How to re-initialize a class object in Python?

Question

I have the following Python class

class MyClass:
    def __init__(self, my_list = []):
        self.my_list = my_list
    
    def addItem(self, item):
        self.my_list.append(item)

And I'm trying to create several objects from that class. However, my_list continues to be shared between the different instances.

For example, with the following two initializations

first_instance = MyClass()
first_instance.addItem("One")
first_instance.addItem("Two")

print(first_instance.my_list)

second_instance = MyClass()
print(second_instance.my_list)

I get the following output

['One', 'Two']
['One', 'Two']

I would have expected the second_instance.my_list to produce an empty list.

What am I missing?

score 4 · Accepted Answer · answered Apr 02 '22 at 11:04

4

Change to

class MyClass:
    def __init__(self, my_list = None):
        self.my_list = my_list if my_list is not None else []

answered Apr 02 '22 at 11:04

md2perpe

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It works, thanks! But what's wrong with my implementation? – Tarek Oraby Apr 02 '22 at 11:07
1

@TarekOraby. The default values are assigned to the method when the method is created. Therefore all calls will share the same object. – md2perpe Apr 02 '22 at 11:12
1

@TarekOraby. Here's another thread about this: https://stackoverflow.com/q/1132941/1412534 – md2perpe Apr 02 '22 at 11:13

How to re-initialize a class object in Python?

1 Answers1