How to use Python type hints with Django QuerySet?

Question

Is it possible to specify type of records in Django QuerySet with Python type hints? Something like QuerySet[SomeModel]?

For example, we have model:

Answer 1

I made this helper class to get a generic type hint:

from django.db.models import QuerySet
from typing import Iterator, Union, TypeVar, Generic

T = TypeVar("T")

class ModelType(Generic[T]):
    def __iter__(self) -> Iterator[Union[T, QuerySet]]:
        pass

Then use it like this:

def somefunc(row: ModelType[SomeModel]):
    pass

This reduces the noise everytime I use this type and it make it usable between models (like ModelType[DifferentModel]).

Answer 2

I've found myself that using typing.Sequence to solve a similar issue:

from typing import Sequence


def print_emails(users: Sequence[User]):
    for user in users:
        print(user.email)


users = User.objects.all()


print_emails(users=users)

As far as I know from docs:

A Sequence is anything that supports len() and .getitem(), independent of its actual type.

Answer 3

This is an improved helper class of Or Duan.

from django.db.models import QuerySet
from typing import Iterator, TypeVar, Generic

_Z = TypeVar("_Z")  

class QueryType(Generic[_Z], QuerySet):
    def __iter__(self) -> Iterator[_Z]: ...

This class is used specifically for QuerySet object such as when you use filter in a query.
Sample:

from some_file import QueryType

sample_query: QueryType[SampleClass] = SampleClass.objects.filter(name=name)

Now the interpreter recognizes the sample_query as a QuerySet object and you will get suggestions such as count() and while looping through the objects, you will get suggestions for the SampleClass

Note
This format of type hinting is available from python3.6 onwards.

You can also use django_hint which has hinting classes specifically for Django.

Answer 4

IMHO, the proper way to do it is to define a type that inherits QuerySet and specify a generic return type for the iterator.

    from django.db.models import QuerySet
    from typing import Iterator, TypeVar, Generic, Optional

    T = TypeVar("T")


    class QuerySetType(Generic[T], QuerySet):  # QuerySet + Iterator

        def __iter__(self) -> Iterator[T]:
            pass

        def first(self) -> Optional[T]:
            pass

        # ... add more refinements

Then you can use it like this:

users: QuerySetType[User] = User.objects.all()
for user in users:
   print(user.email)  # typing OK!
user = users.first()  # typing OK!

Answer 5

from typing import Iterable

def func(queryset_or_list: Iterable[MyModel]): 
    pass

Both of queryset and list of model instance is iterable object.

Answer 6

from typing import (TypeVar, Generic, Iterable, Optional)
from django.db.models import Model
from django.db.models import QuerySet
_T = TypeVar("_T", bound=Model)


class QuerySetType(Generic[_T], QuerySet):
    def __iter__(self) -> Iterable[_T]:
        pass

    def first(self) -> Optional[_T]:
        pass