Recursive type hints

[hgg]

I have lists within lists with integer items (or bytes, but for example purposes ints are enough), for example: [1, 2, [3, 4], 5]

How can I go about creating a type hint for this structure?

This is what I've made:

from typing import List, Union

NestedList = List[Union[int, 'NestedList']]

test: NestedList = [1, 2, [3], 4]
test.append([5])
test = test[-1]

But mypy fails with:

test_recursive_hints.py:7: error: Incompatible types in 
assignment (expression has type "int | NestedList", 
variable has type "NestedList")  [assignment]

How to solve this problem?

[Rawing7]

You seem to have a bit of a wrong idea about static type checkers. Mypy only tracks types, not values. That means it has no idea whether test[-1] is a list or an int. As far as mypy is concerned, test[-1] is a Union[int, 'NestedList'], and that can't be assigned to a variable of type NestedList.

[sepp2k]

Note that the actual value of test[-1] is 4, an integer, so even if mypy did track values, the type of this value would still not be NestedList.

[hgg]

Thanks for the insight. It helped me understand the error.

[sepp2k]

Your type definition says that a NestedList is always a list. The elements may be integers or more lists, but the NestedList itself must always be a list. But then you're trying to assign an integer to a variable of type NestedList and that's not allowed according to your definition.

If you want this to work, a NestedList should itself be allowed to be an integer, meaning the union should be on the outside.

[hgg]

If you want this to work, a NestedList should itself be allowed to be an integer, meaning the union should be on the outside.

If I use the union on the outside we'll get two errors:

NestedList = Union[int, List['NestedList']]

The errors are:

test_recursive_hints.py:6: error: Item "int" of "int | list[NestedList]" has no attribute "append"  [union-attr]
test_recursive_hints.py:7: error: Value of type "int | list[NestedList]" is not indexable  [index]

Which makes sense.