Classes and Objects

You will be able to design a class that protects its data: it validates on creation, computes values on demand and shares some state.

So far a record was a dict or tuple and the functions guarding it lived elsewhere; a class keeps data and rules together. class X(Exception): pass from raising-exceptions was already one, with no body.

Methods receive the instance

A method is a function in a class body; the object before the dot becomes its first argument, self.

Class attributes versus instance attributes

A name assigned in the class body is shared; one assigned on self belongs to one object. This is the mutable-default trap from arguments-and-scope again:

The fix is self.members = [] in __init__. A second trap: self.x += 1 reads the class value but writes a new instance attribute:

To update shared state, name the class: Ticket.issued += 1.

Alternative constructors and utilities

@classmethod receives the class as cls; it is the home of alternative constructors. @staticmethod receives nothing extra: a plain function filed in the class. Here @ is only syntax; function-decorators explains it.

Invariants: validate, and order the side effects

An invariant is a fact that must always hold, like "never below absolute zero". Check it in __init__ and raise ValueError; no object is returned, but statements before the raise already ran:

Validate first, then change state.

Properties

@property makes a method read like an attribute. Without a setter it is read-only and recomputed on each access, so it cannot go stale. @name.setter validates every assignment, including the one in __init__:

Privacy is a convention

_name means "internal". __name is renamed to _ClassName__name; that avoids clashes in subclasses and is not security.

Objects are references

== on instances is identity until special-methods changes it; is asks whether two names point at one object:

Common mistakes

• A list in the class body (guests = []): a guest booked into one room shows up in every room.
• Counting before validating: count reports 1 where 0 rooms exist.
• self.count += 1 as a shared counter: the class total stays 0.
• Storing free seats in __init__: correct until the first booking, then stale. Compute it in a property.

Your exercise

Build the booking system around a Room class: shared counter, parse constructor, computed free. The usual slip is counting a room before its capacity is validated.