When to use "while" or "for" in Python

I am finding problems in when I should use a while loop or a for loop in Python. It looks like people prefer using a for loop (less code lines?). Is there any specific situation which I should use one or the other? Is it a matter of personal preference? The codes I have read so far made me think there are big differences between them.

Solution 1:

Yes, there is a huge difference between while and for.

The for statement iterates through a collection or iterable object or generator function.

The while statement simply loops until a condition is False.

It isn't preference. It's a question of what your data structures are.

Often, we represent the values we want to process as a range (an actual list), or xrange (which generates the values) (Edit: In Python 3, range is now a generator and behaves like the old xrange function. xrange has been removed from Python 3). This gives us a data structure tailor-made for the for statement.

Generally, however, we have a ready-made collection: a set, tuple, list, map or even a string is already an iterable collection, so we simply use a for loop.

In a few cases, we might want some functional-programming processing done for us, in which case we can apply that transformation as part of iteration. The sorted and enumerate functions apply a transformation on an iterable that fits naturally with the for statement.

If you don't have a tidy data structure to iterate through, or you don't have a generator function that drives your processing, you must use while.

Solution 2:

while is useful in scenarios where the break condition doesn't logically depend on any kind of sequence. For example, consider unpredictable interactions:

 while user_is_sleeping():
     wait()

Of course, you could write an appropriate iterator to encapsulate that action and make it accessible via for – but how would that serve readability?¹

In all other cases in Python, use for (or an appropriate higher-order function which encapsulate the loop).

¹ assuming the user_is_sleeping function returns False when false, the example code could be rewritten as the following for loop:

for _ in iter(user_is_sleeping, False):
    wait()