How Python generators work

generators.py

def x(items):
    print('before loop')
    for x in items:
        print ('before yield')
        yield "xx %s yy" % x
        print('after yield')
    print('after loop')

for i in x(list((1,2,3,4))):
    print "zz %s aa" % i

output.txt

[user@computer]$ python generators.py
before loop
before yield
zz xx 1 yy aa
after yield
before yield
zz xx 2 yy aa
after yield
before yield
zz xx 3 yy aa
after yield
before yield
zz xx 4 yy aa
after yield
after loop

At yield(), the generator emits the value yield()ed, then waits for the next iterator call, when the code resumes immediately after the previous call to yield()