Form`, if the field is not a sub-model, or
            the result of `Depends` if it is.

        """
        # 支持字段的类型还是 BaseModel
        if issubclass(field.type_, BaseModel):
            # This is a sub-model.
            assert hasattr(field.type_, "as_form"), (
                f"Sub-model class for {field.name} field must be decorated with"
                f" `as_form` too."
            )
            return Depends(field.type_.as_form)
        else:
            return Form(field.default) if not field.required else Form(...)

    new_params = [
        inspect.Parameter(
            field.alias,
            inspect.Parameter.POSITIONAL_ONLY,
            default=make_form_parameter(field),
        )
        for field in cls.__fields__.values()
    ]

    async def _as_form(**data):
        return cls(**data)

    sig = inspect.signature(_as_form)
    sig = sig.replace(parameters=new_params)
    _as_form.__signature__ = sig
    setattr(cls, "as_form", _as_form)
    return cls

上面这个装饰器还支持参数类型递归

使用示例：

@as_form
class Nested(BaseModel):
    foo: int
    bar: str

@as_form
class Outer(BaseModel):
    inner: Inner
    baz: float

@app.post("/test")
async def test_form(form: Outer = Depends(Outer.as_form)):
    return {"foo": form.inner.foo, "bar": form.inner.bar, "baz": form.baz}

这样我们提交的Form表单也能使用pydantic的BaseModel验证了。