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。