Allow assignment to an empty tuple

[tyehle]

Fixes #5594. Simply removes the check in the semantic analyzer. I made a best guess on what to do about the tests.

[JelleZijlstra]

Shouldn't this still be an error saying 1 is not iterable or similar?

Also, I would like to see a new test that does things like () = [] or a, () = list() and asserts that there are no errors.

[JelleZijlstra]

Not sure we need this test; these tests that print out internal data structures are harder to maintain and harder to map to user-visible behavior.

[gvanrossum]

@tyehle This still looks like an easy win -- but it does seem reasonable to ask for some test that indeed works. Also note that in Python 2, assignment to () is not legal -- the parser generates the SyntaxError so a simple test that this in fact is rejected in Python 2 would make sense.

[tyehle]

Hmm. I did some more tests, and it looks like () = [] produces a syntax error all the way up through python 3.5. Is I'm not sure how to write tests that check for different behavior on different versions of python. Is this even possible?

[JelleZijlstra]

You can pass mypy's --python-version flag in the test by putting # flags: --python-version 3.6 as the first line.

[msullivan]

@tyehle do you think you'll have a chance to return to this?

[tyehle]

@msullivan I forgot about this. I think I'll take another crack at it :)

[tyehle]

I can't figure out how to make these two tests pass

() = 1  # E: 'builtins.int' object is not iterable
[typing fixtures/typing-full.pyi]

I don't know what is required to make the tests realize that int isn't an instance of Iterable. It throws KeyError: 'typing' which I suspected was related to the way the tests know about the typing module, or possibly the fact that the function type_is_iterable is checking is_subtype(type, self.named_generic_type('typing.Iterable', ....

# flags: --python-version 3.5
() = []  # E: can't assign to ()

The test passing with python 3.5 makes sense because I removed that check. I'm honestly not sure why it correctly produces an error when given --python-version 2.7.

[msullivan]

Sorry, I missed your comments here and dropped this.

When does it throw KeyError: 'typing'? If you try to run that test after your change?

[tyehle]

Yeah it throws KeyError: 'typing' after the change. I would expect it to throw an error complaining that ints aren't iterable instead.

[msullivan]

I apologize that I took a year and a half to look at this again. Adding a builtins helped with the stupid typing thing. We run a lot of tests in a pretty broken mypy environment for speed, unfortunately. I moved the tests back into an existing file.

Thanks.