How to break nested for loop in Python?

Question

I wonder how to get out from loop like this:

for a in range(95):
    for b in range(95):
        for c in range(95):
            for d in range(95):
                ...
                do some computings
                ...
                if condition:
                    task completed

After task is completed all loops and computings are continued. They have to be broke but I don't know how - single break statement after "task completed" will end only innermost for loop but it will be invoked again multiple times - so we gain nothing.

In C I would do a=b=c=d=95 but in python that wouldn't work. Of course I can use while loop instead but then I have to use X+=1 statements and it would look awful.

Any help?

About the loop: I use it to break a 4-char password using brute-force. It isn't a real purpose - used only for tests.

Or you could wrap the whole thing inside a `try ... except` block and `raise` an exception to break out of the innermost loop. However, Sayse's solution is cleaner. — Roberto Reale, Sep 09 '14 at 10:22

score 12 · Accepted Answer · answered Sep 09 '14 at 10:23

12

Using itertools product:

from itertools import product
for a, b, c, d in product(range(95), range(95), range(95), range(95)):
    print a, b, c, d
    if a == 1: break

Shorter version, thanks Ashwini:

for a, b, c, d in product(range(95), repeat=4):
    print a, b, c, d
    if a == 1: break

answered Sep 09 '14 at 10:23

perreal

94,503
21
155
181

5

`product(range(95), repeat=4)...` – Ashwini Chaudhary Sep 09 '14 at 10:25
2

I like this best. Loops often don't need to be nested. – RemcoGerlich Sep 09 '14 at 10:27
It looks really nice. Probably this is what I've searched - and it reduces indentations as well! – user3616181 Sep 09 '14 at 10:27
1

And to add: sometimes you can split off the loop and the if/break into one generator with an informative name. Here's a cool video about Python loops: http://nedbatchelder.com/text/iter.html – RemcoGerlich Sep 09 '14 at 10:30

semptic · Answer 2 · 2014-09-09T11:23:44.560

4

You can use a exception to break out of nested loops:

class Break(Exception): pass

try:
    for a in range(95):
        for b in range(95):
            for c in range(95):
                for d in range(95):
                    # ...
                    # do some computings
                    # ...
                    if condition:
                        # task completed
                        raise Break()
except Break:
    # do some stuff

or with StopIteration:

try:
    for a in range(95):
        for b in range(95):
            for c in range(95):
                for d in range(95):
                    # ...
                    # do some computings
                    # ...
                    if condition:
                        # task completed
                        raise StopIteration()
except StopIteration:
    # do some stuff

edited Sep 09 '14 at 11:23

answered Sep 09 '14 at 10:23

semptic

645
4
15

I think that using exception to do flow control is usually a bad idea. – Jan Spurny Sep 09 '14 at 10:28
1

It's de rigueur for python. – khelwood Sep 09 '14 at 10:34
2

`StopIteration` exception best fits this situation: https://docs.python.org/2/library/exceptions.html#exceptions.StopIteration – Don Sep 09 '14 at 10:43
I'm not sure raising `StopIteration` from the body of the loop (instead of the iterator itself) is a great design. – chepner Sep 09 '14 at 11:10

How to break nested for loop in Python?

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