Python lazy list

Question

I would like create my own collection that has all the attributes of python list and also knows how to save/load itself into/from a database. Also I want to make the load implicit and lazy, as in it doesn't happen at the point of creation of the list, but waits until its first used.

Is there a single__xxx__method I can override to load the list on first usage of any list property (such aslen,getitem,iter...etc) without having to override them all?

Answer 1

Not a single, but 5 is enough:

from collections import MutableSequence

class Monitored(MutableSequence):
    def __init__(self):
        super(Monitored, self).__init__()
        self._list = []

    def __len__(self):
        r = len(self._list)
        print "len: {0:d}".format(r)
        return r

    def __getitem__(self, index):
        r = self._list[index]
        print "getitem: {0!s}".format(index)
        return r

    def __setitem__(self, index, value):
        print "setitem {0!s}: {1:s}".format(index, repr(value))
        self._list[index] = value

    def __delitem__(self, index):
        print "delitem: {0!s}".format(index)
        del self._list[index]

    def insert(self, index, value):
        print "insert at {0:d}: {1:s}".format(index, repr(value))
        self._list.insert(index, value)

The correct way of checking if something implements the whole list interface is to check if it is a subclass of MutableSequence. The ABCs found in the collections module, of which MutableSequence is one, are there for two reasons:

1. to allow you to make your own classes emulating internal container types so that they are usable everywhere a normal built-in is.

2. to use as argument for isinstance and issubclass to verify that an object implements the necessary functionality:

>>> isinstance([], MutableSequence)
True
>>> issubclass(list, MutableSequence)
True

Our Monitored class works like this:

>>> m = Monitored()
>>> m.append(3)
len: 0
insert at 0: 3
>>> m.extend((1, 4))
len: 1
insert at 1: 1
len: 2
insert at 2: 4
>>> m.l
[3, 1, 4]
>>> m.remove(4)
getitem: 0
getitem: 1
getitem: 2
delitem: 2
>>> m.pop(0)   # after this, m.l == [1]
getitem: 0
delitem: 0
3
>>> m.insert(0, 4)
insert at 0: 4
>>> m.reverse()   # After reversing, m.l == [1, 4]
len: 2
getitem: 1
getitem: 0
setitem 0: 1
setitem 1: 4
>>> m.index(4)
getitem: 0
getitem: 1
1