Return Value of None?

Question

I'm trying to solve a problem on CodeWars with the following specifications:

"Write a function, persistence, that takes in a positive parameter num and returns its multiplicative persistence, which is the number of times you must multiply the digits in num until you reach a single digit."

My solution is as follows:

def persistence(n,count = 0):
    n = list(str(n))
    if len(n)  <= 1 :
        count = 0
        return count
    else :
        num_per = 1
        for i in n :
            num_per = num_per * int(i)
        num_dump = num_per
        if len(str(num_dump)) > 1:
            count += 1
            n = num_per
            persistence(n,count)
        else:
            count = count + 1
            return count

When I choose any number with more than a single digit, I get a return value of 'None'. I've narrowed down the problem to the last return statement in the second else clause. The number of counts are calculated correctly,but the return statement still returns a value of 'None'. Any help would be greatly appreciated.

You're getting the `None` from falling off the end of the function, rather than any of your explicit `return` statements. — jasonharper, Jul 04 '19 at 02:57
No, you need to return the result of your recursive call, like any other function call — juanpa.arrivillaga, Jul 04 '19 at 02:58

score 0 · Accepted Answer · answered Jul 04 '19 at 03:06

0

Just add return before your persistence line:

return persistence(n, count)

answered Jul 04 '19 at 03:06

Akaisteph7

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score 0 · Answer 2 · answered Jul 04 '19 at 03:07

Going off of what @jasonharper suggested, your function has an if/else statement which is decided by len(n) <= 1. If the length of n is less than 1, count (which is 0) is returned.

In your else statement however, you have another if/else statement. In your if statement, which is decided by len(str(num_dump)) > 1, you decide to recursively call the same function, however, you return no value. Therefore, your program most likely makes a lot of recursive calls but has no way of storing the values to be returned at the end of the program.

This can be fixed by replacing

persistence(n, count)

with

return persistence(n, count)

Return Value of None?

2 Answers2