fastapi+pydantic query parameter checking for complex arguments - fastapi

Question

Based on this API definition, my api supports queries like:
GET http://my.api.url/posts?sort=["title","ASC"]&range=[0, 24]&filter={"q":"bar"}
where some of the checks needed are
sort[1] is either "asc" or "desc" (case should not matter)
filter has the key "q". filter can have other keys.
range is a list of two integers. range[0] is less then or equal to range[1]
In fastapi path definitions I currently define filter, sort, and range as strings as in the code below, convert them using json.loads, and do checks.
#r.get(
"/users",
response_model=List[User],
response_model_exclude_none=True,
)
async def list_users(
filter: Optional[str] = None,
sort: Optional[str] = None,
range: Optional[str] = None,
...
):
...
How can I use pydantic definitions for checks and API definition instead of just using str, such that checks are done by pydantic, and openapi schema definitions are more descriptive?

Answer 1

There are some ways to do what you want. But FastAPI only allow sequence structures (list, tuples, set, sequence) for Query and Header params, so that &filter={"q":"bar"} it won't work on FastAPI at least for 0.75.0 version where I am working.
If you wish to support a Dictionary you should use POST method with Body params. And then you can wrap your values around a Pydantic model to support validation.
Below there are two ways to implement this in FastAPI.
Solution 1:
class FilterModel(BaseModel):
filter: dict
#router.post(
"/posts"
)
async def list_users(
filters: FilterModel,
sort: Tuple[str, Literal["DESC", "ASC", "desc", "asc"]] = Query(("title", "ASC")),
ranges: Tuple[int, int] = Query((0, 24))
) -> None:
print(filters)
print(sort)
print(ranges)
model = MyModel(sort=sort, range=ranges)
print(model)
This will allow you to call api like:
POST http://my.api.url/posts?sort=title&sort=ASC&ranges=0&ranges=24
With body:
{
"filter"={"q":"bar"}
}
Solution 2:
class MyModel(BaseModel):
sortBy: Optional[str]
sortOrder: Optional[Literal["DESC", "ASC", "desc", "asc"]]
min_range: Optional[int]
max_range: Optional[int]
class FilterModel(BaseModel):
filter: dict
#router.post(
"/posts"
)
async def list_users(
filters: FilterModel,
model: MyModel = Depends()
) -> None:
print(filters)
print(model)
This will allow you to call api like:
POST http://my.api.url/posts?sortBy="title"&sortOrder="ASC"&min_range=0&max_range=24
With body:
{
"filter"={"q":"bar"}
}
If you wish to use GET method and the filter you have to do some hacky things.