Disallow tuple unpacking for strings

[msullivan]

I think it would be a good idea to disallow tuple unpacking from strings, so that the following is caught:

d: Dict[str, str]
for a, b in d:
    stuff

There are a lot of cases involving one or the other of string iteration and tuple unpacking that we probably can't solve without risking too many false positives, but this one should be pretty harmless.

[djb4ai]

Hi @msullivan I would like to take a stab at it, I am new to mypy so if you could give a little guidance.

[msullivan]

I think you'll want to add some sort of special case to check_multi_assignment in checker.py

[djb4ai]

@msullivan can you explain the example? I get it that mypy won't catch this exception but couldn't understand the tuple unpacking of string part.

[JukkaL]

Here is a another example:

s = "foo"
a, b = s

The tuple unpacking refers to the lvalue (a, b in the above example). If this is still unclear, it would probably be useful to study how assignment statements generally work in Python in some detail.