Why does mypy error on returning list[str, int] + list[int]?

Question

From mypy's point of view l1 + l2 is OK. But returning l1 + l2 isn't OK. Why?

I'm using Python 3.11 and mypy 1.16.

def test() -> list[str | int]:
    l1: list[str | int]
    l2: list[int]
    l1 + l2  # OK
    return l1 + l2  # error: Unsupported operand types for + ("list[str | int]" and "list[int]")  [operator]

You can't do foo: list[str | int] = l1 + l2 either; it's not that you can't return it, so much that the result isn't assignable to list[str | int]. Possibly related to e.g. github.com/python/mypy/issues/3351. — jonrsharpe
– jonrsharpe, Commented Aug 1 at 16:00
the error might be because of the type checker’s strictness ont the value that you are returning — Soudagar Minhaz
– Soudagar Minhaz, Commented Aug 1 at 16:05

InSync · Accepted Answer · 2025-08-01 16:29:18Z

4

This is an old bug. It has been reported multiple times over the years, but never fixed. There is a draft PR that aims to resolve it.

A potentially interesting observation is that extracting l1 + l2 to an unannotated variable l causes the error to disappear, but not if l is annotated with list[str | int]:

_(playground)

l1: list[str | int] = []
l2: list[int] = []

l = l1 + l2
reveal_type(l)                # list[int | str]

return l                      # fine

l: list[str | int] = l1 + l2  # error
reveal_type(l)                # list[str | int]

return l                      # fine

As discussed in the aforementioned issue, the workaround is to use unpacking:

return l1 + l2     # error
return [*l1, *l2]  # fine

edited Aug 1 at 16:29

answered Aug 1 at 16:24

InSync

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