python内部函数怎么修改外部函数的值?

def b():
    b = 1
    def bchange():
        b += 1  #UnboundLocalError: local variable 'b' referenced before assignment 
    bchange()
    print b

Python 3:

def b():
  b = 1
  def bchange():
      nonlocal b
      b += 1
  bchange()
  print(b)

Python 2 只能这样（利用 mutable 对象）：

def b():
  b = [1]
  def bchange():
      b[0] += 1
  bchange()
  print b[0]