python关于调用函数外的变量实例

class Solution(object):
  def foo(self, s):
    def bar(a):
      s += a
      print s
    bar("aa")
Solution().foo("ss")

UnboundLocalError: local variable 's' referenced before assignment

class Solution(object):
  def foo(self, s):
    def bar(a):
      print s
    bar("aa")
Solution().foo("ss")