data = []
    def __init__(self, data):
        self.data = data
    def dispersia(self):
        return self.expected_value(map(lambda x: x*x, self.data)) - self.expected_value(self.data)**2
    def expected_value(self,data):
        return sum(data)/len(data)

return sum(data)/len(data)
TypeError: object of type 'map' has no len()

>>> lst = map(lambda x: x, 'abc')
>>> lst
<map object at 0xb73fc8ac>
>>> len(lst)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'map' has no len()
>>> dir(lst)
['__class__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__lt__', '__ne__', '__new__', '__next__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']
>>> lst = list(lst)
>>> len(lst)
3
>>>