__init__ and Instance Attributes

Scenario: Every account must begin with a name and balance in a valid state.

New words, in plain English

• Initializer - __init__, called after a new instance is created
• Instance attribute - data belonging to one object
• Invariant - a rule that should always remain true

Initialize required state in __init__. Methods can enforce invariants rather than allowing callers to edit everything carelessly.

Analogy: A Python program is like a clear set of instructions for a careful helper: names label things, indentation groups steps, and errors explain where the helper became confused.

A small, real example

class Account:
    def __init__(self, owner, balance=0):
        self.owner = owner
        self.balance = balance

    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("amount must be positive")
        self.balance += amount

Tip: __init__ initializes an existing new instance; __new__ actually creates it and is rarely overridden.

Goal: Practice this idea in the py-small-class challenge at /labs/python. Fix the broken program, run it yourself, then use check to prove the real end state.