Mypy cannot infer type of [[], [1]]

[squiddy]

mypy 0.4.5, CPython 3.4.4

Running the code with mypy results in an error:

x = [[], [1]]
for b in x:
    reveal_type(b) # error: Revealed type is 'builtins.object*'
    b + [1]  # error: Unsupported left operand type for + ("object")

This might be me misunderstanding the type system, but I would have expected mypy to ask for type annotations if it can't infer the type, e.g.:

error: Need type annotation for variable

I'm also wondering why it infers an object, when it could (should?) be a list instead.

[gvanrossum]

Thanks for the report. It's not directly related to the for loop, it's just that in this case the item type of the outer list is inferred as the join of (the types of) [] and [1] and for obscure reasons that becomes object instead of list. E.g.

a = [[], [1]]
reveal_type(a)

shows that the type of a is builtins.list[builtins.object*].

You're not getting the error Need type annotation for variable because that is only used for "partial types" (that's a mypy-internal term), and partial types are only inferred for variables directly assigned from None or an empty container. Here we've got something more complicated and it tries to directly infer a type for the outer list, and failing to notice the similarity between an empty list and a list of integers.

I think we can fix this though I'm not sure how important it is or how soon it'll happen.

[rwbarton]

The workaround is to add an explicit type signature to x:

x = [[], [1]] # type: List[List[int]]

[gvanrossum]

Since this can be worked around by adding annotations I don't consider this very urgent, and it's hard to fix (see the closed PR :-).

[Alberth289346]

Looks like the same issue as #5045

[ilevkivskyi]

Duplicate of #230