loops – When to use while or for in Python

loops – When to use while or for in Python

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 isnt preference. Its 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 dont have a tidy data structure to iterate through, or you dont have a generator function that drives your processing, you must use while.

while is useful in scenarios where the break condition doesnt logically depend on any kind of sequence. For example, consider unpredictable interactions:

 while user_is_sleeping():

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):

loops – When to use while or for in Python

The for is the more pythonic choice for iterating a list since it is simpler and easier to read.

For example this:

for i in range(11):
    print i

is much simpler and easier to read than this:

i = 0
while i <= 10:
    print i
    i = i + 1

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