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In this lesson, you’ll learn how to create and use custom functions in Python. So far, we’ve worked with built-in functions like print, len, and input, and we’ve distinguished between functions and methods. In Python, functions can come from the core language, external modules, or be defined by you.
Encapsulating repeated code into functions enhances readability and minimizes potential bugs.

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Why Use Functions?

Repeating blocks of code can make your programs harder to maintain and increase the risk of errors. By defining a function, you bundle the code into a reusable unit, reducing redundancy and improving clarity. Consider the following example where the same input logic is repeated multiple times: 
input1 = int(input("Enter a number: "))
input2 = int(input("Enter a number: "))
input3 = int(input("Enter a number: "))
input4 = int(input("Enter a number: "))
input5 = int(input("Enter a number: "))
To streamline this, we can define a function named input_number:

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Defining the Function

We create a function using Python’s def keyword. The function starts with the function name and parentheses, followed by a colon to indicate the beginning of the function body. In this case, the function prompts the user for a number, converts the input to an integer using int(), and returns the value. Here’s how to rewrite the repetitious code using our new input_number function: 
def input_number():
    return int(input("Enter a number: "))

input1 = input_number()
input2 = input_number()
input3 = input_number()
input4 = input_number()
input5 = input_number()
Each time you call input_number(), the function executes its internal code, prompting the user, converting the input to an integer, and returning the resulting value. For example, the variable input1 holds the number provided by the user.

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Important Consideration

It is crucial to define a function before calling it. If you attempt to use a function before its definition, Python will throw an error: 
>>> input1 = input_number()
def input_number():
    return int(input("Enter a number: "))
NameError: name 'input_number' is not defined
This error occurs because Python processes the code sequentially and has not encountered the definition of input_number when it is called.
Always define your functions before invoking them to avoid NameError issues.
That’s all for this lesson. Keep practicing your function definitions and usage. Learn more about Python functions for additional insights. See you in the next lesson!

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