Understanding Python Functions: Built‑in and Custom Functions

Functions are a fundamental programming concept that enable modular design, and mastering them is essential for any programmer, especially in Python where functional programming is prominently featured.

Built‑in Functions are provided by Python’s standard library and can be called directly without any import; they are often referred to as APIs. For example, print() outputs text to the console and is a classic built‑in function.

Custom Functions are user‑defined blocks of code designed to perform specific tasks, offering encapsulation and reusability. They allow you to package functionality into a “box” that can be invoked whenever needed.

1. Function without parameters

<code># Function definition

def function():
    print("hello world")

function()
</code>

This defines a simple function that prints "hello world" and demonstrates a parameter‑less function call.

2. Function with parameters

<code># This is a function with parameters
def give(username, number):
    print("{}给主播送了{}个西瓜".format(username, number))

give("Tom", 100)
</code>

The function give accepts two arguments, username and number , and prints a formatted message. When called with "Tom" and 100, it outputs the corresponding text.

<code>D:\My_Python\Scripts\python.exe D:/My_Python/021.py
hello world
Tom给主播送了100个西瓜
Process finished with exit code 0
</code>

3. Function with a return value

<code># This is a function that returns a value
def sum(n1, n2):
    result = n1 + n2
    return result

sum(1, 2)
</code>

The sum function adds two numbers and returns the result, illustrating how a function can provide output back to the caller.

Understanding how to define and invoke functions, whether built‑in or custom, is a prerequisite for mastering object‑oriented programming and more advanced Python techniques.