Mike Vincent

Posted on Jun 6

Quark's Outlines: Python User-defined Methods

#python #programming #beginners #tutorial

Quark’s Outlines: Python User-Defined Methods

Overview, Historical Timeline, Problems & Solutions

An Overview of Python User-Defined Methods

What is a Python user-defined method?

When you define a function inside a class, Python does not treat it as just a function. When you call it from an instance, Python changes it into a method. This method knows which object it was called from. It adds that object as the first argument when the function runs.

A Python user-defined method joins a function, a class, and a class instance (or None). It is created when you get a function from a class or an instance. Python binds the instance to the function and forms a method.

Python lets you bind a class function to an instance as a method.

class Box:
    def show(self, word):
        print("Box says:", word)

x = Box()
x.show("hi")
# prints:
# Box says: hi

The method x.show is bound to the instance x. Python passes x as the first argument.

What does "bound method" mean in Python?

When a method is bound, it remembers the instance that called it. A bound method is created when you get a method from an object. It holds a reference to both the function and the instance. Python will pass the instance automatically when you call the method.

If you get the same method from the class, Python gives you an unbound method. That means the function is not tied to any one object.

Python uses bound methods to remember which object to call with.

class Lamp:
    def turn_on(self):
        print("The lamp is now on.")

l = Lamp()
m = Lamp()

a = l.turn_on
b = m.turn_on

a()
b()
# prints:
# The lamp is now on.
# The lamp is now on.

Each bound method remembers which Lamp it came from.

A Historical Timeline of Python User-Defined Methods

Where do Python user-defined methods come from?

Python user-defined methods grew from early ideas in object-oriented design. In many languages, methods are just functions that get special treatment when called from an object. Python made this clear by letting you see and work with method objects directly.

People made objects that could hold behavior

1967 — Simula 67 added classes and methods to programming. Methods were functions that acted on data from one object.

1983 — Smalltalk-80 refined this idea with message-passing and method binding at runtime.

People designed Python's class method rules

1991 — Python 0.9.0 introduced classes, functions inside them, and method binding.

2000 — Unbound methods changed in Python 2.2. You could now get function objects back from classes.

2010 — Bound and unbound methods unified in Python 3. Now, calling C.f just gives the function. Only instance access makes it a method.

2025 — Method handling stable. Python methods remain clear, open, and part of the object model.

Problems & Solutions with Python User-Defined Methods

How do you use Python user-defined methods the right way?

When you write a class in Python, you often define functions inside it. These become methods when accessed from an instance. These problems show how methods bind, behave, and help organize object behavior.

Problem: How do you pass one method to another function in Python?

You are writing a program where objects have named actions. You want to pass a method to another function to run it later. You are not sure how to keep the object tied to the method.

Problem: You want to pass a method while keeping its object context.

Solution: Python binds the method to the instance, so you can pass it and call it later.

Python lets you pass bound methods as function objects.

class Bell:
    def ring(self):
        print("Ding!")

b = Bell()

def trigger(action):
    action()

trigger(b.ring)
# prints:
# Ding!

The method b.ring is passed like a function. It still knows b is the object.

Problem: How do you call a method from the class in Python?

You have a class with a function. You want to call that function, but not from an instance. You want to see what happens if you call it using the class itself.

Problem: You want to call a method without using an instance.

Solution: Python gives you the function itself from the class. You must pass the instance by hand.

Python gives you the function when called from the class.

class Box:
    def show(self, msg):
        print("Box shows:", msg)

b = Box()
Box.show(b, "hello")
# prints:
# Box shows: hello

You must pass the instance yourself, or Python will not know what self is.

Problem: How do you tell if a method is bound in Python?

You want to inspect a method in your code. You are not sure if it is bound to an instance. You want a way to check if it already holds a reference to an object.

Problem: You need to know if a method is bound or unbound.

Solution: Python bound methods have a __self__ attribute. It holds the instance.

Python lets you inspect the method to see its binding.

class Light:
    def flash(self): pass

l = Light()
print(Light.flash.__self__)  # error: unbound method has no __self__
print(l.flash.__self__)
# prints:
# <__main__.Light object at 0x...>

Calling __self__ shows what object the method is bound to. Class access returns the function, not a method.

Problem: How do you store a method in a variable in Python?

You want to call a method often. You do not want to type the full obj.method() each time. You want to assign the method to a short name and reuse it.

Problem: You want to store a method to reuse it in many calls.

Solution: Python lets you assign a bound method to a variable.

Python lets you keep methods as first-class objects.

class Volume:
    def up(self): print("Volume up")

v = Volume()
raise_volume = v.up
raise_volume()
# prints:
# Volume up

You can treat the method as a value. It keeps its link to the object.

Problem: How do you define a method inside a class in Python?

You are new to classes. You want to define a function that works on objects of the class. You want Python to pass the object automatically.

Problem: You want to define a method in your own class.

Solution: Python defines methods with def inside a class and uses self as the first name.

Python lets you define methods that act on one object.

class Dog:
    def bark(self):
        print("Woof!")

fido = Dog()
fido.bark()
# prints:
# Woof!

The self argument points to the object that called the method.

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Mike Vincent is an American software engineer and app developer from Los Angeles, California. More about Mike Vincent

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