python引用變量的順序： 當前作用域局部變量->外層作用域變量->當前模塊中的全局變量->python內置變量

一、global

global關鍵字用來在函數或其他局部作用域中使用全局變量。但是如果不修改全局變量也可以不使用global關鍵字。

代碼如下:
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關鍵字用來在函數或其他作用域中使用外層(非全局)變量。

代碼如下:
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來實現類似的counter。如下：

代碼如下:
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