How to use Python type hints with Django QuerySet?

One solution may be using Union typing class.

from typing import Union, List
from django.db.models import QuerySet
from my_app.models import MyModel

def somefunc(row: Union[QuerySet, List[MyModel]]):
    pass

Now when you slice the row argument it will know that the returned type is either another list of MyModel or an instance of MyModel, whilst also hinting that the methods of the QuerySet class are available on the row argument too.

There's a special package called django-stubs (the name follows PEP561) to type your django code.

That's how it works:

# server/apps/main/views.py
from django.http import HttpRequest, HttpResponse
from django.shortcuts import render

def index(request: HttpRequest) -> HttpResponse:
    reveal_type(request.is_ajax)
    reveal_type(request.user)
    return render(request, 'main/index.html')

Output:

» PYTHONPATH="$PYTHONPATH:$PWD" mypy server
server/apps/main/views.py:14: note: Revealed type is 'def () -> builtins.bool'
server/apps/main/views.py:15: note: Revealed type is 'django.contrib.auth.models.User'

And with models and QuerySets:

# server/apps/main/logic/repo.py
from django.db.models.query import QuerySet

from server.apps.main.models import BlogPost

def published_posts() -> 'QuerySet[BlogPost]':  # works fine!
    return BlogPost.objects.filter(
        is_published=True,
    )

Output:

reveal_type(published_posts().first())
# => Union[server.apps.main.models.BlogPost*, None]

Complete tutorial: https://sobolevn.me/2019/08/typechecking-django-and-drf
Types for django: https://github.com/typeddjango/django-stubs
Types for drf: https://github.com/typeddjango/djangorestframework-stubs

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]).

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.

How to use Python type hints with Django QuerySet?

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