Passing a list as an argument with * leads to confusing error

[JelleZijlstra]

(Originally reported at python/typeshed#814, which gives an example from real code.)

def foo(a: int, b: str = '') -> None: ...

foo(1)  # OK
foo(*[1])  # E: Argument 1 to "foo" has incompatible type *List[int]; expected "str"

Ideally this shouldn't be an error at all (this works fine at runtime), but I imagine it's hard for mypy to accept this code since it doesn't know how long the list is. However, we could at least fix the error message, which talks about "argument 1" needing to be a str when really it's argument 2 that is the problem.

[gvanrossum]

IIRC the argument numbers are for the call site, not the definition site. But regardless the error is horrible. (And indeed, mypy probably will never support this. It's also kind of silly. :-)

[elazarg]

There is a confusion inside mypy about the terms Argument vs Parameter, but this case specifically gets it right.

[gvanrossum]

(That confusion does not just exist inside mypy -- the whole world is inconsistent in its use of these terms. When the distinction is really important the only solution is to say "actual" vs. "formal" parameter, or if you want to be even clumsier but rely less on convention, you could say "call-site" vs. "definition-time" or something.)