How Can I Prevent Shared Class Data Among Object Instances in Python?

Preventing Shared Class Data Among Instances

In object-oriented programming, it's crucial to maintain distinct data for different instances of a class. However, by default, class-level variables are shared among all instances, which can lead to unexpected behaviors.

Understanding the Issue

Consider the following code:

class a:
    list = []

x = a()
y = a()

x.list.append(1)
y.list.append(2)
x.list.append(3)
y.list.append(4)

print(x.list) # prints [1, 2, 3, 4]
print(y.list) # prints [1, 2, 3, 4]

Instead of obtaining separate lists for x and y, both instances share the same list. This occurs because list is declared as a class variable, so all instances reference the same underlying object.

Solution: Instance Variables

To avoid shared data, instance variables should be utilized. Instance variables are defined within the constructor (__init__) of the class. Here's the corrected code:

class a:
    def __init__(self):
        self.list = []

x = a()
y = a()

x.list.append(1)
y.list.append(2)
x.list.append(3)
y.list.append(4)

print(x.list) # prints [1, 3]
print(y.list) # prints [2, 4]

In this case, the list attribute is initialized for each instance in the constructor, creating separate copies. The print statements now accurately reflect the intended behavior with distinct lists for x and y.

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