Why does using an integer value as a 'while' loop condition work in Python?

Question

The following code will output infinite lines of "test".

foo = 5
while foo:
    print("bar")

The other day I came across an answer here about digit sums. This was the code shown in the answer:

def digit_sum(t):
    s = 0
    while t:
        s += t % 10
        t //= 10
    return s

The part I'm focusing on is the "while t:" part. How and why does this work?

Could you explain your question in more detail? Because, as of now, my answer would be something like: "That's just the way it is". — Moon Cheesez, Jul 02 '16 at 15:48
You could have found the answer by reading the docs: https://docs.python.org/3/library/stdtypes.html#truth — Phillip, Jul 02 '16 at 15:48
Partly historical from C (and possibly earlier?) that zero is considered False. The `bool` class is a subclass of `int`, so they are closely related. Note that character `"0"` (zero) is True in Python, but False in Perl. — cdarke, Jul 02 '16 at 15:59
@LauraCookson: integer 0 is false in python, but character "0" is true. — cdarke, Mar 02 '17 at 15:41
@cdardke Well fancy that! I'm going to tell that to everyone in my class (school) and blow their minds. — Parallax Sugar, Mar 02 '17 at 15:44

score 4 · Accepted Answer · answered Jul 02 '16 at 15:49

The while condition tests for truth. Any non-zero numeric value is considered true. See the Truth Value Testing section in the Python documentation:

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:

None

False

zero of any numeric type, for example, 0, 0L, 0.0, 0j.

any empty sequence, for example, '', (), [].

any empty mapping, for example, {}.

instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False.

All other values are considered true — so objects of many types are always true.

Bold emphasis mine.

In your sample while loop, t trends to 0 (integer division by 10), so eventually while t: ends because t is considered false.

Note that `__nonzero__` is Python 2, renamed `__bool__` in Python 3. — cdarke, Jul 02 '16 at 16:01
I guess my question was not taken very well, but thanks a lot for the answer. — UnderscoreZero, Jul 02 '16 at 16:08

score 0 · Answer 2 · answered Jul 02 '16 at 18:06

You already got useful answers, but I just wanted to answer your question in a way that can be easily understood by someone who is a beginner in Python. You could rewrite your code as:

def digit_sum(t):
    s = 0
    while t!=0:
        s += t % 10
        t = t//10
    return s

'while t' is equivalent to 'while t!=0', meaning that the loop will end when t is equal to 0.

In your for loop, 't //= 10' is equivalent to 't = t // 10' ('//' is a floor division operator and returns an integer). So the value of t becomes smaller each time the loop is executed, until it eventually reaches the value of 0. At this point, the 'while t' condition is False and the loop ends.

Why does using an integer value as a 'while' loop condition work in Python?

2 Answers2