Django gotchas

[hmpf]

For lack of a better place to put it, let's put unexpected/weird behavior of vanilla Django here.

[hmpf]

request.POST and request.GET are not quite dicts

They are QueryDicts, a dict subclass with some weirdities. Do not assume that you can give a normal dict to something that wants
a QueryDict and vice versa. Make sure to type annotate when something must have a QueryDict! Do not assume that something that is annotated as Dict can take a QueryDict!

Weirdities:

1. Immutability. You cannot add anything to a QueryDict.
2. Every value is a list. If you use direct access (that is: dict[key]) you will quietly get the first member of the list.
3. To access the entire list of values there are extra methods. See https://docs.djangoproject.com/en/5.1/ref/request-response/#querydict-objects
4. You can make a QueryDict mutable with the .copy()-method, which is not quite the same thing as copy.deepcopy().

You're supposed to feed the QueryDict to a form. If the form is valid (form.is_valid()), then form.cleaned_data will be a normal dict. However, primitive types (like a model id) will have been replaced with their cleaned equivalents (like a model instance).

I assure you, they used to be even weirder, so take care with ancient Django code.

[hmpf]

Attributes (not only the fields!) on model and form classes are magic

They do not behave the same at all on a class as compared to on an instance.

This works:

class MyModel(models.Model):
    ..

    objects = MyManager()

This does not:

class MyModel(models.Model):
    ..
MyModel.objects = MyManager()

Don't ask me how I know.

[lunkwill42]

Models use metaclasses, so all kinds of magic may ensue at class definition time that would not work after the class is already defined.