Recursive function in python returns None

Question

I have the following simple code to compute the sum of values until the indicated number. The total is stored inside result.

def ComputeMath(number, result):
   if number > 0:
       result = number + result
       print('if', result)
       ComputeMath(number-1, result)
   else:
       print("else", result)
       return result;

output = ComputeMath(5,0)
print(output)

For some reason output is always None. Printing it inside the function works fine. I just cannot figure out why this is happenning. Here is the output:

In the `if` branch you're _calling_ `ComputeMath()` but you're not _returning_ the result. — John Gordon, Oct 21 '22 at 23:14

cards · Answer 1 · 2022-10-21T23:13:41.587

The conditional split the code flow/execution among two paths but only one of them has a return, hence the other one will implicitly return None

def ComputeMath(number, result):
   if number > 0:
       result = number + result
       print('if', result)
       return ComputeMath(number-1, result)
   else:
       print("else", result)
       return result

output = ComputeMath(5,0)
print(output)

Here another possibility, a return for "all":

def ComputeMath(number, result):
   if number > 0:
       result = number + result
       print('if', result)
       ComputeMath(number-1, result)
   else:
       print("else", result)
   return result

Another way to look at the problem is not to print the output since the print is already in the body of the function.

def ComputeMath(...): # as in the question
   ....

output = ComputeMath(5,0) # the output maybe None
if output is not None:
    print(output)

Recursive function in python returns None

1 Answers1