Question

I have a class that should act as a list of differences (deltas), as calculated from another list. For example, if I have a list of positions, I can use my class to have a dynamically created list of displacements.

For that, I’ve used the __getitem__ method the following way:

class Deltas(object):
    def __init__(self, ref):
        self.ref = ref

    def __getitem__(self, index):
        if index == 0:
            return 0
        else:
            return self.ref[index] - self.ref[index-1]

samplelist = [1,2,3,5,7,9]

deltas = Deltas(samplelist)

If I print individual elements, it works fine, but if I print the whole list, it gives me:

> print deltas
<__main__.Deltas object at 0x7f7aa93e3b50>

I’d like to know what other method I have to implement to get this:

> print deltas
[0, 1, 1, 2, 2, 2]

Answer 1

For example, adding this method would work:

def __repr__(self):
    return [x for x in self].__repr__()

Or, alternatively:

def __repr__(self):
    strs = []
    for x in self:
        strs.append(str(x))
    return '[%s]' % ', '.join(strs)