CODEWITHSUNDEEP
pythonpython-3.x
phawk
phawk

Reputation: 23

Using return statement in generators

I'm trying to learn python, reading by the book Learning Python, and came across the section of using the return statement in generators, and I'm finding it difficult to wrap my head around it.

It says that while using the return statement in a generator function, it will produce a StopIteration exception to be raised, effectively ending the iteration. If a return statement were actually to make the function return something, it would break the iteration protocol.

Here's the code sample

def geometric_progression(a, q):
    k = 0
    while True:
        result = a * q**k
        if result <= 100000:
            yield result
        else:
            return
        k += 1

for n in geometric_progression(2,5):
    print(n)

Can anyone please explain it, and also how to use it further in any other context. Provide extra examples, if you may.

Upvotes: 2

Views: 2035

Answers (1)

mgilson
mgilson

Reputation: 309831

return in a generator is just syntactic sugar for raise StopIteration. In python 3.3(?)+, you can also return a value (return value == raise StopIteration(value)).

As for why you'd want to put it there, obviously raising a StopIteration means that the generator will stop yielding items. In other words, the generator is finished executing -- just like return signals the end of a function's execution. Similarly, anything which is iterating over the generator (e.g. a for loop) will understand that it should stop trying to get values from the generator.

The return value bit was added to support some workflows using co-routines IIRC. It's a pretty advanced feature but finds lots of usefulness in various async contexts.

Upvotes: 4

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