Subclass Python list to Validate New Items

Question

I want a python list which represents itself externally as an average of its internal list items, but otherwise behaves as a list. It should raise a TypeError i

Answer 1

Here's how to create a subclass using the MutableSequence abstract base class in the collections module as its base class (not fully tested -- an exercise for the reader ;-):

import collections

class AveragedSequence(collections.MutableSequence):
    def _validate(self, x):
        try: return float(x)
        except: raise TypeError("Can't add {} to AveragedSequence".format(x))
    def average(self):  return sum(self._list) / len(self._list)
    def __init__(self, arg):  self._list = [self._validate(v) for v in arg]
    def __repr__(self):  return 'AveragedSequence({!r})'.format(self._list)
    def __setitem__(self, i, value):  self._list[i] = self._validate(value)
    def __delitem__(self, i):  del self._list[i]
    def insert(i, value):  return self._list.insert(i, self._validate(value))
    def __getitem__(self, i):  return self._list[i]
    def __len__(self):  return len(self._list)
    def __iter__(self):  return iter(self._list)
    def __contains__(self, item):  return item in self._list

if __name__ == '__main__':
    avgseq = AveragedSequence(range(10))
    print avgseq
    print avgseq.average()
    avgseq[2] = 3
    print avgseq
    print avgseq.average()
    # ..etc