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Mastering `__init__` in Python

OOP Core

The __init__ Method

`__init__` sets up each instance. Validate inputs, wire dependencies, and keep state consistent.

Review signaturesHandle inheritance

Signatures​

class Invoice:
    def __init__(self, *, total: float, customer: str, currency: str = "USD"):
        if total < 0:
            raise ValueError("Total must be positive")
        self.total = total
        self.customer = customer
        self.currency = currency
  • Use keyword-only parameters for clarity.
  • Avoid heavy business logic; keep initialization focused.

Calling super()​

class PriorityInvoice(Invoice):
    def __init__(self, *, priority_level: int, **kwargs):
        super().__init__(**kwargs)
        self.priority_level = priority_level
  • Always call super().__init__ when inheriting to ensure base initialization runs.

Next up in your learning path

Dunder methods guide

Customize object behavior beyond __init__.

Next

Instance vs class variables

Know where attributes live.

Design

Dataclasses

Automate init generation for data containers.

Modern