无法修改全局变量

list_m = None
# 
# 1.[2,0,2,0]--->[2,2,0,0]
# 1.[2,0,0,2]--->[2,2,0,0]

def zero_to_end():

 print(list_m)
 for r in range(-1,-len(list_m)-1,-1):
 if list_m[r]==0:
 del list_m[r]
 list_m.append(0)

zero_to_end()
print(list_m)

def merge():
 '''
 先将中间的元素移到末尾
 再将相邻元素合并
 :return:
 '''
 zero_to_end()
 for i in range(0,len(list_m)-1):
 if list_m[i]==list_m[i+1]:
 list_m[i]+=list_m[i+1]
 del list_m[i+1]
 list_m.append(0)
merge()
print(list_m)
list_m = None
map=[
 [2,0,0,2],
 [4,4,2,2],
 [2,4,0,4],
 [0,0,2,2],
]



def move_left():
 for line in map:
 global list_m
 list_m = line
 # merge() #merge方法开始操作


最后的这函数for line in map: global list_m list_m = line 这句话赋值给全局变量list_m 但就是赋值不到啊 list_m一直就是None 这是为什么啊？？

来源：oschina

链接：https://my.oschina.net/u/4406496/blog/4260720