False positive for `defaultdict(defaultdict)`

[erictraut]

Discussed in https://github.com/microsoft/pyright/discussions/8839

^{Originally posted by **matt-the-midnight-hacker** August 26, 2024} Hi I just updated pyright to 1.1.377 and I get now type errors for `defaultdict(defaultdict)` assignments. I'm not sure if it is a bug or a typeshed issue or just me. The code to reproduce what I mean would be: ``` from typing import DefaultDict from collections import defaultdict class MyClass: pass d: DefaultDict[int, DefaultDict[int, MyClass]] = defaultdict(defaultdict) d[1][2] = MyClass() ``` I get error: `error: Expression of type "defaultdict[Unknown, defaultdict[Unknown, defaultdict[Unknown, Unknown]]]" is incompatible with declared type "DefaultDict[int, DefaultDict[int, MyClass]]` How to do the `defaultdict` instantiation here to fulfill the type I've declared? fyi. I've also asked this on [stackoverflow](https://stackoverflow.com/questions/78917316/how-to-instantiate-defaultdict-to-fulfill-defaultdictint-defaultdictint-mycl)

[tofetpuzo]

A work around would be something like this

from typing import DefaultDict
from collections import defaultdict

class MyClass:
    pass

d: DefaultDict[int, DefaultDict[int, MyClass]] = defaultdict(lambda: defaultdict(MyClass))

d[1][2] = MyClass()

Test for it could be

d: DefaultDict[int, DefaultDict[int, MyClass]] = defaultdict(lambda: defaultdict(MyClass))

def func(a: DefaultDict[int, DefaultDict[int, MyClass]]):
    reveal_type(a)  # pyright should now reveal Runtime type is 'defaultdict'

func(d)

[erictraut]

This is addressed in pyright 1.1.380.