Loop with conditions in python

Question

Consider the following code in C:

for(int i=0; i<10 && some_condition; ++i){
    do_something();
}

I would like to write something similar in Python. The best version I can think of is:

i = 0
while some_condition and i<10:
    do_something()
    i+=1

Frankly, I don't like while loops that imitate for loops. This is due to the risk of forgetting to increment the counter variable. Another option, that addressess this risk is:

for i in range(10):
    if not some_condition: break
    do_something()

Important clarifications

1. some_condition is not meant to be calculated during the loop, but rather to specify whether to start the loop in the first place

2. I'm referring to Python2.6

Which style is preferred? Is there a better idiom to do this?

Answer 1

This might not be related, but there's what I'm used to do... If some_condition is simple enough, put it in a function and filter items you iterate over:

def some_condition(element):
    return True#False

for i in filter(some_condition, xrange(10)):
    pass

You can use this approach also when you iterate over some list of elements.

selected = filter(some_condition, to_process)
for i, item in enumerate(selected):
    pass

Again, this might not be your case, you should choose method of filtering items depending on your problem.

Answer 2

In general, the " range + break " style is preferred - but in Python 2.x, use xrange instead of range for iteration (this creates the values on-demand instead of actually making a list of numbers).

But it always depends. What's special about the number 10 in this context? What exactly is some_condition ? Etc.

Response to update:

It sounds as though some_condition is a "loop invariant", ie will not change during the loop. In that case, we should just test it first:

if some_condition:
  for i in xrange(10):
    do_something()

Answer 3

for loops with a constant upper bound are a bit rare in Python. If you are iterating over somearray , you might do:

for i in xrange(len(somearray)):
    if not some_condition:
        break
    do_sth_with(i, somearray[i])

or, better:

for i, item in enumerate(somearray):
    if not some_condition:
        break
    do_sth_with(i, item)