def funcA(i): if i%3==0: print "Oh! No!", print i break for i in range(100): funcA(i) print "Pass", print i
I know script above won’t work. So, how can I write if I need put a function with break or continue into a loop?
A function cannot cause a break or continue in the code from which it is called. The break/continue has to appear literally inside the loop. Your options are:
- return a value from funcA and use it to decide whether to break
- raise an exception in funcA and catch it in the calling code (or somewhere higher up the call chain)
- write a generator that encapsulates the break logic and iterate over that instead over the
By #3 I mean something like this:
def gen(base): for item in base: if item%3 == 0: break yield i for i in gen(range(1, 100)): print "Pass," i
This allows you to put the break with the condition by grouping them into a generator based on the “base” iterator (in this case a range). You then iterate over this generator instead of over the range itself and you get the breaking behavior.
Elaborating BrenBarns answer:
break fortunately will not propagate.
break is to break the current loop, period. If you want to propagate an event, then you should
raise an exception. Although, raising the exception to break the loop is a really ugly way to break loops and a nice way to break your code.
KISS! The simplest would be to check the condition directly in the loop
def my_condition(x): return x == 4 for i in xrange(100): if my_condition(i): break print i
If, for some reason, you want to propagate an exception, then you use it like this
# exception example for i in xrange(100): if i == 4: raise Exception("Die!") print i
As mentioned, it is a really ugly design. Imagine you forget to catch this exception, or you change its type from
MyBreakException and forget to change it somewhere in
try/except higher part of the code…
The generator example has its merits, it makes your code more functional style (which I presonally adore)
# generator example def conditional_generator(n, condition): for i in xrange(n): if condition(i): break else: yield i for i in conditional_generator( 100, my_condition ): print i
…which is similar to
takewhile, mentioned by eumiro
def funcA(i): if i%3==0: print "Oh! No!", print i return True else: return False for i in range(100): if funcA(i): break print "Pass", print i
Break won’t propagate between functions, you need to put it directly within the loop somewhere.