Conditional import of izip is problematic

[gvanrossum]

Test code:

try:
    from itertools import izip
except ImportError:
    izip = zip

In Python 3 mode, this complains that itertools doesn't define izip. This is correct but maybe inside try/except ImportError this should not be an error.

In Python 2 mode, this gives the dreaded "Incompatible types" errror:

/Users/guido/mypy_tests/mypy_izip.py:4: error: Incompatible types in assignment (expression has type overloaded function, variable has type overloaded function)

I guess this is another case of #649 that isn't handled yet.

[JukkaL]

Yeah, we could consider this is special case of #649, but we can keep ImportError related problems where mypy should ignore a block separate in this issue. #649 can cover cases where all conditional definitions should take effect.

Another related case that is pretty common in Python 2 std library code is shown below. In this case both imports work, but mypy can't deal with a multiply defined class. Maybe we should just ignore one of the class definitions here for now, and maybe give an optional warning in the future:

try:
    from cStringIO import StringIO
except ImportError:
    from StringIO import StringIO

[AlexWaygood]

Closing as a duplicate of #1153, which (although it's a newer issue) has more detailed discussion and is less Python 2-specific (refs #12237)