While function with string argument

Question

I am trying to learn Python and can't get my head around why this while loop I found breaks:

while string:
    print("Test")
    string = string[3:]

With for example

string = "123456789"

The output is:

Test
Test
Test

The way I understood the while function is, that it breaks only when the expression following the while command is False. But how can string = string[3:] be False, I mean it just changes string form being "123456789" to being "456789"?

Thanks for any tips!

Python, like JavaScript and PHP has "falsy" values. An empty-string is "falsy", for example. Your string starts off as "123456789", then it's "456789", then it's "789", and then it's "", which is considered false and the while loop breaks. — Dai
– Dai, Commented Oct 21, 2020 at 4:31
If you don't understand how a program works by looking at it, grab a sheet of grid-lined paper and execute it by-hand. You don't need a computer to do computations :) — Dai
– Dai, Commented Oct 21, 2020 at 4:32
Empty strings are Falsy. freecodecamp.org/news/truthy-and-falsy-values-in-python — susenj
– susenj, Commented Oct 21, 2020 at 4:37

Mohsin Ali · Accepted Answer · 2020-10-21 04:46:19Z

1

after the first iteration, we've

string = 456789

after the second iteration

string = 789

after that when we assign string[3:] (now this becomes '' empty string) which is falsy value and loop exits

answered Oct 21, 2020 at 4:46

Mohsin Ali

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1 Comment

PythonLearner101 Over a year ago

Thank you! I forgot that it removes parts of the string every iteration.

Digvijay Shekhawat · Accepted Answer · 2020-10-21 05:08:21Z

0

Since you have put String = string[3:] In the iteration, every time the loop executes it will remove the first 3 digits making it [4,5,6,7,8,9] after first iteration Then [7,8,9] after second iteration And finally [] after third iteration Which makes it false

answered Oct 21, 2020 at 5:08

Digvijay Shekhawat

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