Other languages

Now, put aside whatever dislike you may have for other programming languages, and consider the syntax of labeled break and continue. In Perl:

ALOOP: foreach $a (@a_array){
    ...
    BLOOP: foreach $b (@b_array){
        ...
        if (condition_one($a,$b)){
            last BLOOP; # same as plain old last;
        }
        ...
        if (condition_two($a,$b)){
            last ALOOP;
        }
        ...
    }
    ...
}

(Notes: Perl uses last instead of break. The BLOOP labels could be omitted; last and continue apply to the innermost loop by default.)

PHP uses a number denoting the number of loops to break out of, rather than a label:

foreach ($a_array as $a){
    ....
    foreach ($b_array as $b){
        ....
        if (condition_one($a, $b)){
            break 1;  # same as plain old break
        }
        ....
        if (condition_two($a, $b)){
            break 2;
        }
        ....
    }
    ...
}

C/C++, Java, and Ruby all have similar constructions.

The control flow regarding when to break out of the outer (a) loop is fully encapsulated in the break statement which gets executed when the break condition is satisfied. The depth of the break statement does not matter. Control flow is not spread out. No extra variables, exceptions, or re-checking or storing of control conditions is required. There is no danger that code will get inadvertently inserted after the end of the inner (b) loop and before the break condition is re-checked inside the outer (a) loop. These are the benefits that labeled break and continue would bring to Python.

Proposal A - Explicit labels

The for and while loop syntax will be followed by an optional as or label (contextual) keyword [2] and then an identifier, which may be used to identify the loop out of which to break (or which should be continued).

The break (and continue) statements will be followed by an optional identifier that refers to the loop out of which to break (or which should be continued). Here is an example using the as keyword:

for a in a_list as a_loop:
    ...
    for b in b_list as b_loop:
        ...
        if condition_one(a, b):
            break b_loop  # same as plain old break
        ...
        if condition_two(a, b):
            break a_loop
        ...
    ...

Or, with label instead of as:

for a in a_list label a_loop:
    ...
    for b in b_list label b_loop:
        ...
        if condition_one(a, b):
            break b_loop  # same as plain old break
        ...
        if condition_two(a, b):
            break a_loop
        ...
    ...

This has all the benefits outlined above. It requires modifications to the language syntax: the syntax of break and continue syntax statements and for and while statements. It requires either a new conditional keyword label or an extension to the conditional keyword as. [3] It is unlikely to require any changes to existing Python programs. Passing an identifier not defined in the local scope to break or continue would raise a NameError.

Proposal B - Numeric break & continue

Rather than altering the syntax of for and while loops, break and continue would take a numeric argument denoting the enclosing loop which is being controlled, similar to PHP.

It seems more Pythonic to me for break and continue to refer to loops indexing from zero, as opposed to indexing from one as PHP does.

for a in a_list:
    ...
    for b in b_list:
        ...
        if condition_one(a,b):
            break 0  # same as plain old break
        ...
        if condition_two(a,b):
            break 1
        ...
    ...

Passing a number that was too large, or less than zero, or non-integer to break or continue would (probably) raise an IndexError.

This proposal would not require any changes to existing Python programs.

Proposal C - The reduplicative method

The syntax of break and continue would be altered to allow multiple break and continue statements on the same line. Thus, break break would break out of the first and second enclosing loops.

for a in a_list:
    ...
    for b in b_list:
        ...
        if condition_one(a,b):
            break  # plain old break
        ...
        if condition_two(a,b):
            break break
        ...
    ...

This would also allow the programmer to break out of the inner loop and continue the next outermost simply by writing break continue, [4] and so on. I'm not sure what exception would be raised if the programmer used more break or continue statements than existing loops (perhaps a SyntaxError?).

I expect this proposal to get rejected because it will be judged too difficult to understand.

This proposal would not require any changes to existing Python programs.

Proposal D - Explicit iterators

Rather than embellishing for and while loop syntax with labels, the programmer wishing to use labeled breaks would be required to create the iterator explicitly and assign it to an identifier if he or she wanted to break out of or continue that loop from within a deeper loop.

a_iter = iter(a_list)
for a in a_iter:
    ...
    b_iter = iter(b_list)
    for b in b_iter:
        ...
        if condition_one(a,b):
            break b_iter  # same as plain old break
        ...
        if condition_two(a,b):
            break a_iter
        ...
    ...

Passing a non-iterator object to break or continue would raise a TypeError; and a nonexistent identifier would raise a NameError. This proposal requires only one extra line to create a labeled loop, and no extra lines to break out of a containing loop, and no changes to existing Python programs.

Proposal E - Explicit iterators and iterator methods

This is a variant of Proposal D. Iterators would need be created explicitly if anything other that the most basic use of break and continue was required. Instead of modifying the syntax of break and continue, .break() and .continue() methods could be added to the Iterator type.

a_iter = iter(a_list)
for a in a_iter:
    ...
    b_iter = iter(b_list)
    for b in b_iter:
        ...
        if condition_one(a,b):
            b_iter.break()  # same as plain old break
        ...
        if condition_two(a,b):
            a_iter.break()
        ...
    ...

I expect that this proposal will get rejected on the grounds of sheer ugliness. However, it requires no changes to the language syntax whatsoever, nor does it require any changes to existing Python programs.