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The while loop

In the preceding pages, we saw the for loop in action. It's incredibly useful when you need to loop over a sequence or a collection. The key point to keep in mind, when you need to be able to discriminate which looping construct to use, is that the for loop rocks when you have to iterate over a finite amount of elements. It can be a huge amount, but still, something that ends at some point.

There are other cases though, when you just need to loop until some condition is satisfied, or even loop indefinitely until the application is stopped, such as cases where we don't really have something to iterate on, and therefore the for loop would be a poor choice. But fear not, for these cases, Python provides us with the while loop.

The while loop is similar to the for loop, in that they both loop, and at each iteration they execute a body of instructions. What is different between them is that the while loop doesn't loop over a sequence (it can, but you have to write the logic manually and it wouldn't make any sense, you would just want to use a for loop), rather, it loops as long as a certain condition is satisfied. When the condition is no longer satisfied, the loop ends.

As usual, let's see an example that will clarify everything for us. We want to print the binary representation of a positive number. In order to do so, we can use a simple algorithm that collects the remainders of division by 2 (in reverse order), and that turns out to be the binary representation of the number itself:

6 / 2 = 3 (remainder: 0) 
3 / 2 = 1 (remainder: 1) 
1 / 2 = 0 (remainder: 1) 
List of remainders: 0, 1, 1. 
Inverse is 1, 1, 0, which is also the binary representation of 6: 110

Let's write some code to calculate the binary representation for the number 39: 100111₂:

# binary.py
n = 39
remainders = []
while n > 0:
    remainder = n % 2  # remainder of division by 2
    remainders.insert(0, remainder)  # we keep track of remainders
    n //= 2  # we divide n by 2

print(remainders)

In the preceding code, I highlighted n > 0, which is the condition to keep looping. We can make the code a little shorter (and more Pythonic), by using the divmod function, which is called with a number and a divisor, and returns a tuple with the result of the integer division and its remainder. For example, divmod(13, 5) would return (2, 3), and indeed 5 * 2 + 3 = 13:

# binary.2.py
n = 39
remainders = []
while n > 0:
    n, remainder = divmod(n, 2)
    remainders.insert(0, remainder)

print(remainders)

In the preceding code, we have reassigned n to the result of the division by 2, and the remainder, in one single line.

Notice that the condition in a while loop is a condition to continue looping. If it evaluates to True, then the body is executed and then another evaluation follows, and so on, until the condition evaluates to False. When that happens, the loop is exited immediately without executing its body.

If the condition never evaluates to False, the loop becomes a so-called infinite loop. Infinite loops are used, for example, when polling from network devices: you ask the socket whether there is any data, you do something with it if there is any, then you sleep for a small amount of time, and then you ask the socket again, over and over again, without ever stopping.

Having the ability to loop over a condition, or to loop indefinitely, is the reason why the for loop alone is not enough, and therefore Python provides the while loop.

By the way, if you need the binary representation of a number, check out the bin function.

Just for fun, let's adapt one of the examples (multiple.sequences.py) using the while logic:

# multiple.sequences.while.py
people = ['Conrad', 'Deepak', 'Heinrich', 'Tom']
ages = [29, 30, 34, 36]
position = 0
while position < len(people):
    person = people[position]
    age = ages[position]
    print(person, age)
    position += 1

In the preceding code, I have highlighted the initialization, condition, and update of the position variable, which makes it possible to simulate the equivalent for loop code by handling the iteration variable manually. Everything that can be done with a for loop can also be done with a while loop, even though you can see there's a bit of boilerplate you have to go through in order to achieve the same result. The opposite is also true, but unless you have a reason to do so, you ought to use the right tool for the job, and 99.9% of the time you'll be fine.

So, to recap, use a for loop when you need to iterate over an iterable, and a while loop when you need to loop according to a condition being satisfied or not. If you keep in mind the difference between the two purposes, you will never choose the wrong looping construct.

Let's now see how to alter the normal flow of a loop.