python 传入参数返回的时候好像有些时候会出现莫名其妙的循环

def handle_field(name, s_len, s):
   #some code
   #return s would error but return not....
   #return s

for ...
  s += handle_field(name, s_len, s)'

后来发现

def foo(s)
    .....
    return s

这样传入参数与传出参数一样的话会出问题