[Tutor] for/else question

[Scott Widney]

> I seem to recall a discussion of this on comp.lang.python some time 
> ago, which intimated that it was included as much for symmetry (with 
> if/else) as for practical need.  (The implementation of code suites
> is such that it is essentially free -- 'else' works the same whether
> it follows a 'for' or an 'if'.)
> 
> That said, I can see cases where it's important to know whether
> you've broken out of a loop or not.  You might need to take an 
> action only if there are no errors in your loop, for instance. 
> You could use a sentinel variable, but it's cleaner to avoid that,
> and for/else allows you to avoid it.

This is probably a moot point (I doubt that they would change this) but I
think it's the use of the word 'else' that is confusing. Here is how I tend
to think of the 'if' statement:

    If a condition is true:
        then do something;
    otherwise:
        do something else.

This is not symmetrical with 'for: else:' in my mind. Grammatically
speaking, it seems backward, and therein lies the tension. Here is my brain
map of the 'for' statement:

    For each item in a group:
        do something;
    afterwards (provided nothing went wrong):
        do this too.

"Otherwise" doesn't map cleanly to "afterwards". If we swapped the terms we
would get this:

    If a condition is true: then do something;
    afterwards: do something else.

    For each item in a group: do something;
    otherwise: do this.

It just doesn't fit. But there is a term in another statement in Python that
seems closer to what I'm thinking:

    Try:
        something that might not work;
    if it fails:
        do this;
    finally (whether or not it worked):
        do this too.

This "feels" more comfortable. Finally and afterwards are semantic
neighbors. So if I think 'finally' in my head, but type 'else' when dealing
with a for statement, it seems to relieve the pressure. And now we return to
our regularly scheduled program....


Scott