Confusing error message when class definition duplicated across if-else

[sangsta]

I have a block of code that looks like this:

if foo:
    class A(object):
        ...
else:
    class A(object):
        ...

mypy complains for the second class definition:

error: Name 'A' already defined

However, this is technically not true.

[gvanrossum]

Yeah, this is a common issue. There are plans to fix this but they haven't landed yet. For now, just put # type: ignore on the second class statement.

[gnprice]

It seems like there are (at least) two or three things that could be done here:

• We don't allow these, and in many cases it'd be helpful if we did. This is complicated to get right, though. This is covered in existing issues.
• The error message is confusingly worded, because at runtime the class is never "already" defined.
• The error message isn't specific about where the other definition is -- which, when the code isn't a tiny example program, leaves the user to wonder if the puzzling message is really referring to that other definition up in the if or to some other place where it really was "already" defined, if they've even spotted the previous definition yet.

I think fixing the last point would in itself mitigate the confusing wording a lot, because it'd be totally explicit what other definition the checker has in mind and the user can figure out that that might be a problem even when they're never duplicated at runtime. So if nothing else, we should add a previously defined here note on that error.

[JukkaL]

The message is now Name 'A' already defined on line 2, which partially addresses the issue. If this is still too confusing, we can reopen the issue.