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:
-
to allow you to make your own classes emulating internal container types so that they are usable everywhere a normal built-in is.
-
to use as argument for
isinstance
andissubclass
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