when you append a list to a list, something needs to be noted:
1 a = 1 2 b = [] 3 for i in range(4): 4 a = a + b 5 b.append(a)
the result is right, but when the a is a list like a=[1,1,1,1]:
1 a = [-1,-1,-1,-1] 2 b = [] 3 for i in range(4): 4 a[i] = 1 5 b.append(a)
the result will be wrong with what we need.
Due to the append use the object, and the result will be the object's result.