[Python] global，nonlocal

（1）global：修改全局变量

a=1
def f():
    a=2

    def g():
        global a
        a=3
        print(a)    #3

    g()
    print(a)    #2

f()
print(a)    #3

（2）nonlocal：修改外层词法变量

a=1
def f():
    a=2

    def g():
        nonlocal a
        a=3
        print(a)    #3

    g()
    print(a)    #3

f()
print(a)    #1

（3）default：创建局部变量

a=1
def f():
    a=2

    def g():
        a=3
        print(a)    #3

    g()
    print(a)    #2

f()
print(a)    #1