[python] Understanding __getitem__ method

__getitem__ can be used to implement "lazy" dict subclasses. The aim is to avoid instantiating a dictionary at once that either already has an inordinately large number of key-value pairs in existing containers, or has an expensive hashing process between existing containers of key-value pairs, or if the dictionary represents a single group of resources that are distributed over the internet.

As a simple example, suppose you have two lists, keys and values, whereby {k:v for k,v in zip(keys, values)} is the dictionary that you need, which must be made lazy for speed or efficiency purposes:

class LazyDict(dict):
    
    def __init__(self, keys, values):
        self.keys = keys
        self.values = values
        super().__init__()
        
    def __getitem__(self, key):
        if key not in self:
            try:
                i = self.keys.index(key)
                self.__setitem__(self.keys.pop(i), self.values.pop(i))
            except ValueError, IndexError:
                raise KeyError("No such key-value pair!!")
        return super().__getitem__(key)

Usage:

>>> a = [1,2,3,4]
>>> b = [1,2,2,3]
>>> c = LazyDict(a,b)
>>> c[1]
1
>>> c[4]
3
>>> c[2]
2
>>> c[3]
2
>>> d = LazyDict(a,b)
>>> d.items()
dict_items([])