Incompatible types for fixed-length list/tuple creation

[headtr1ck]

Bug Report

When creating lists or tuples of a fixed (initial) length using multiplication with a constant (i.e. 3*[1]) mypy evaluates the type as "Tuple[XXX, ...]", even though the length of the list/tuple is known.
This leads to incompatibilities when assigning values to existing variables that were constructed "classically" (i.e. [1, 1, 1]).

I don't know if this is intentional or actually a bug.
If the type will be changed to "Tuple[XXX, XXX, XXX]" this could lead to long expressions for long lists (i.e. 1e4*[1]).
Is it possible to remove the incompatibility or will this lead to other inconsistencies?

To Reproduce

Here is a minimal example on how to reproduce the problem:

var = (1, 2, 3)
var = 3 * (1,)

I know, that this is quite useless code, but a more often encountered code example with the same problem would be:

if cond:
  var = (1, 2, 3)
else:
  var = 3 * (1,)

Expected Behavior

Both expressions should evaluate to Tuple[int, int, int] OR be compatible.

Actual Behavior

This raises:
Incompatible types in assignment (expression has type "Tuple[int, ...]", variable has type "Tuple[int, int, int]")

Your Environment

• Mypy version used: 0.812
• Mypy command-line flags: /
• Mypy configuration options from mypy.ini (and other config files): /
• Python version used: 3.9.1
• Operating system and version: redhat enterprise linux 7

Comment

I have no idea what the operation 5*(1,) is called, so I could not find anything about this problem. Sorry, if this was asked/resolved before.

[Akuli]

This should be a typeshed issue. We can improve this by adding overloads for tuple.__mul__ in typeshed/stdlib/builtins.pyi.

[headtr1ck]

I think currently it is not possible to add this feature since fixed length tuples with integer generics are still a problem, see the open discussion #3345.
So an overload of the type

    def __mul__(self, n: SupportsIndex) -> Tuple[n *(_T_co, )]: ...

is not possible yet.
Any other suggestions?

[Akuli]

You're right. This isn't possible, not even with Literal, because apparently typeshed only defines arbitrary-length tuples.

[Akuli]

This now works correctly on master, and the resulting type is a tuple of 3 ints.

(env310) akuli@akuli-desktop:~/mypy$ cat a.py
var = (1, 2, 3)
var = 3 * (1,)
(env310) akuli@akuli-desktop:~/mypy$ python3 -m mypy a.py
Success: no issues found in 1 source file
(env310) akuli@akuli-desktop:~/mypy$ echo 'reveal_type(var)' >> a.py
(env310) akuli@akuli-desktop:~/mypy$ python3 -m mypy a.py
a.py:3: note: Revealed type is "Tuple[builtins.int, builtins.int, builtins.int]"

[ethanhs]

Confirmed as fixed in master.