Runtime Error

A runtime error is a type of error that occurs during the execution of a program. These errors, unlike syntax errors which are detected when a program is parsed, are only detected when a certain problematic line is executed.

Defining Runtime Error

Runtime errors are caused by actions such as dividing a number by zero, accessing an out-of-bounds index of a list, or trying to use a null reference. They cause the program to terminate abnormally or prematurely.

It’s important to note that even a program that is free of syntax errors may still cause a runtime error. For instance, consider the following example:

1def division():
2    num1 = int(input("Enter a number: "))
3    num2 = int(input("Enter another number: "))
4    return num1/num2

In this program, if a user enters 0 for num2, a ZeroDivisionError, a type of runtime error, will occur.

Handling Runtime Errors

Python includes built-in error handling functionality that allows you to respond to, or “catch”, runtime errors. It uses try and except blocks to encapsulate and handle errors without forcing the program to exit abruptly. The following code demonstrates error handling for the previous example:

 1def division():
 2    try:
 3        num1 = int(input("Enter a number: "))
 4        num2 = int(input("Enter another number: "))
 5        return num1/num2
 6    except ZeroDivisionError:
 7        print("Error: Division by zero is undefined. Please enter a non-zero number.")
 8        return None

In this program, if a user attempts to divide by zero, the program will catch and handle the ZeroDivisionError, print a user-friendly error message, and continue to run smoothly.


Not all runtime errors can be prevented as they may depend on user input or external resources, but they should always be handled gracefully.