Changing a value in one list changes the values in another list with a different memory ID

Question

I have been beating my head against a wall over this problem. I create a list and make 4 copies, only one of which shares the same memory index. If I change the original list, is somehow changes 3 of those copies as well, 2 of which have a different memory index. Only if I make a list using the same command as the original, am I able to create a list that is not impacted by changes to the original. How is this possible? Here is the output from my console:

>>> orig=[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]
>>> id(orig)
151498220
>>> copy1=orig   #same index
>>> id(copy1)
151498220
>>> copy2=orig[:]   #different index
>>> id(copy2)
151498348
>>> copy3=list(orig)   #different index
>>> id(copy3)
151503020
>>> copy4=[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]
>>> id(copy4)
151498636
>>> orig[0][1]=34
>>> copy1
[[0, 34, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]   #expected, same memory index
>>> copy2
[[0, 34, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]   #WTF?!?!?
>>> copy3
[[0, 34, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]   #ARGH!!!
>>> copy4
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]   #workaround?
>>> id(orig)
151498220
>>> id(copy1)
151498220
>>> id(copy2)
151498348
>>> id(copy3)
151503020
>>> id(copy4)
151498636

The memory indices did not change and yet the lists were altered. Only copy1 should have changed as it has the same memory index as orig.

Answer 1

That's because you are just creating a shallow copy. You need to create a deep copy instead.

As per copy module doc:

• A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.
• A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

You can verify it by comparing the id of inner list:

>>> orig=[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]
>>> id(orig)
151498220
>>> copy2=orig[:]   #different index
>>> id(copy2)
151498348

>>> id(copy2[0]) == id(orig[0])  # inner list have same id
True

You can create a deepcopy using copy.deepcopy(x):

>>> import copy
>>> 
>>> copy3 = copy.deepcopy(orig)
>>> 
>>> id(copy3[0]) == id(orig[0])   # inner list have different id
False

>>> orig[0][3] = 34
>>> 
>>> orig
[[0, 34, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]
>>> copy3
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 123, 0]]