Type inference of Tuples returns object instead

[eddieschoute]

While interacting with zip I encountered a variant of the following error:

error: No overload variant of "zip" matches argument types [builtins.list[builtins.int*], builtins.object*]

which is emitted by mypy on the case of

out = list(
    list(zip([1, 2], ordered_item))
    for item in [(1, 2), (4, 5)]
    for ordered_item in [item, tuple(reversed(item))]
    )
print(out)

This code runs normally on python 3.6.5, outputting:

[[(1, 1), (2, 2)], [(1, 2), (2, 1)], [(1, 4), (2, 5)], [(1, 5), (2, 4)]]

Expected Behavior

Mypy should infer that ordered_item is a tuple, or at least an iterable so that zip can be applied on it.

It may also be interesting to consider the case where ordered_item is heterogenous over different Iterables (i.e. not only tuples). Would Iterable be inferred then?

Actual Behavior

Mypy infers the object type on ordered_item which you understandably cannot zip.

By explicitly casting item to tuple the error message is resolved:

out = list(
    list(zip([1, 2], ordered_item))
    for item in [(1, 2), (4, 5)]
    for ordered_item in [tuple(item), tuple(reversed(item))]
    )
print(out)

Environment

OS: macOS 10.13
Python: 3.6.5
mypy: 0.590

[ilevkivskyi]

Yes, this looks like a bug. Most likely it is because join of tuples is still pretty basic (in particular a case of joining fixed length tuple and variable length tuple is not covered).

[ilevkivskyi]

A simple repro:

x: Tuple[int, int]
y: Tuple[int, ...]
lst = [x, y]
reveal_type(lst[0])  # Revealed type is 'builtins.object*'

Clearly the type should be Tuple[int, ...] in this case.

[beamerblvd]

Is this fix available in a release yet?

[ethanhs]

@beamerblvd I'm not sure I understand. I don't think there is a fix for this yet

[beamerblvd]

@ethanhs, my mistake. I saw a red Closed and thought it was for this issue, but it was for another issue having reference this.

[ilevkivskyi]

This already appeared five times, so I am raising priority to high (especially since IIUC we are not going to infer unions for ternary expressions). This should be not hard to fix if someone wants to try.

[intgr]

Another minimal reproducer:

tup1: tuple = ()
tup2 = tup1 if input() else ()
reveal_type(tup2)   # N: Revealed type is 'builtins.object'

What is even happening here? 😄

[JelleZijlstra]

@intgr the underlying problem is that internally in mypy, the types for fixed-length tuples (like Tuple[int, int] and variadic tuples (Tuple[int, ...]) are completely separate, so when mypy tries to join these two types, it lands on object instead of something more reasonable.

[JukkaL]

@intgr Do you think that this issue can be closed now, after you've PRs were merged?

[intgr]

Yes, that should solve all the cases involving tuples. Also #8074 can be closed, as it falls back to Sequence[] type.

[ethanhs]

Closing as it was fixed in #8335. I'm very happy to see this issue fixed :)