Instance state and behavioral contracts

The idea

A class can group state with the operations permitted on it. __init__ initializes a new instance, and methods receive that instance as self. Put per-instance mutable collections inside __init__. A list placed directly in the class body is shared until an instance shadows that attribute, which often surprises developers expecting each object to have a private list.

Methods should enforce the rules that make the object meaningful. A reservation must validate its amount and available capacity before decreasing the remaining count. A leading underscore is a convention for internal implementation, not a security boundary. Properties can expose a controlled interface, but callers and tests should still exercise invalid inputs and verify that failure leaves the object unchanged.

class Basket:
    def __init__(self):
        self.items = []
    def add(self, item):
        self.items.append(item)
a, b = Basket(), Basket()
a.add("pear")
print(a.items, b.items)

['pear'] []

Each constructor call creates a fresh items list.