Can someone explain this python behavior to me?

Question

Can someone explain this behavior? When I run the code, it prints 10, then 20. Why is list_of_classes being changed even though I only changed the value of bob? Shouldn't I have to update the list with the new values?

class wooo():
    def __init__(self,x,y,name):
        self.x=x
        self.y=y
        self.name=name

bob=wooo(10,10,"bob")
joe=wooo(10,10,"joe")
list_of_classes=[bob,joe]
print(list_of_classes[0].x)
bob.x=20
print(list_of_classes[0].x)

Actual Output

10
20

Expected Output

10
10

Martijn Pieters · Accepted Answer · 2014-06-27 15:10:47Z

Your lists contain references to the objects, not copies.

list_of_classes[0] is a reference to the same object that bob references. You can create more references to the same object and the attribute change would be visible through all those references:

>>> class wooo():
...     def __init__(self,x,y,name):
...         self.x=x
...         self.y=y
...         self.name=name
... 
>>> bob=wooo(10,10,"bob")
>>> guido = bob
>>> guido.x
10
>>> guido.x = 20
>>> bob.x
20
>>> guido is bob
True

If you wanted to add copies of a class to the list, use the copy module to create a deep copy of your instance:

>>> import copy
>>> robert = copy.deepcopy(bob)
>>> robert.x
20
>>> bob.x = 30
>>> robert.x
20
>>> robert is bob
False

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