I've encountered this code from Most pythonic way of counting matching elements in something iterable
r = xrange(1, 10)
print sum(1 for v in r if v % 2 == 0) # 4
print sum(1 for v in r if v % 3 == 0) # 3
r is iterated once. and then it's iterated again. I thought if an iterator is once consumed then it's over and it should not be iterated again.
Generator expressions can be iterated only once:
r = (7 * i for i in xrange(1, 10))
print sum(1 for v in r if v % 2 == 0) # 4
print sum(1 for v in r if v % 3 == 0) # 0
enumerate(L) too:
r = enumerate(mylist)
and file object too:
f = open(myfilename, 'r')
Why does xrange behave differently?
Because the xrange object produced by calling xrange() specifies an __iter__ that provides a unique version of itself (actually, a separate rangeiterator object) each time it's iterated.
>>> x = xrange(3)
>>> type(x)
<type 'xrange'>
>>> i = x.__iter__()
>>> type(i)
<type 'rangeiterator'>