How Do Variable Scopes Work in Python Classes?

Understanding Variable Scopes in Python Classes

Declaring variables in Python classes can be confusing at first. This article aims to clarify the different scopes and nuances associated with variable declaration in class contexts.

General Scope Rules:

• Class Variables: Declared outside functions within a class, these variables are accessible to all class functions.
• Local Variables: Declared inside functions within a class, these variables are only accessible within that function.
• Instance Variables: Declared using self.(variable name) inside functions within a class, these variables are accessible to all class functions.

Instance Variables vs. Global Variables:

Instance variables (declared using self.(variable name)) behave differently from global variables (declared outside any function or class). While everything declared within a class is technically public, instance variables are bound to the individual object instance.

Example:

class Test:
    a = None  # Class variable
    b = None  # Class variable

    def __init__(self, a):
        self.a = a  # Instance variable

In this example, the class variables a and b are accessible to all class functions. However, the instance variable self.a is specific to each object instance. Setting self.a in __init__ does not affect the class variable a.

Protected Variables:

Although Python does not explicitly define private or protected variables, variables prefixed with an underscore (_) are considered protected. Technically, they are still accessible outside the class, but mangling their names discourages access.

Private Variables:

Variables prefixed with double underscores (__) are considered private. Their names are mangled to make it difficult to access them directly outside the class.

Additional Nuances:

• Variables assigned within local functions but outside self.(variable name) are not available outside that function.
• Mangle-proof protected and private variables are not completely inaccessible outside the class but should not be relied upon for true encapsulation.

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