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

The array.array class will take care of the float part:

class AverageList(array.array):
    def __new__(cls, *args, **kw):
        return array.array.__new__(cls, 'd')
    def __init__(self, values=()):
        self.extend(values)
    def __repr__(self):
        if not len(self): return 'Empty'
        return repr(math.fsum(self)/len(self))

And some tests:

>>> s = AverageList([1,2])
>>> s
1.5
>>> s.append(9)
>>> s
4.0
>>> s.extend('lol')
Traceback (most recent call last):
  File "", line 1, in 
    s.extend('lol')
TypeError: a float is required