Special case assignment to '_': always infer 'Any'

[gvanrossum]

Fixes #465. Also see discussion there about the exact specification and concerns.

[ilevkivskyi]

I would add at least one of the two safety rules:

• check that the variable is never read at the same scope (this is free from problems I mentioned in the issue)
• only allow this at a local (function or method) scope

[ilevkivskyi]

Why do you have only tests at the function scope? Is it something intentional?

[gvanrossum]

Oops, I thought it would all be treated the same, but it isn't. Will fix. (Since I've seen a fair number of global uses of this idiom.)

[ilevkivskyi]

I would add _() + 'hi' to check that this generates an error.

[gvanrossum]

I still am hesitant about the safety rules -- I suspect there are too many different places. I'd almost rather add it behind a flag.

[ilevkivskyi]

Oh, pilot error, pushed the test updates (I hope).

Maybe now I am missing something, but I still don't see the tests updates in this PR.

[gvanrossum]

I'm guessing I committed but didn't push (or the push failed and I didn't notice). I pushed again, we now have 67d84aa and the tests for that are running.

[gvanrossum]

Recording a few ideas from Jukka:

• only do it on the second assignment?
• create a new Var on each assignment?
• do this analysis in semanal.py, using is_func_scope()?

Also need tests:

• multiple assignments to _ on same line (e.g. _, _, x = <tuple>)
• make sure import ... as _ doesn't trigger this
• make sure _: <type> = <expr> overrides

[ilevkivskyi]

Some thoughts about these ideas (not sure if I should comment these, but just in case).

only do it on the second assignment?

I am not sure I understand the benefit of this.

create a new Var on each assignment?

I would say it is a bad idea to have two variables with the same name.

do this analysis in semanal.py, using is_func_scope()?

I like this idea, this is similar to what I was thinking about this.

[gvanrossum]

This is ready for another review. I moved the special-casing to semanal.py, where it seems to work fine, and limited it to locals only. There's one quirk: except Exception as _: doesn't treat _ as Any because the checker overrides the type. The code in the checker looks a bit complex so I'd rather keep this as a limitation rather than trying to fix it. As the tests show, it works fine with the target in a for loop or in a with statement.

[ilevkivskyi]

Looks good!

[ilevkivskyi]

First isinstance() should be unnecessary, since we are already in a huge if isinstance(lval, NameExpr) block. Also the last check self.is_func_scope() is not needed, because it is equivalent to self.locals[-1] is not None where we are now. But it is up to you.

[gvanrossum]

OK, I'll remove the redundant checks. If the tests pass I'll merge.