How Can I Avoid Shared Class Data Issues in Object-Oriented Programming?

Overcoming Shared Class Data Issue

In object-oriented programming, it's desirable to keep class data distinct for each instance. However, a common issue arises when class data is accidentally shared among instances, leading to unexpected behavior.

The Problem:

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]

In this example, instances x and y of class a share the same list. As a result, appending elements to x.list also adds them to y.list, violating the intended behavior.

The Solution:

To prevent shared class data, instance members should be declared inside individual instance methods instead of the class declaration. In Python, the __init__ method is commonly used for this purpose.

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

By initializing the list variable within the __init__ method, each instance of a will have its own independent copy of the list.

Expected Behavior:

With this modification, the intended behavior can be achieved:

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]

Separating class data using this approach guarantees that each instance has its own unique set of data, eliminating the issue of shared data among instances.

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