python返回none而不是True / False

Question

I have two programs for searching using binary search in Python

Program 1:

def bin(alist,x):
    if len(alist)==0:
        return False
    else:
        mid=len(alist)//2
        if (alist[mid]==x):
            return True 
        else:
            if alist[mid] < x:
                #print(alist[mid+1:],x)
                bin(alist[mid+1:],x)
            else:   
                #print(alist[:mid],x)
                bin(alist[:mid],x)

print (bin([2,3,5,8,9],8))
print (bin([2,3,5,8,9],7))

Program output:

None
None

Program 2:

def bin(alist,x):
     if len(alist)==0:
        return False
     else:
        mid=len(alist)//2
        if (alist[mid]==x):
           return  True
        else:
            if alist[mid]<x:
                return bin(alist[mid+1:],x)
            else:
                return bin(alist[:mid],x)
print(bin([1,5,7,8,9],10))
print(bin([1,4,5,8,9],8))

Program output:

False
True

Why is it so?

Answer 1

In your program 1, only when the list is empty or the value you are searching for in the middle of the list, it would returns you boolean value, that's because you explicitly say return if len(alist)==0 and return True when it meets if (alist[mid]==x): , you have to do the same for the rest conditions as well

def bin(alist,x):
    if len(alist)==0:
        return False
    else:
        mid=len(alist)//2
        if (alist[mid]==x):
            return True 
        else:
            if alist[mid] < x:
                #print(alist[mid+1:],x)
                bin(alist[mid+1:],x)   # -------> return 
            else:   
                #print(alist[:mid],x)
                bin(alist[:mid],x)  # -------> return 

When you invoke your bin() method recursively and expect a boolean value, you have to add return in the above highlighted lines.