Well, thats because python, to save memory and processing time with lists, passes by reference, not by copy. The way to do what you want is this: >>> a=[[5]] >>> b=[a[0][:]] >>> a[0][0]=3 >>> b [[5]] However, if you are going to be passing around lists, and want copies to modify, then do this: b=a[:] to do a one-level copy of the list's contents. Any lists in the list, as above, will also be passed by reference, not copy. If you do have nested lists, then take a look at the copy module.