QueryDict __getitem__ wrong type

[StefanBrand]

Bug report

What's wrong

In our code base I get Type of "self.request.query_params["start_date"]" is "str | List[object]" after upgrading django-stubs from 1.8.0 to 1.12.0.

Reproducible example:

from django.http import QueryDict


query_params = QueryDict()
reveal_type(query_params["foo"])

Type of "query_params["foo"]" is "str | List[object]"

According to Django docs, QueryDict.__getitem__ always returns one value, never a list:

Returns the value for the given key. If the key has more than one value, it returns the last value.

How is that should be

Type of "query_params["foo"]" should be "str"

System information

• OS: python:3.9.1 Docker image
• python version: 3.9.1
• django version: 4.0.1
• mypy version: 0.931
• django-stubs version: 1.12.0
  - django-stubs-ext version:

[sobolevn]

СС @intgr

[intgr]

According to Django docs, QueryDict.__getitem__ always returns one value, never a list:

Returns the value for the given key. If the key has more than one value, it returns the last value.

You would think so, but actually:

In [6]: from django.http.request import QueryDict
In [7]: q = QueryDict(mutable=True)

In [8]: q.setlist('foo', ['value'])
In [9]: q['foo']
Out[9]: 'value'

In [10]: q.setlist('bar', [])
In [11]: q['bar']
Out[11]: []                # <-- !?!?

I would argue this is a bug in Django, but I don't think this has been reported upstream.

#909 added the hint that QueryDict.__getattr__ may return list, and #925 added the _ImmutableQueryDict hack to work around the common usages where lists shouldn't occur. See #925 (comment) for relevant discussion.

Type of "self.request.query_params["start_date"]" is "str | List[object]"

It looks like you're using django-rest-framework, query_params comes from djangorestframework-stubs.

This can be fixed by using _ImmutableQueryDict there. I have opened a pull request: typeddjango/djangorestframework-stubs#237

(But if this continues to cause problems, personally I'm not opposed to reverting the complexity created for _ImmutableQueryDict and going back to the un-pedantic but "wrong" type hint that __getattr__ returns str.)

[intgr]

Cc: @sterliakov