My function for binary search returns None, I can't figure out why

Question

I am implementing binary search in Python, I created a function for it but somehow, I am kinda stuck in between. My funtion returns "None" as the output and I can't figure out why it is doing that. My code here:

x = [1,2,3,4,5,6,7,8,9,10]
y = 9

def bin_search(s_list, key):
    print(s_list)
    m = s_list[len(s_list)//2]
    if m == key:
        return 1
    elif m < key:
        bin_search(s_list[s_list.index(m)+1:],key)
    else:
        bin_search(s_list[0:s_list.index(m)],key)

print( bin_search(x, y)) It gives, the folowing output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[7, 8, 9, 10]
None

Answer 1

When you make the recursive calls make sure to return the values so they're passed up the stack to the caller. Just writing bin_search(...) ignores the return value.

if m == key:
    return 1
elif m < key:
    return bin_search(s_list[s_list.index(m)+1:],key)
else:
    return bin_search(s_list[0:s_list.index(m)],key)

Answer 2

I would like to add to @Greg's answer, that if you want to retuen 1 to main calling function then you should return your return value after recursion. Like:

def bin_search(s_list, key):
    print(s_list)
    m = s_list[len(s_list)//2]
    if m == key:
        return 1
    elif m < key:
        return bin_search(s_list[s_list.index(m)+1:],key)
    else :
        return bin_search(s_list[0:s_list.index(m)],key)

x = [1,2,3,4,5,6,7,8,9,10]
y = 7    
print( bin_search(x, y))

You should also handle for the condition of no key found.

Answer 3

This is because your code is working as expected:

def bin_search(s_list, key):
    print(s_list)
    m = s_list[len(s_list)//2]
    if m == key:
        print('if')
        return 1
    elif m < key:
        print('elif')
        print(s_list[s_list.index(m)+1:])
        bin_search(s_list[s_list.index(m)+1:],key)
    else:
        print('else')
        print(s_list[0:s_list.index(m)])
        bin_search(s_list[0:s_list.index(m)],key)

Adding in debug prints helps to pinpoint what's going on.

print( bin_search(x, y)) 
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
elif
[7, 8, 9, 10]
[7, 8, 9, 10]
if
None

So there is actually an instance where m==key.