Are linked variables linked for all perpertuity or for specific scenarios?

Question

In a quiz I received the following code to find the result:

list1 = [1,2,3,4];
list2 = list1;
list1.append(5);
print(list2)

>>> [1,2,3,4,5] was the result.
Now that seemed strange to me since the append happened after the assignment and I would have assumed that the changes should only occur on list1. Do they remain linked forever or is the append a special situation?

Answer 1

"Reference" means that when you copy a variable containing a list, you only copy the memory address where it's stored... so modifying any referenced copy, will be effective for all referenced copy...

To make a real copy of a list ( not by reference, but cloning items, so the list would be modifiable without affecting original ), you need to use the slice notation to explicitly ask interpreter to made a deep copy:

list1 = [1,2,3,4];
list2 = list1[:];
list1.append(5);
print(list2)

>>> [1,2,3,4] is the result.

http://pythoncentral.io/how-to-slice-listsarrays-and-tuples-in-JUMP_LINK__&&__python__&&__JUMP_LINK/

Answer 2

@Amaras A:

That's a complement... as you've said the essential by giving the real answer... mine just specify what's the concept of reference, and how to workaround ;)

Answer 3

list1 and list2 are actually REFERENCES to the same memory object. since they are references, change in one implies the same change in the other.

Answer 4

Are they not 2 separate lists? And therefore different?

Answer 5

Thanks visph. That's a better explanation than mine