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

Answer 1

Usually I tend to think of a loop structure like this:

for item in my_sequence:
    if logic(item):
        do_something(item)
        break

To be a lot like a variable number of if/elif statements:

if logic(my_seq[0]):
    do_something(my_seq[0])
elif logic(my_seq[1]):
    do_something(my_seq[1])
elif logic(my_seq[2]):
    do_something(my_seq[2])
....
elif logic(my_seq[-1]):
    do_something(my_seq[-1])

In this case the else statement on the for loop works exactly like the else statement on the chain of elifs, it only executes if none of the conditions before it evaluate to True. (or break execution with return or an exception) If my loop does not fit this specification usually I choose to opt out of using for: else for the exact reason you posted this question: it is non-intuitive.

Answer 2

If you pair else with for, it could be confusing. I don't think the keyword else was a great choice for this syntax, but if you pair else with if which contains break, you can see it actually makes sense. else is barely useful if there is no preceding if statement and I believe this is why the syntax designer chose the keyword.

Let me demonstrate it in human language.

for each person in a group of suspects if anyone is the criminal break the investigation. else report failure.

Answer 3

My gotcha moment with the loop's else clause was when I was watching a talk by Raymond Hettinger, who told a story about how he thought it should have been called nobreak. Take a look at the following code, what do you think it would do?

for i in range(10):
    if test(i):
        break
    # ... work with i
nobreak:
    print('Loop completed')

What would you guess it does? Well, the part that says nobreak would only be executed if a break statement wasn't hit in the loop.

Answer 4

When does an if execute an else? When its condition is false. It is exactly the same for the while/else. So you can think of while/else as just an if that keeps running its true condition until it evaluates false. A break doesn't change that. It just jumps out of the containing loop with no evaluation. The else is only executed if evaluating the if/while condition is false.

The for is similar, except its false condition is exhausting its iterator.

continue and break don't execute else. That isn't their function. The break exits the containing loop. The continue goes back to the top of the containing loop, where the loop condition is evaluated. It is the act of evaluating if/while to false (or for has no more items) that executes else and no other way.

Answer 5

The way I think about it, the key is to consider the meaning of continue rather than else.

The other keywords you mention break out of the loop (exit abnormally) whilst continue does not, it just skips the remainder of the code block inside the loop. The fact that it can precede loop termination is incidental: the termination is actually done in the normal way by evaluation of the loop conditional expression.

Then you just need to remember that the else clause is executed after normal loop termination.

Answer 6

If you think of your loops as a structure similar to this (somewhat pseudo-code):

loop:
if condition then

   ... //execute body
   goto loop
else
   ...

it might make a little bit more sense. A loop is essentially just an if statement that is repeated until the condition is false. And this is the important point. The loop checks its condition and sees that it's false, thus executes the else (just like a normal if/else) and then the loop is done.

So notice that the else only get's executed when the condition is checked. That means that if you exit the body of the loop in the middle of execution with for example a return or a break, since the condition is not checked again, the else case won't be executed.

A continue on the other hand stops the current execution and then jumps back to check the condition of the loop again, which is why the else can be reached in this scenario.