Break big method into 2 methods, first containing a “for” loop and second a “break”

Question

A method grew too big for its own good, and I need to break it up into two separate methods.

def big_method(dct):

    # Initial code
    # ...
    for i in dct:
        # More code
        # ...
        for j in dct[i]:    
            # Inner code
            # ...
            # ...
            # and a break
            if j == 'something':
                break
            # Inner code ended here.

The problem is that my first method will contain a loop, and the second method needs to contain a break:

def first_small(dct, second_method):
    # Initial code
    # ...
    for i in dct:
        # More code
        # ...
        for j in dct[i]:
            second_method()


def second_small(j):

    # Plenty of code
    # ...
    # ...
    # and a break
    if j == 'something':
        break    
    # Inner code ended here.

second_small() would raise a SyntaxError: 'break' outside loop.

To fix it, I can replace break in second_small() with a custom exception and make first_small() handle that exception, and break its loop when it finds it:

def first_small(dct, second_method):
    # Initial code
    # ...
    for i in dct:
        # More code
        # ...
        for j in dct[i]:
            try:
                second_method(j=j)
            except MyException:
                break


def second_small(j):

    # Inner code
    # ...
    # ...
    # and a break
    if j == 'something':
        raise MyException
    # Inner code ended here.

However, I am not so sure this is the cleanest way to achieve that.

Question:
How should I split a method into two others, where first method will contain a loop and second has to break that loop?

Answer 1

Forget the outer loop around for a moment, and think about the meaning of your method second_small, the abstraction it represents (I assume in your real code, you picked a more meaningful name). Then what you do in that method should depend on what

 if j == 'something':
     ...

means, in contrast to that abstraction. For example, if j == 'something' is a real error case, throw an exception. If it means "I successfully found the result of a search", use a boolean return value and return True. If it means "could not find it", return False. If it means "validation succeeded", return True.

A boolean return value might be also the better alternative if the condition means "might be an error, not sure, only the caller can decide". A boolean is probably the wrong alternative if you have to provide some kind of error message.

It should be clear in case of using a boolean, your code will finally look like this:

 for j in dct[i]:
      if second_method(j):
            break

def second_method(j):
    # ...
    if j == 'something':
        return True
    # ...