How can I make sense of the `else` clause of Python loops? [closed]

Answer 1

# tested in Python 3.6.4
def buy_fruit(fruits):
    '''I translate the 'else' below into 'if no break' from for loop '''
    for fruit in fruits:
        if 'rotten' in fruit:
            print(f'do not want to buy {fruit}')
            break
    else:  #if no break
        print(f'ready to buy {fruits}')


if __name__ == '__main__':
    a_bag_of_apples = ['golden delicious', 'honeycrisp', 'rotten mcintosh']
    b_bag_of_apples = ['granny smith', 'red delicious', 'honeycrisp', 'gala', 'fuji']
    buy_fruit(a_bag_of_apples)
    buy_fruit(b_bag_of_apples)

'''
do not want to buy rotten mcintosh
ready to buy ['granny smith', 'red delicious', 'honeycrisp', 'gala', 'fuji']
'''

Answer 2

Others have already explained the mechanics of while/for...else, and the Python 3 language reference has the authoritative definition (see while and for), but here is my personal mnemonic, FWIW. I guess the key for me has been to break this down into two parts: one for understanding the meaning of the else in relation to the loop conditional, and one for understanding loop control.

I find it's easiest to start by understanding while...else:

while you have more items, do stuff, else if you run out, do this

The for...else mnemonic is basically the same:

for every item, do stuff, but else if you run out, do this

In both cases, the else part is only reached once there are no more items to process, and the last item has been processed in a regular manner (i.e. no break or return). A continue just goes back and sees if there are any more items. My mnemonic for these rules applies to both while and for:

when breaking or returning, there's nothing else to do,
and when I say continue, that's "loop back to start" for you

– with "loop back to start" meaning, obviously, the start of the loop where we check whether there are any more items in the iterable, so as far as the else is concerned, continue really plays no role at all.