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