I am working with a Python object that implements __add__, but does not subclass int. MyObj1 + MyObj2 works fine, but sum([MyObj1, MyObj2]) led to a TypeError, becausesum() first attempts 0 + MyObj. In order to use sum(), my object needs __radd__ to handle MyObj + 0 or I need to provide an empty object as the start parameter. The object in question is not designed to be empty. Before anyone asks, the object is not list-like or string-like, so use of join() or itertools would not help. Edit for details: the module has a SimpleLocation and a CompoundLocation. I'll abbreviate Location to Loc. A SimpleLoc contains one right-open interval, i.e. [start, end). Adding SimpleLoc yields a CompoundLoc, which contains a list of the intervals, e.g. [[3, 6), [10, 13)]. End uses include iterating through the union, e.g. [3, 4, 5, 10, 11, 12], checking length, and checking membership. The numbers can be relatively large (say, smaller than 2^32 but commonly 2^20). The intervals probably won't be extremely long (100-2000, but could be longer). Currently, only the endpoints are stored. I am now tentatively thinking of attempting to subclass set such that the location is constructed as set(xrange(start, end)). However, adding sets will give Python (and mathematicians) fits. Questions I've looked at: python's sum() and non-integer values why there's a start argument in python's built-in sum function TypeError after overriding the __add__ method I'm considering two solutions. One is to avoid sum() and use the loop offered in this comment. I don't understand why sum() begins by adding the 0th item of the iterable to 0 rather than adding the 0th and 1st items (like the loop in the linked comment); I hope there's an arcane integer optimization reason. My other solution is as follows; while I don't like the hard-coded zero check, it's the only way I've been able to make sum() work. # ... def __radd__(self, other): # This allows sum() to work (the default start value is zero) if other == 0: return self return self.__add__(other) In summary, is there another way to use sum() on objects that can neither be added to integers nor be empty? Continue reading...
