Why does it work?

Question

So I'm learning Python and working through a list of program ideas. Of course I wrote the obligatory FizzBuzz, which worked, but was basically if elif else blablabla. I googled it to see if there are other ways and found this dank one-liner:

for i in range(1,101): 
    print("Fizz" * (i % 3 == 0) + "Buzz" * (i % 5 == 0) or i)

There are no ifs, no elifs, nothing. I googled "string concatenation" and found info on the * symbol, but don't understand how it's working in this case. Can somebody explain?

Answer 1

There are no ifs, no elifs, nothing.

Sure there are! Just disguised. Looking for string concatenation (i.e. +) won't help you, because * is repetition. Specifically, string * n gives you a string that is n copies of string in a row. Further, a boolean value can be implicitly converted to an integer: True becomes 1 and False becomes 0. So

"Fizz" * (i % 3 == 0)

means "One Fizz if i % 3 == 0, none if not." Same with Buzz.

Finally, that or i at the end means that if you got the empty string because both parts came up empty, then you get i instead. or really means "the value of the left hand side unless the left hand side is a false value, in which case return the value of the right hand side."

This trick gets used elsewhere, too. Python doesn't have a direct equivalent to C's ?: operator, but you can get close to one with a two-element tuple and an index operation because of the bool-to-integer conversion I mentioned above. So C's

a? b: c

which means "b if a is true, otherwise c" becomes this in Python:

(c, b)[a]

Answer 2

Break it out and you'll understand it.

def does_it_fizz(num):
    return num % 3 == 0

def does_it_buzz(num):
    return num % 5 == 0

for num in range(1, 101):
    print("Fizz" * does_it_fizz(num) + "Buzz" * does_it_buzz(num) or num)

String multiplication repeats the string, so 'a' * n is aaaaa...n times...a. if i % 3 != 0, then does_it_fizz(i) returns 0. "any string" * 0 == "". If the number should neither Fizz nor Buzz, you get print("" or num). The empty string is Falsey, but num is always Truthy, so it prints num