Custom class instance copying

Question

Hi,

I'm new to programming and Python. The problem I have is with removing list elements that are instances of custom class.

import copy

class some_class:
    pass

x = some_class()
x.attr1 = 5
y = some_class()
y.attr1 = 5

z = [x,y]
zcopy = copy.deepcopy(z)
z.remove(zcopy[0])

This returns: ValueError: list.remove(x): x not in list

Is there a simple way to remove element from list by using reference from deepcopied list?

edit: Thanks for your answers. I found some solution using indexing. It's not pretty but it does the job:

import copy

class some_class:
    pass

x = some_class()
x.attr1 = 5
y = some_class()
y.attr1 = 5

z = [x,y]
zcopy = copy.deepcopy(z)
del z[zcopy.index(zcopy[0])]

Answer 1

No, because the call to deepcopy creates a copy of the some_class instance. That copy, zcopy[0] is a different object from the original, z[0], so when you try to remove zcopy[0] from the list z, it rightly complains that the copy doesn't exist in the original list. Furthermore, there is no link between the copied object and the original object, which is the intent of deepcopy.

I suppose you could implement a __deepcopy__ method in your class, which returns a copy that maintains some reference to the original object. Then you could use that reference to get the original object, z[0], from the copy, zcopy[0]. It strikes me as a rather odd thing to do, though, and probably not a good idea. Without further information, I'd suggest just using copy.copy instead of copy.deepcopy.