[Python-ideas] Re: Idea: Extend "for ... else ..." to allow "for ... if break ..." else

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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