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Understanding Python Class and Instance Variables

In Python, class and instance variables play crucial roles in defining and modifying object behaviors. However, their distinct characteristics can lead to confusion. This article aims to clarify their differences and explain why a list can appear as both a class and instance variable.

Class Variables vs Instance Variables

Class variables are declared at the class level and are shared by all instances of that class. They are typically defined outside of the constructor (__init__()) method. Instance variables, on the other hand, are specific to each instance and are defined within the constructor using the self attribute.

Example 1: Class Variable

The following code creates a class with a class variable list and an instance variable self.list:

<code class="python">class testClass():
    list = []  # Class variable
    def __init__(self):
        self.list.append('thing')  # Instance variable</code>

In this example, testClass.list is a class variable that all instances of testClass will share. When the constructor is called, it appends 'thing' to self.list, creating an instance variable for each instance. As a result, when you access p.list and f.list, they both output ['thing'] because they share the same class variable.

Example 2: Instance Variable

In contrast, when you remove the list class variable and redefine it within the constructor:

<code class="python">class testClass():
    def __init__(self):
        self.list = []
        self.list.append('thing')</code>

Each instance of testClass will now have its own self.list instance variable. When you call p.list and f.list, they both output ['thing'], but they are separate lists that are not shared between instances.

Python's Variable Resolution

This behavior is due to Python's variable resolution process. When you access self.list, Python first checks for the name list in the instance object. If it's not found, it searches the class definition.

In the first example, self.list resolves to the class variable because there is no list name defined in the instance. In the second example, self.list resolves to the instance variable because it is defined within the constructor.

Conclusion

Understanding the differences between class and instance variables is essential for working effectively with Python objects. Class variables are shared by all instances, while instance variables are specific to each object. Python's variable resolution process determines which variable is accessed when you use self.variable_name.

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