Python—global vs local variables

# num1 is a global variable


num1 = 1
print num1


# num2 is a local variable


def fun():
    num1 = 2
    num2 = num1 + 1
    print num2
    
fun()


# the scope of global num1 is the whole program, num 1 remains defined
print num1


# the scope of the variable num2 is fun(), num2 is now undefined
print num2

结果：

1
3
1
 NameError: name 'num2' is not defined

example2：

num = 4

def fun1():
    global num
    num = 5
    
def fun2():
    global num
    num = 6

# note that num changes after each call with no obvious explanation    
print num
fun1()
print num
fun2()
print num

结果：

4
5
6

在函数外定义的是全局变量，函数内部定义的是局部变量。如果全局变量在函数中使用了并更改了其值，则会生成一个同名的局部变量。所以在函数中引用全局变量并改变其值，要加上global关键字。