UnboundLocalError: local variable 'c' referenced before assignment

问题代码：

def outer():
    c = 0
    def inner():
        c += 1
        print 'inner'
        print c
    print 'outer'
    return inner()

outer()

报错：

UnboundLocalError: local variable 'c' referenced before assignment

解决方法：
1.python2.7使用global

c = 0
def outer():
    def inner():
        global c            # python2.7使用global
        c += 1
        print 'inner'
        print c
    print 'outer'
    return inner()

outer()
outer()
outer()

输出：

outer
inner
1
outer
inner
2
outer
inner
3

2.python3使用nonlocal

def outer():
    c = 0
    def inner():
        nonlocal c          # python3使用nonlocal
        c += 1
        print ('inner')
        print (c)
    print ('outer')
    return inner()

outer()
outer()
outer()

参考：https://www.cnblogs.com/thinking-jxj/p/7681415.html

另外，

def outer():
    c = 0
    def inner():
        print 'inner'
        print c             # 直接访问不修改值，不报错
    print 'outer'
    return inner()

outer()
outer()
outer()

输出：

outer
inner
0
outer
inner
0
outer
inner
0