lsst.afw.image.image.image.ImageF has no __idiv__ method defined, so if one tries to do im \= value without having done from __future__ import division (which makes /= point to __itruediv__, then an AttributeError gets raised. Other undesired operators (e.g., __add__, __mul__) raise NotImplementedError, which seems more appropriate. Possible solutions would be to either have __idiv__ raise NotImplementedError (with a helpful message suggesting to please do the __future__ import) or to redirect __idiv__ to __itruediv__.