What\'s the Difference Between Variables Defined Inside and Outside `__init__()` in Python?

Delving into the Distinction Between Variables Inside and Outside __init__()

Consider the following two classes:

<code class="python">class WithClass ():
    def __init__(self):
        self.value = "Bob"
    def my_func(self):
        print(self.value)

class WithoutClass ():
    value = "Bob"

    def my_func(self):
        print(self.value)</code>

At first glance, these classes appear identical. However, a subtle distinction lies in the placement of the value variable. In the first class (WithClass), value is initialized within the __init__() method, while in the second class (WithoutClass), it is declared outside the method.

Variables Outside __init__() (Class Attributes) vs. Variables Inside __init__() (Instance Attributes)

The placement of the variable determines whether it is a class attribute or an instance attribute.

• Class attributes: Variables declared outside __init__() are shared by all instances of the class. They can be accessed using the class name, such as WithoutClass.value.
• Instance attributes: Variables created inside __init__() and prepended with self. are specific to each object instance. They can be accessed only through the object itself, such as instance_of_WithClass.value.

Consequences of Placement

This distinction has ramifications for code behavior and maintenance.

• Usage within methods: Class attributes are available to all methods within the class, regardless of where they are declared. Instance attributes, on the other hand, must be declared before they can be used within a method.
• Access from outside the class: Class attributes can be accessed from outside the class using the class name. Instance attributes, however, can only be accessed through an instance of the class.
• Manipulating attributes: Class attributes affect all instances of the class. Modifying a class attribute will change its value for all instances. In contrast, instance attributes only affect the specific object they belong to.

Choice of Placement

The choice of where to place a variable depends on how you want it to behave within the class. If you want a shared value that remains constant across instances, use a class attribute. If you want a unique value for each instance, create it within __init__() as an instance attribute.

By understanding the distinction between variables inside and outside __init__(), you can design classes with clear and predictable behavior, avoiding potential confusion and pitfalls.

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