Can't assign to ()

[tyehle]

Mypy incorrectly complains about assignments to (). The simplest example of this is

() = []

This code executes without a problem, but mypy says error: can't assign to ()

A more real world use case might be code like this

(a, b), () = [[1, 2], []]

In this case the assignment to () is a concise way of asserting that the second list is empty.

• What is the actual behavior/output?
  error: can't assign to ()
• What is the behavior/output you expect?
  No type checking errors
• What are the versions of mypy and Python you are using?
  A build from the latest commit as of writing this, 614090b
• What are the mypy flags you are using?
  No flags

If this is a change you decide would be good then I believe it could be implemented by removing the the check on line 1930 of mypy/semanal.py and updating the tests.

[JelleZijlstra]

Thanks for the report! This seems like a real bug, but low-priority because little real-world code would do it. If somebody submits a PR to fix it, we'll take a look.

[PeterJCLaw]

Note that assignment to an empty tuple isn't valid syntax Python in 3.5, so mypy isn't totally wrong. It does seem to be valid syntax in 3.6 though.

Not sure if I should report this separately or just add it here -- assignment to [] is disallowed by mypy, but is valid in 3.5 (and 3.6, I don't have other versions to hand to test with).
Assuming that that spelling is valid in all supported versions it would be great if mypy could allow it. Currently for code [] = foo() mypy emits the error can't assign to (), which is doubly confusing as that source code doesn't feature any tuples.

For reference, I hit this because I wanted to unpack a (nested) iterable I expected to be empty:

[], kwargs = mock_thing.call_args