1. 内部函数，不修改全局变量可以访问全局变量
2. 内部函数，修改同名全局变量，则python会认为它是一个局部变量
3. 在内部函数修改同名全局变量之前调用变量名称（如print sum），则引发Unbound-LocalError
   

如果内部函数有引用外部函数的同名变量或者全局变量,并且对这个变量有修改.那么python会认为它是一个局部变量,又因为函数中没有sum的定义和赋值，所以报错。

测试：

import threading
import time

COUNT=10

def printHello():
	print(COUNT)
	COUNT = COUNT -1

def main():
	threads = []
	for i in range(0,1):
		t = threading.Thread(target=printHello)
		threads.append(t)
	for t in threads:
		t.start()
	for t in threads:
		t.join()

if __name__ == '__main__':
	main()

import threading
import time

LOCK = threading.Lock()
COUNT=10

def printHello():
	global COUNT
	print(COUNT)
	LOCK.acquire()
	COUNT = COUNT -1
	LOCK.release()

def main():
	threads = []
	for i in range(0,10):
		t = threading.Thread(target=printHello)
		threads.append(t)
	for t in threads:
		t.start()
	for t in threads:
		t.join()

if __name__ == '__main__':
	main()