Type Check Django App - Euro Python 2021

who am I?

Types at runtime

from django.contrib.auth.models import User


type(User.objects.filter(
	email='foo@example.com'))

# output
django.db.models.query.QuerySet
type(("127.0.0.1", 8000))

# output
tuple

Static Checker Type

# filename.py
addr = "127.0.0.1"
port = 8000

reveal_type((addr, port))
$mypy filename.py
note: Revealed type is 
  'Tuple[builtins.str, builtins.int]'
# filename.py
from django.contrib.auth.models import User

reveal_type(User.objects.filter(
  email='foo@example.com'))
$ mypy filename.py
note: Revealed type is 
  'django.contrib.auth.models.UserManager
  [django.contrib.auth.models.User]'
  

mypy config

$cat mypy.ini
...
plugins =
  mypy_django_plugin.main,
  
[mypy.plugins.django-stubs]
django_settings_module = "yourapp.settings"

Annotation Syntax

Variable & Function

from datetime import date
lang: str = "Python"
year: date = date(1989, 2, 1)
def sum(a: int, b: int) -> int:
  return a + b

class Person:
  def __init__(self, name: str, age: int, 
               is_alive: bool):
    self.name = name
    self.age = age
    self.is_alive = is_alive

Annotating Django Code

View

from django.http import (HttpRequest, HttpResponse, 
			HttpResponseNotFound)

def index(request: HttpRequest) -> HttpResponse:
    return HttpResponse("hello world!")

def view_404(request: HttpRequest) -> HttpResponse:
    return HttpResponseNotFound(
      'Page not found')

def view_404(request: 
             HttpRequest) -> HttpResponseNotFound:
    return HttpResponseNotFound(
      'Page not found')

# bad - not precise and not useful
def view_404(request: HttpRequest) -> object:
	return HttpResponseNotFound(
      'Page not found')

Method Resolution Order

HttpResponse.mro()
[django.http.response.HttpResponse,
 django.http.response.HttpResponseBase,
 object]


HttpResponseNotFound.mro()
[django.http.response.HttpResponseNotFound,
 django.http.response.HttpResponse,
 django.http.response.HttpResponseBase,
 object]

LSP

The LSP states 
that in an object-oriented program, 
substituting a superclass 
object reference with an object 
of any of its subclasses, 
the program should not break.

Django Models

Create

from django.db import models
from django.utils import timezone


class Question(models.Model):
    question_text = models.CharField(max_length=200)
    pub_date = models.DateTimeField("date published")


def create_question(question_text: str) -> Question:
    qs = Question(question_text=question_text, 
                  pub_date=timezone.now())
    qs.save()
    return qs

Read


def get_question(question_text: str) -> Question:
    return Question.objects.filter(
      question_text=question_text).first()
error: Incompatible return value type 
(got "Optional[Any]", expected "Question")
from typing import Optional

def get_question(question_text: str) -> 
	Optional[Question]:
    return Question.objects.filter(
      question_text=question_text).first()

mypy config

# mypy.ini
strict_optional = False

def get_question(question_text: str) -> 
	Question:
    return Question.objects.filter(
      	question_text=question_text).first()

Filter

Example

In [8]: Question.objects.all()
Out[8]: <QuerySet [<Question: Question object (1)>, 
                   <Question: Question object (2)>]>

In [9]: Question.objects.filter()
Out[9]: <QuerySet [<Question: Question object (1)>, 
                   <Question: Question object (2)>]>
Box with cup cake

def filter_question(text: str) -> 
	QuerySet[Question]:
    return Question.objects.filter(
      text__startswith=text)
  
def exclude_question(text: str) -> 
	QuerySet[Question]:
    return Question.objects.exclude(
      text__startswith=text)

all, reverse, none, complex_filter, 
union, order_by, distinct, 
defer, only, using, extra, 
select_related, select_for_update, 
prefetch_related

Methods returning QuerySet

Aggregate


class Publisher(models.Model):
    name = models.CharField(max_length=300)
   
class Book(models.Model):
    name = models.CharField(max_length=300)
    pages = models.IntegerField()
    # use integer field in production
    price = models.DecimalField(max_digits=10, 
                                decimal_places=2)
    rating = models.FloatField()
    publisher = models.ForeignKey(Publisher)
    pubdate = models.DateField()

Example

def get_avg_price():
    return Book.objects.all().aggregate(
      avg_price=Avg("price"))

print(get_avg_price())
{'avg_price': Decimal('276.666666666667')}

Annotated Code

from decimal import Decimal


def get_avg_price() -> dict[str, Decimal]:
    return Book.objects.all().aggregate(
      avg_price=Avg("price"))

Annotate Method

Count books by pubslisher

def count_by_publisher():
    return Publisher.objects.annotate(
      num_books=Count("book").values(
        'name', 'num_books'))

Result

In [17]: [i for i in count_by_publisher()]
Out[17]: [{'name': 'Penguin', 'num_books': 2}, 
          {'name': 'vintage', 'num_books': 1}]
  

Returning QuerySet

def count_by_publisher():
    return Publisher.objects.annotate(
      num_books=Count("book"))


def print_pub(num_books=0):
    if num_books > 0:
        res = count_by_publisher().filter(
          num_books__gt=num_books)
    else:
        res = count_by_publisher()
    for item in res:
        print(item.name, item.num_books)

Example Output

In [42]: print_pub()
Penguin 2
vintage 1

Annotation

from typing import TypedDict
from collections.abc import Iterable

class PublishedBookCount(TypedDict):
    name: str
    num_books: int

def count_by_publisher() -> 
	Iterable[PublishedBookCount]:
  ...

Error

# mypy output
scratch.py:46: error: Incompatible return value 
    type (got "QuerySet[Any]", expected
"Iterable[PublishedBookCount]")
        return Publisher.objects.annotate(
          num_books=Count("book"))
               ^
scratch.py:51: error: 
      "Iterable[PublishedBookCount]" has no attribute "filter"
       res = count_by_publisher().filter(
         num_books__gt=num_books)

QuerySet

def count_by_publisher() -> QuerySet[Publisher]:
   ...
  
def print_pub(num_books: int=0) -> None:
    ...
    for item in res:
        print(item.name, item.num_books)
# mypy output

scratch.py:55: error: "Publisher" has 
      no attribute "num_books"
      print(item.name, item.num_books)

Suggestion

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    class TypedPublisher(Publisher):
        num_books = models.IntegerField()

        class meta:
            abstract = True


def count_by_publisher() -> QuerySet[TypedPublisher]:
    return Publisher.objects.annotate(
      num_books=Count("book"))


def print_pub(num_books: int=0) -> None:
    if num_books > 0:
        res = count_by_publisher().filter(
          num_books__gt=num_books)
    else:
        res = count_by_publisher()
    for item in res:
        print(item.name, item.num_books)

Tools

pyannotate

Pyannotate

  • Automatically insert type-hints into the code

 

  • pytest-annotate infer type from test cases

 

Source Code

from django.http import (HttpResponse, 
		HttpResponseNotFound)

# Create your views here.
# annotate the return value
def index(request):
    return HttpResponse("hello world!")

def view_404_0(request):
    return HttpResponseNotFound(
      'Page not found')

Test Code

from polls.views import *
from django.test import RequestFactory

def test_index():
    request_factory = RequestFactory()
    request = request_factory.post('/index')
    index(request)


def test_view_404_0():
    request_factory = RequestFactory()
    request = request_factory.post('/404')
    view_404_0(request)

Run test command

$DJANGO_SETTINGS_MODULE="mysite.settings" 
PYTHONPATH='.' poetry run pytest 
-sv polls/tests.py 
--annotate-output=./annotations.json
$cat annotations.json
[...
    {
        "path": "polls/views.py",
        "line": 7,
        "func_name": "index",
        "type_comments": [
            "(django.core.handlers.wsgi.WSGIRequest) -> 
          django.http.response.HttpResponse"
        ],
        "samples": 1
    },
    {
        "path": "polls/views.py",
        "line": 10,
        "func_name": "view_404_0",
        "type_comments": [
            "(django.core.handlers.wsgi.WSGIRequest) -> 
          django.http.response.HttpResponseNotFound"
        ],
        "samples": 1
    }
]

Apply the changes

$poetry run pyannotate --type-info 
./annotations.json 
-w polls/views.py --py3
from django.http import HttpResponse, HttpResponseNotFound
from django.core.handlers.wsgi import WSGIRequest
from django.http.response import HttpResponse
from django.http.response import HttpResponseNotFound

def index(request: WSGIRequest) -> HttpResponse:
    return HttpResponse("hello world!")

def view_404_0(request: WSGIRequest) -> 
	HttpResponseNotFound:
    return HttpResponseNotFound(
      '<h1>Page not found</h1>')

Auto-modified file

Learning Resource

Python Typing Koans

Thank you!

Twitter: @kracetheking

Github: kracekumar

Type Check Django App - Euro PyCon 2021

By Kracekumar

Type Check Django App - Euro PyCon 2021

How to annotate the optional python static type hints in the Django project.

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