为什么此递归求和函数返回None？

Question

def recursiveadd(x,sum1):
    if x > 0:
        sum1 += x
        recursiveadd(x-1,sum1)
    else:
        return sum1

print recursiveadd(100,0)

Inserting a "print sum1" after the addition shows that sum1 is being increased, so I don't understand why the function returns None. The only thing I can think of is that sum1 is somehow being reset to 0 before being returned, but I have no idea why that would be.

Answer 1

You need to write

def recursiveadd(x,sum1):
    if x > 0:
        sum1 += x
        return recursiveadd(x-1,sum1)
    else:
        return sum1

print recursiveadd(100,0)

Answer 2

In [52]: def rec(x, sum = None):
if sum == None:
    sum = 0
if x > 0:
    sum += x
    return rec(x - 1, sum)
else:          
    return sum

....:

In [53]: rec(100)
Out[53]: 5050