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Class and Instance Attributes: Variables Inside and Outside of __init__()

In object-oriented programming, class attributes and instance attributes play crucial roles. But what's the difference between placing a variable inside vs. outside the __init__() method? Let's delve into this distinction.

Consider the given code snippets:

<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>

Variables Outside __init__()

Variables defined outside __init__() belong to the class. They are common to all instances created from that class. For instance, in the WithoutClass class:

<code class="python">WithoutClass.value = "Bob"</code>

Accessing this variable via any instance of the WithoutClass class will return the same value "Bob."

Variables Inside __init__()

In contrast, variables defined inside __init__(), prefixed with self., belong to each individual object. When setting a variable within __init__(), it becomes an instance attribute specific to that object.

<code class="python">WithClass().value = "Alice"  # Unique to this instance</code>

Accessing this same variable from another instance of the WithClass class will yield a different value:

<code class="python">WithClass().value = "Bob"  # Unique to this instance</code>

Implications

The choice of where to declare a variable has implications for its behavior.

• Class-wide Variables: Placing a variable outside __init__() makes it a class attribute, shared among all instances. This can be useful for global settings or properties that apply consistently across all objects.
• Instance-specific Variables: Defining a variable inside __init__() with self. prefix creates an instance-specific attribute. Each object has its own copy of that variable, allowing customization for individual objects.

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