11.3 Instance methods

Instance methods are fundamental to Python classes, allowing objects to interact with their own data. They define behaviors specific to each instance, accessing and modifying attributes using the 'self' keyword.

These methods come in various types, including constructors, regular instance methods, class methods, and static methods. Each serves a unique purpose, from initializing objects to performing operations on the class itself or providing utility functions.

Instance Methods in Python Classes

Constructor method with parameters

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• [__init__()](https://www.fiveableKeyTerm:__init__())

  special instance method called when creating new instance of class
  • Initializes attributes of instance
  • First parameter always

    self

    refers to instance being created
• Takes multiple parameters to initialize instance attributes
  • Parameters defined after

    self

    separated by commas
  • Each parameter assigned to instance attribute using

    self.attribute_name = parameter_name

    • self.name = name

      assigns value of

      name

      parameter to

      name

      attribute of instance
• Optional parameters included by assigning default values
  • Default values specified using

    =

    operator after parameter name (

    def __init__(self, name, age=0):

    )
  • If argument not provided when creating instance, default value used

Instance methods and attribute access

• Instance methods access and modify instance attributes using

  self

  keyword

  • self.attribute_name

    refers to attribute of current instance
  • Modifying

    self.attribute_name

    changes value of attribute for that specific instance

    • self.name = "New Name"

      changes

      name

      attribute of instance to

      "New Name"

• Can also access class attributes directly using class name

  • ClassName.attribute_name

    refers to class attribute
  • Modifying

    ClassName.attribute_name

    changes value of attribute for all instances of class

    • Car.wheels = 5

      changes

      wheels

      attribute for all instances of

      Car

      class
• If instance attribute has same name as class attribute, accessing

  self.attribute_name

  refers to instance attribute, overriding class attribute for that instance
• Method invocation occurs when an instance method is called on an object

Types of class methods

• Instance methods defined within class and operate on individual instances
  • Take

    self

    as first parameter referring to instance on which method is called
  • Access and modify instance attributes using

    self

    • def drive(self): self.speed += 10

      increases

      speed

      attribute of instance by

      10

• Class methods defined using

  [@classmethod](https://www.fiveableKeyTerm:@classmethod)

  decorator and operate on class itself
  • Take

    [cls](https://www.fiveableKeyTerm:cls)

    as first parameter referring to class
  • Access and modify class attributes using

    cls

    • @classmethod def from_string(cls, car_str): ...

      creates instance of class from string representation
  • Often used for alternative constructors or methods that operate on class as a whole
• Static methods defined using

  [@staticmethod](https://www.fiveableKeyTerm:@staticmethod)

  decorator and do not have access to instance or class attributes
  • Do not take

    self

    or

    cls

    as parameters
  • Independent of instances and class, used for utility functions or operations that don't require access to instance or class data

    • @staticmethod def miles_to_kilometers(miles): return miles * 1.60934

      converts miles to kilometers without needing instance or class

Object-Oriented Programming Concepts

• Encapsulation: Instance methods help achieve encapsulation by bundling data and methods that operate on that data within a class
• Inheritance: Subclasses inherit instance methods from their parent classes, allowing for code reuse and specialization
• Polymorphism: Instance methods can be overridden in subclasses, enabling different implementations of the same method name across related classes