A simple coroutine example

gistfile1.py

def lazy_writer(): 
    with open("out.txt", "w") as o: 
        while True: 
            line = (yield) 
            # do whatever you like to line here 
            out = "%s%s\n" % (line, " added") 
            o.write(out)

wl = lazy_writer() 
# initialise the coroutine before you send anything in 
wl.next() 

for line in open("in.txt", "r"): 
    wl.send(line.rstrip()) 

wl.close()