Generators in Python (genexps)

generator.py

def gen(num):
    n = 0
    while n < num:
        yield n
        n += 1

# returns a generator object
g = gen(3)

# returns 0
g.next()

# returns 1 
g.next()

# returns 2
g.next()

# raise StopIteration
g.next()

# prints 0 1 2
for i in gen(3):
    print i

# Turn a list comprehension into a generator expression by replacing the
# square brackets ("[ ]") with parentheses. 
doubles = (2 * n for n in xrange(3))
for i in doubles:
    # prints 0 2 4
    print i