Ned Batchelder: Loop Like A Native

[Rick Johnson]

On Saturday, August 6, 2016 at 10:43:01 PM UTC-5, Steven D'Aprano wrote:

> Yes. The two ways of ending the loop are distinct and different:
> 
> - reach the end, and stop;
> - bail out early.
> 
> 
> When you read a book, there are two ways of stopping:
> 
> - reach the end, and run out of pages to read, so you stop;
> - give up reading early, and just put the book away.
> 
> (Or possibly throw the book across the room.)
> 
> 
> Why would you treat these two cases in the same way?
> 
> interested = True
> for page in book:
>     if interested:
>         read(page)
>     if bored_now():
>         interested = False

This algorithm does not exit early when `interested` becomes false, instead, it iterates every page of the book regardless of the value of `interested`. Was this intentional, or merely a mistake?

> finished = False
> while not finished:
>     try:
>         page = next(book)
>     except StopIteration:
>         finished = True
>     else:
>         read(page)
>         if bored_now():
>             finished = True

That kind of code, whilst being "somewhat" idiomatic python, is horrific. If the intent is to iterate the pages of a book until the end is reached *OR* until the "reader's interest wanes", then a simple for-loop will suffice.

for page in book:
    isReaderInterested = reader.read_page(page)
    if not isReaderInterested:
        break

Simple is better than complex.