Question

Is it possible to overload [] (__getitem__) Python operator and chain methods using the initial memory reference.

Imagine I have a class Math that accepts a list of integer numbers, like this:

class Math(object):
    def __init__(self, *args, **kwargs):
        assert(all([isinstance(item, int) for item in list(args)]))
        self.list = list(args)

    def add_one(self):
        for index in range(len(self.list)):
            self.list[index] += 1

And I want to do something like this:

instance = Math(1,2,3,4,5)
instance[2:4].add_one()

After executing this code instance.list should be [1,2,4,5,5], is this possible?

I know I could do something like add_one(2,4), but this is not the style of API I would like to have if possible.

Thanks

Answer 1

As Winston mentions, you need to implement an auxiliary object:

class Math(object):
    def __init__(self, *args, **kwargs):
        self.list = list(args)

    def __getitem__(self, i):
        return MathSlice(self, i)

class MathSlice(object):
    def __init__(self, math, slice):
        self.math = math
        self.slice = slice

    def add_one(self):
        for i in xrange(*self.slice.indices(len(self.math.list))):
            self.math.list[i] += 1


instance = Math(1,2,3,4,5)
instance[2:4].add_one()

print instance.list

How you share the math object with the MathSlice object depends on what you want the semantics to be if the math object changes.