lazy.py

# return double of first number from collection which is greater than 3 and even
# this is Python 3 srcipt, in Python 2 you have to use itertools.imap and itertools.ifilter

def is_gt_3(number):
    print('is_gt_3 called...')
    return number > 3

def is_even(number):
    print('is_even called...')
    return number % 2 == 0

def double_it(number):
    print('is_double_it called...')
    return number * 2

data  = [1, 2, 3, 5, 4, 6, 7, 8, 9, 10]
result = map(double_it, filter(is_even, filter(is_gt_3, data)))
print(next(result))

result.txt

$ python3 lazy.py
is_gt_3 called...
is_gt_3 called...
is_gt_3 called...
is_gt_3 called...
is_even called...
is_gt_3 called...
is_even called...
is_double_it called...
8