Python list return 'None'

Question

Code:

puzzle1= [
          [7,0,0,0,0,0,2,1,8],
          [0,4,8,6,2,9,0,0,0],
          [0,0,3,0,0,1,0,0,0],
          [0,0,7,0,0,8,0,3,2],
          [0,0,9,7,0,6,5,0,0],
          [6,8,0,1,0,0,7,0,0],
          [0,0,0,2,0,0,4,0,0],
          [0,0,0,4,1,5,8,7,0],
          [3,5,4,0,0,0,0,0,6]
          ]  
def eliminate_values(puzzle):
    redo = False
    for i in range(9):
        for j in range(9):
            if puzzle[i][j]==0 or isinstance(puzzle[i][j], list):
                puzzle[i][j] = []
                for num in range(1,10):
                   num_check = True;
                   for x in range(9):
                       if puzzle[i][x]==num:
                           num_check = False
                       if puzzle[x][j]==num:
                           num_check = False
                       if i<3:
                           aa=0
                       elif i<6 and i>2:
                           aa=3
                       else:
                           aa=6
                       if j<3:
                           bb=0
                       elif j<6 and j>2:
                           bb=3
                       else:
                           bb=6
                       for a in range(3):
                           for b in range(3):
                               if puzzle[a+aa][b+bb]==num:
                                   num_check = False
                   if num_check:
                       puzzle[i][j].append(num)
                if len(puzzle[i][j]) == 1:
                    puzzle[i][j] = puzzle[i][j][0]
                    redo = True;
    if redo:
        eliminate_values(puzzle)
    else:
        print(puzzle)
        return puzzle

puzzle=eliminate_values(puzzle1)
print(puzzle)

Console:

[[7, 9, 6, 3, 5, 4, 2, 1, 8], [1, 4, 8, 6, 2, 9, 3, 5, 7], [5, 2, 3, 8, 7, 1, 9, 6, 4], [4, 1, 7, 5, 9, 8, 6, 3, 2], [2, 3, 9, 7, 4, 6, 5, 8, 1], [6, 8, 5, 1, 3, 2, 7, 4, 9], [8, 7, 1, 2, 6, 3, 4, 9, 5], [9, 6, 2, 4, 1, 5, 8, 7, 3], [3, 5, 4, 9, 8, 7, 1, 2, 6]]
None

Comments:

I am new to python but i dont understand why print IS working within the function and NOT after it is returned to the main program. (expecting it to print twice but only prints once then 'None')

Answer 1

@tobias_k is right.

In every recursive function you have a base case and a recursive case. The base case is when you reach the end of your recursion and return the final value from your recursive function. The recursive case is where the function calls itself again.

You need to be returning in both cases though.

If you don't, then even if you're eventually hitting your base case, the return value of the base case doesn't get passed up the stack.

ie:

def recursiveDecrement(x):
    if x > 0:
        print("Recursive case. x = %s" %x)
        recursiveDecrement(x - 1)
        print("I should have returned...x = %s" %x)
    else:
        print("Base case. x = %s" %x)
        return x

If I call recursiveDecrement(5) my output would be:

Recursive case. x = 5
Recursive case. x = 4
Recursive case. x = 3
Recursive case. x = 2
Recursive case. x = 1
Base case. x = 0
I should have returned...x = 1
I should have returned...x = 2
I should have returned...x = 3
I should have returned...x = 4
I should have returned...x = 5

However, once the base case is hit, the method just continues to execute and at the end nothing is returned and x is still equal to 5.

Change your if statement to return in both cases and everything should work.

if redo:
    return eliminate_values(puzzle)
else:
    return puzzle

Answer 2

If redo is True , you are recursively calling your function, and somewhere down the stack, once redo is False , you print and return the result. However, this result is not propagated up the call stack, thus the outermost function call will return nothing, ie None , which is then printed. For this, you also have to return the result of the recursive call:

    if redo:
        return eliminate_values(puzzle)  # try again and return result
    else:
        return puzzle  # result found in this try, return it

Alternatively, instead of using recursion, you could wrap your function body in a while loop.