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python中global与nonlocal比较

2017-08-14

python中global与nonlocal比较

python引用变量的顺序：当前作用域局部变量->外层作用域变量->当前模块中的全局变量->python内置变量

一、global

global关键字用来在函数或其他局部作用域中使用全局变量。但是如果不修改全局变量也可以不使用global关键字。

代码如下:
gcount = 0

def global_test():
    print (gcount)

def global_counter():
    global gcount
    gcount +=1
    return gcount

def global_counter_test():
    print(global_counter())
    print(global_counter())
    print(global_counter())

二、nonlocal

nonlocal关键字用来在函数或其他作用域中使用外层(非全局)变量。

代码如下:
def make_counter():
    count = 0
    def counter():
        nonlocal count
        count += 1
        return count
    return counter

def make_counter_test():
mc = make_counter()
print(mc())
print(mc())
print(mc())

也可以使用generator来实现类似的counter。如下：

代码如下:

def counter_generator():
    count = 0
    while True:
        count += 1
        yield count

def counter_generator_test():
# below is for python 3.x and works well
citer = counter_generator().__iter__()
i = 0
while(i < 3) :
    print(citer.__next__())
    i+=1

def counter_generator_test2():
#below code don't work
#because next() function still suspends and cannot exit
#it seems the iterator is generated every time.
j = 0
for iter in counter_generator():
    while(j < 3) :
      print(iter)
      j+=1