py.user.next
для len() нужен список

>>> len(xrange(6))
6
>>> len(x for x in xrange(6))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'generator' has no len()

>>> type(xrange(5))
<type 'xrange'>
>>> type(x for x in xrange(5))
<type 'generator'>

odnochlen
А посчитать количество элементов, не сохраняя все в список, можно?

sum(1 for x in xrange(5))

generator
A function which returns an iterator.
…
Each yield temporarily suspends processing, remembering the location execution state (including local variables and pending try-statements). When the generator resumes, it picks-up where it left-off (in contrast to functions which start fresh on every invocation).

>>> matr = [[1, -1, 2, -2, 3], [1, -1, 2, -2, -3]]
>>> sum(i < 0 for i in sum(matr, []))
5
>>>

odnochlen

>>> len(xrange(6))
6
>>>

>>> range(1, 10)[2:5]
range(3, 6)
>>>