#Case 1
myList=[1,2,3,4]
old=myList
myList=[5,6,7,8]
print(old)
#Case 2
myList=[1,2,3,4]
old=myList
myList[0]=10
print(old)
#Case 3
myList=[1,2,3,4]
old=myList.copy()
myList[0]=10
print(old)
[1, 2, 3, 4]
[10, 2, 3, 4]
[1, 2, 3, 4]
For me the case 3 is the safe case and Case 2 is clear. However, I am not able to clearly understand why in case 1 old is not changed.
In case 1, we are re-assigning a brand new list to the name myList. The original list that was assigned to myList is not affected by this operation; myList is now simply pointing to a different object
This becomes clear when we look at the ids of the objects:
>>> myList = [1,2,3,4]
>>> print(id(myList))
47168728
>>> old = myList
>>> print(id(old))
47168728
>>> myList = [5,6,7,8]
>>> print(id(myList))
47221816
>>> print(id(old))
47168728
Writing old = myList does not bind the two variables inextricably; it assigns the value of myList to old at that point in time. By reassigning myList to a new list afterwards, you are then making myList and old point to different values.
