Classes: State plus Behavior

A class bundles state (attributes) with the behavior that operates on it (methods). __init__ runs at creation; self is the instance being worked on:

class Book:
    def __init__(self, title, pages):
        self.title = title
        self.pages = pages

    def reading_time(self, minutes_per_page=2):
        return self.pages * minutes_per_page

book = Book("Fluent Python", 780)
book.reading_time()        # 1560 — method call binds book as self

Two dunders every serious class defines

• __repr__: unambiguous, for developers — ideally code that rebuilds the object.
• __str__: readable, for end users (falls back to __repr__ if absent).

class Book:
    def __init__(self, title):
        self.title = title

    def __repr__(self):
        return f"Book({self.title!r})"

    def __str__(self):
        return self.title

In a REPL or traceback you see the repr; in print() you see the str. Without them you get <Book object at 0x...> — useless when debugging.

Class attributes vs instance attributes

An attribute assigned at class level is shared by all instances (constants, counters). Instance attributes (assigned via self.) belong to one object. Default to instance attributes; reach for class attributes only for genuinely shared data.