Tip 58 Yield, Do Not Return

Pythonic Programming — by Dmitry Zinoviev (69 / 116)

👈 Chain Function Calls | TOC | Return and Apply Functions 👉

★★★2.7, 3.4+ Once you return from a Python function, you can never come back again and continue from the next line after the return statement. This behavior is well understood and rarely causes lamentations — except when you want to implement a generator.

A generator is an object that, when asked, “lazily” produces one item at a time. From a caller’s perspective, it looks like a function with internal memory that remembers where it stopped after returning the previous result and resumes from the next line — something that I claimed earlier not to be possible. Some standard generator functions are shown in Tip 17, ​Avoid range() in Loops​, but you can quickly write generators yourself. All you need is to replace return with yield. A function with a yield returns a generator object. You can use the object as a parameter to another function or explicitly elicit the generated values by applying the built-in functions next (one item) or list (all items):

​ ​def​ ​fortune_teller​(attempts=2):
​     ​for​ _ ​in​ range(attempts):
​         ​yield​ bool(random.randint(0, 1))
​     ​return​ ​'Do not call me again!'​
​ oracle = fortune_teller(2) ​# The generator created​
​ next(oracle)
​=> ​False​
​ next(oracle)
​=> ​True​