When running 2to3 -d on this doctest (from this file[0] in SymPy):

        >>> class SzUpKet(Ket):
        ...     def _represent_SzOp(self, basis, **options):
        ...         return Matrix([1,0])
        ...

2to3 adds an extra space in the last line. This then raises an error for our automated whitespace tests (and is generally annoying). I haven't seen this happen anywhere else (and SymPy is a big codebase). 

It's really a minor issue, though (I can't set the priority myself, though).

[0] https://github.com/sympy/sympy/blob/master/sympy/physics/quantum/represent.py