Being able to break multiple loops and having "labelled" breaks would be
achievable using `except`, i.e. adding `except` to the loop statements
before `else` like this:
for elem in iterable:
> ...
> if should_break(elem):
> raise SomeException
> except SomeException as e:
> handle_break_behaviour(e)
> else:
> print("Did not break")
>
would be sugar for:
try:
> for elem in iterable:
> ...
> if should_break(elem):
> raise SomeException
> except SomeException as e:
> handle_break_behaviour(e)
> else:
> print("Did not break")
>
I (and others) have suggested this before and no one has said it's a
*bad *option,
it's just been ignored, despite seeming to be an intuitive way to
accomplish `else` clarity, "labelled" breaks and breaking from multiple
loops. Is there a reason that this suggestion is worse / no better than
adding special break syntax?
As for an example for labelled breaks how about something of the form:
def insert_ordered_no_duplicates(duplicate_free_list, item):
> for i, elem in enumerate(duplicate_free_list):
> if elem == item:
> break already_present
> if elem > item:
> break location_found
> case already_present:
> return duplicate_free_list
> case location_found:
> new_list = duplicate_free_list[:]
> new_list.insert(i, item)
> return new_list
> new_list = duplicate_free_list[:]
> new_list.append(item)
> return new_list
>
which you can conceivably have a nested case where you don't want to let
duplicate inserts even be attempted like this:
def insert_many_ordered_strictly_no_duplicates(dup_free_list, items):
> new_list = dup_free_list[:]
> for item in items:
> for i, elem in new_list:
> if elem == item:
> break already_present
> if elem > item:
> break location_found
> case already_present:
> break duplicates_attempted
> case location_found:
> new_list.insert(i, item)
> else:
> new_list.append(item)
> case duplicates_attempted:
> return dup_free_list[:]
> return new_list
>
On Wed, 5 Aug 2020 at 13:40, Rob Cliffe via Python-ideas <
python-ideas@python.org> wrote:
> I note that both of these examples could be handled by having a way to
> break out of 2 loops at a time.
> Rob
>
> On 29/07/2020 12:33, Jonathan Fine wrote:
>
> Thank you all, particularly Guido, for your contributions. Having some
> examples will help support the exploration of this idea.
>
> Here's a baby example - searching in a nested loop. Suppose we're looking
> for the word 'apple' in a collection of books. Once we've found it, we stop.
>
> for book in books:
> for page in book:
> if 'apple' in page:
> break
> if break:
> break
>
> However, suppose we say that we only look at the first 5000 or so words in
> each book. (We suppose a page is a list of words.)
>
> This leads to the following code.
>
> for book in books:
> word_count = 0
> for page in book:
> word_count += len(page)
> if word in page:
> break
> if word_count >= 5000:
> break found
> if break found:
> break
>
> At this time, I'd like us to focus on examples of existing code, and
> semantics that might be helpful. I think once we have this, the discussion
> of syntax will be easier.
>
> By the way, the word_count example is as I typed it, but it has a typo.
> Did you spot it when you read it? (I only noticed it when re-reading my
> message.)
>
> Finally, thank you for your contributions. More examples please.
>
> --
> Jonathan
>
>
>
>
>
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