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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