Python timer generator for simple duration measurement

timer.py

import datetime

def timer(print_=True):
    last = datetime.datetime.now()

    while True:
        now = datetime.datetime.now()
        delta = now - last
        if print_:
            print 'Duration: %s' % delta

        yield delta
        last = now

if __name__ == '__main__':
    import time
    t = timer()
    print 'Roughly zero'
    t.next()
    time.sleep(1)
    print 'Roughly one'
    t.next()