Python Classes and Objects

An object bundles data (attributes, comprising its state) with methods (functions that act on that data) — the object has a notion of self.

Defining a class

class ClassName():
    def __init__(self, ...):
        self.attribute = ...

    def some_method(self, ...):
        return ...

• Class names use CamelCaps by convention.
• __init__ is the initializer, run automatically when an instance is created.
• Every method’s first parameter is self — Python supplies it automatically; never pass an argument for it explicitly.

Instantiation and attributes

>>> p = ClassName(...)      # creates an instance, passing args to __init__
>>> p.attribute              # dot notation accesses instance variables

Equality vs. aliasing

• Two separately-constructed instances with identical attributes are not == by default (equality compares identity unless a class defines otherwise).
• Assigning q = p makes q an alias for the same object as p — mutating one is visible through the other, and p == q holds because they’re literally the same object.

Private variables, getters, and setters

A leading underscore (self._value) signals that an attribute is intended for internal use only — a convention, not an enforced restriction; nothing stops external code from reading or writing it directly (which is bad practice).

Prefer exposing controlled access via methods:

def get_value(self) -> int:            # getter
    return self._value

def set_value(self, x: int) -> None:   # setter
    if x < 0:
        raise ValueError
    self._value = x

Python’s @property/@<name>.setter decorators let a getter/setter pair be used with ordinary attribute syntax (obj.name / obj.name = value) instead of explicit method calls.

Encapsulation

Encapsulation means keeping an object’s data safe inside the class, exposed only through its attributes and methods.