Adding type hints to the Django ORM

Last updated 283 days ago by Will McGugan

django

It occurred to me that Django's ORM could do with a bit of a revamp to make use of recent developments in the Python language.

The main area where I think Django's models are missing out is the lack of type hinting (hardly surprising since Django pre-dates type hints). Adding type hints allows Mypy to detect bugs before you even run your code. It may only save you minutes each time, but multiply that by the number of code + run iterations you do each day, and it can save hours of development time. Multiply that by the lifetime of your project, and it could save weeks or months. A clear win.

Typing Django Models

I'd love to be able to use type hints with the Django ORM, but it seems that the magic required to create Django models is just too dynamic and would defy any attempts to use typing. Fortunately that may not necessarily be the case. Type hints can be inspected at runtime, and we could use this information when building the model, while still allowing Mypy to analyze our code. Take the following trivial Django model:

python class Foo(models.Model): count = models.IntegerField(default=0)

The same information could be encoded in type hints as follows:

python class Foo(TypedModel): count: int = 0

The TypedModel class could inspect the type hints and create the integer field in the same way as models.Model uses IntegerField and friends. But this would also tell Mypy that instances of Foo have an integer attribute called count.

But what of nullable fields. How can we express those in type hints? The following would cover it:

python class Foo(TypedModel): count: Optional[int] = 0

The Optional type hint tells Mypy that the attribute could be None, which could also be used to instruct TypedModel to create a nullable field.

So type hints contain enough information to set the type of the field, the default value, and wether the field is nullable--but there are other pieces of information associated with fields in Django models; a CharField has a max_length attribute for instance:

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