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The __init__ method is a special method (often referred to as a “dunder” or double-underscore method) in Python that acts as the instance initializer. It is automatically invoked by the Python runtime immediately after an object is instantiated in memory, serving to initialize the newly created object’s state by binding attributes to the specific instance.

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Syntax and Signature

The method is defined within a class block. Its signature must include at least one parameter, conventionally named self, which represents the bound instance being initialized. 
class Definition:
    def __init__(self, arg1, arg2):
        self.arg1 = arg1
        self.arg2 = arg2

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Technical Mechanics

  • Initialization vs. Construction: Unlike constructors in languages like C++ or Java, __init__ does not allocate memory or create the object. The actual object creation is handled by the __new__ method. The __new__ method returns an uninitialized instance, which is then passed as the first argument (self) to __init__ for state mutation.
  • Return Type Constraint: The __init__ method must strictly return None. Attempting to return any other value will result in a TypeError at runtime. Its sole purpose is to mutate the self object, not to yield a new value.
  • Implicit Invocation: When a class is called as a function (e.g., instance = Definition(val1, val2)), Python internally translates this into a two-step process:
    1. obj = Definition.__new__(Definition, val1, val2)
    2. if isinstance(obj, Definition): obj.__init__(val1, val2)

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Inheritance Behavior

When dealing with class hierarchies, Python does not automatically invoke the __init__ method of a superclass if the subclass overrides it. If a subclass defines its own __init__ method, it must explicitly invoke the superclass’s initializer using the super() proxy object to ensure the inherited state is properly initialized. 
class BaseClass:
    def __init__(self):
        self.base_state = 1

class DerivedClass(BaseClass):
    def __init__(self):
        # Explicitly invoke the superclass initializer
        super().__init__()
        self.derived_state = 2
If the subclass does not define an __init__ method, it inherits the __init__ method from its superclass via standard Method Resolution Order (MRO), and the superclass initializer is called implicitly upon instantiation.
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