Python def Keyword

The def keyword is used to define user-defined functions. Functions help organize code into reusable blocks, making programs easier to read, maintain, and reuse. They can accept input values through parameters, perform specific tasks and optionally return results.

Example:

Hello

Explanation:

• def func(): Defines a function named func.
• print("Hello"): Code inside the function that runs when called.
• func(): Calls the function, printing Hello to the output.

The below diagram shows the basic structure of a Python function, including its name, parameters, body, and optional return value.

Syntax

def function_name(parameters):
# Code to execute
return value # Optional

• def: Keyword to define a function
• function_name: Name of the function
• parameters: Optional input values (can be empty)
• return: Optional, sends a value back from the function
• The indented block is executed when the function is called

Note: Defining a function using def does not execute it immediately. The function runs only when it is called using its name followed by parentheses.

Examples

Example 1: In this example, we create a function sub() using the def keyword, which calculates the difference between two numbers.

subtraction of 90 and 50 is 40

Explanation:

• def sub(x, y): Defines a function named sub that takes two parameters x and y.
• return (x-y): Returns the difference between x and y.
• res = sub(a, b): Calls the function with a and b, storing the result in res.

Example 2: In this example, we define a user-defined function fun() using the def keyword. The function takes a parameter n and prints the first n prime numbers.

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29

Explanation:

• fun(n): Prints the first n prime numbers.
• x = 2, count = 0: Initialize number and prime counter.
• while count < n: Loop until n primes are found.
• for d in range(2, int(x**0.5)+1): Check if x is divisible by any number up to √x.
• if x % d == 0: break: Not prime, skip.
• else: If prime, print x and increment count.
• x += 1: Check next number.

Using Default Arguments

Default arguments allow a function parameter to have a predefined value. If no argument is provided during the function call, the default value is used automatically.

Hello Guest
Hello Alice

Explanation:

• Default Value: The parameter name is assigned "Guest" as its default value.
• Optional Argument: Calling greet() without an argument uses the default value.
• Override Default: Passing "Alice" replaces the default value during the function call.

Passing Function as an Argument

In Python, functions are first-class objects, which means you can pass functions as arguments to other functions, allowing you to call it inside that function.

25

Explanation:

• def fun(func, arg): Defines a function that takes another function func and an argument arg.
• return func(arg): Calls the passed function with the given argument and returns the result.
• def square(x): Defines a function that returns the square of x.
• res = fun(square, 5): Passes the square function and 5 to fun.

Using *args

*args allows a function to accept a variable number of positional arguments, which are collected into a tuple, making the function flexible to handle multiple inputs.

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Explanation:

• def fun(*args): Defines a function that can take any number of positional arguments, stored as a tuple args.
• fun(1, 2, 3, 4, 5): Calls the function with 5 positional arguments.

Using **kwargs

**kwargs lets a function accept any number of keyword arguments. These arguments are collected into a dictionary, with keys as argument names and values as their corresponding values.

name: Olivia
age: 30
city: New York

Explanation:

• def fun(**kwargs): Defines a function that can take any number of keyword arguments, stored as a dictionary kwargs.
• fun(name="Leo", age=30, city="New York"): Calls the function with 3 keyword arguments.

Using def Inside a Class

Inside a class, functions are called methods. You define them using def just like regular functions, but they usually take self as the first parameter to access the object’s attributes and other methods.

Name - Harry and Age - 30.

Explanation:

• def __init__(self, name, age): Constructor method called when a new object is created.
• self.name = name and self.age = age: Initialize object attributes.
• def greet(self): Defines a method greet that can access the object’s attributes.
• p1.greet(): Calls the greet method on p1, outputting:

Related Articles:

• How to Define and Call a Function in Python
• Python User Defined Functions