In Python, find item in list of dicts, using bisect

Question

I have a list of dicts, something like this:

test_data = [
    { 'offset':0, 'data':1500 },
    { 'offset':1270, 'data':120 },
    { 'offset':2117, 'data':30 },
    { 'offset':4055, 'data':30000 },
]

The dict items are sorted in the list according to the 'offset' data. The real data could be much longer.

What I want to do is lookup an item in the list given a particular offset value, which is not exactly one of those values, but in that range. So, a binary search is what I want to do.

I am now aware of the Python bisect module, which is a ready-made binary search—great, but not directly usable for this case. I'm just wondering what is the easiest way to adapt bisect to my needs. Here is what I came up with:

import bisect

class dict_list_index_get_member(object):
    def __init__(self, dict_list, member):
        self.dict_list = dict_list
        self.member = member
    def __getitem__(self, index):
        return self.dict_list[index][self.member]
    def __len__(self):
        return self.dict_list.__len__()

test_data_index_get_offset = dict_list_index_get_member(test_data, 'offset')
print bisect.bisect(test_data_index_get_offset, 1900)

It prints:

2

My question is, is this the best way to do what I want, or is there some other simpler, better way?

Answer 1

The usual pattern here is similar to sorting by an attribute, decorate, operate, and undecorate. So in this case you'd just need to decorate and then call. However you'd want to avoid doing this since decorate would be O(n) whereas you want this to be O(logn). Therefore I'd consider your method best.