Attributes and Instance Variables

Behavior Belongs with Data

A method is a function that belongs to a class. Its purpose is to perform behavior for an object, while self identifies the particular object receiving that behavior. This connection lets a method work with the object's attributes and helps bundle behavior with data in an organized, maintainable way.

Defining a Method Inside a Class

class Person:
    def say_hi(self):
        print("Hi!")

The def keyword defines the function. Because the def block is placed inside Person, say_hi is a method of that class. The first parameter is named self. In this example, self is the only parameter, so the method needs no additional information from the caller.

How Python Supplies self

class Person:
    def say_hi(self):
        print("Hi!")

p = Person()
p.say_hi()

Hi!

Connecting self to Attributes

An object's attributes are data associated with that object. self gives a method access to the attributes of the particular object that called it. Therefore, the same method definition can operate on different instances: self identifies which object's data the method should use.

class Person:
    def introduce(self):
        print(self.name)

p = Person()
p.name = "Asha"
p.introduce()

Asha

In this generated example, self.name refers to the name attribute on p because p is the object that called introduce. The important link is not the particular name value; it is the path from the method to the calling object's attributes through self.

Calling Without Extra Arguments

A method whose definition contains only self is called without an argument written inside the parentheses. Python supplies the instance automatically. The caller writes p.say_hi(), not p.say_hi(p), because the object named p is already the object on which the method is invoked.

class Person:
    def say_hi(self):
        print("Hi!")

p = Person()
p.say_hi()
Person().say_hi()

Hi!
Hi!

The two calls produce the same output. In the first call, p is stored and then used to invoke say_hi. In the second, Person() creates a temporary Person object and the method is called on it immediately. Because that temporary object is not stored in a variable for later use, it is discarded after the call.

Mistakes with self

• Leaving self out of the method definition

  The method has no parameter that can receive the object automatically passed during an instance call.

  Fix: Write def say_hi(self): even when the method needs no other parameters.

• Passing the instance manually in ordinary dot notation

  The object p is already supplied as self when the method is invoked through p.

  Fix: Use p.say_hi() when say_hi takes only self.

• Treating self as a separate object created by the method

  self refers to the specific object on which the method was called.

  Fix: Trace the receiver of the dot call. In p.say_hi(), self refers to p.

• Assuming a method with only self cannot work with object data

  self gives the method access to the calling object's attributes.

  Fix: Use self to refer to attributes associated with the instance.

Practice the Binding

Key Takeaways

1. A method is a function that belongs to a class.
2. Every method definition requires self, even when there are no additional parameters.
3. When an instance invokes a method with dot notation, Python automatically passes that instance as self.
4. Inside the method, self identifies the object's attributes and lets the method act on behalf of that object.
5. Methods bundle behavior with data, supporting more organized and maintainable object-oriented programs.