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

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.

Answer 2

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

Answer 3

You actually can do what you want if you import the annotations module:

from __future__ import annotations
from django.db import models
from django.db.models.query import QuerySet

class MyModel(models.Model):
    pass

def my_function() -> QuerySet[MyModel]:
    return MyModel.objects.all()

Neither MyPy nor the Python interpreter will complain or raise exceptions on this (tested on python 3.7). MyPy will probably be unable to type-check it, but if all you want is to document your return type, this should be good enough.