Python RuntimeError – Tutorial with Examples

Python RuntimeError - Tutorial with Examples

Python RuntimeError is a built-in exception class that is raised when no other exception is raised. It usually indicates an error in the program’s logic, and it is raised automatically by Python when it detects an issue. This tutorial will explore what Python RuntimeError is and how it can be used in Python programming.

What is a Runtime Error in Python?

In general, errors are classified into two types: compile-time errors and runtime errors. Compile-time errors are detected by the compiler at the time the source code is compiled, while runtime errors are detected by the interpreter when the program is executing.

A runtime error, as the name suggests, is an error that is detected while the program is running. Runtime errors in Python are usually caused by user input or the program’s logic. In Python, when a runtime error is encountered, a Built-in Exception class named RuntimeError is raised. This exception can be handled using the try-except block.

Python RuntimeError – Syntax

The syntax for raising a RuntimeError is as follows:

<b>raise RuntimeError("error message")</b>

The code above raises a RuntimeError. We can describe the error with a string argument passed to the exception.

Python RuntimeError Examples

Example 1: RuntimeError with try-except block

We can handle the RuntimeError using the try-except block, just like the other exceptions. In the example below, we will try to open a non-existing file, and when the program tries to read from the file, Python will raise a RuntimeError because the file is not available. We will use a try-except block to handle the exception and display a custom message when the exception is raised.

    with open("non-existing-file.txt", "r") as f:
except RuntimeError:
    print("File not found. Please check if the file exists and if the spelling is correct.")

The output of the code above will be:

File not found. Please check if the file exists and if the spelling is correct.

As you can see in the example above, we used a try-except block to trap the RuntimeError, and when Python raised the RuntimeError, the except block was executed and displayed a custom message to the user.

Example 2: RuntimeError without try-except block

In this example, we will create our own function that can raise a RuntimeError based on the input parameter. The following function accepts a single parameter and will raise a RuntimeError if the value is less than 0.

def validate_number(num):
    if num < 0:
        raise RuntimeError("Number cannot be a negative value.")
        print("Number is valid")

Now we can call the function and pass in any integer value to the function. If we pass in a negative value, the function will raise a RuntimeError.

validate_number(10) # Output: Number is valid
validate_number(-1) # Output: "RuntimeError: Number cannot be a negative value."

The first call to the validate_number function passes in 10, which is valid, and therefore prints “Number is valid.” The second call to the function passes in -1, which is less than zero and, therefore, raises a RuntimeError with the message “Number cannot be a negative value.”

Example 3: RuntimeError with Uncaught Exception

In this example, we will use the os module, which is the built-in operating system module in Python, to trigger a RuntimeError. The os.urandom function generates a specified number of random bytes using the operating system’s random number generator.

import os


The code above will raise a RuntimeError when the specified number of bytes to generate is not supported by the operating system. For example, if we set the number of bytes to generate to 1, the function may not be able to generate just one byte and may raise a RuntimeError with a message “Not enough entropy to generate security token.”

The output of the code above will be:

NotImplementedError: Wrong number of bytes read from OpenSSL (was 0, should be 100)

As you can see in the example above, we did not handle the exception with a try-except block, so Python raised the error and displayed the default RuntimeError message.


In this tutorial, we explored what Python RuntimeError is, and how it can be used to trap runtime errors. We also saw how to raise a RuntimeError and handle it with a try-except block. Python provides many built-in exceptions like RuntimeError to catch errors and handle them in a more user-friendly way. Runtime errors can be tricky to debug, so it’s essential to have good exception handling in place when writing Python code.

Leave a Reply

Your email address will not be published. Required fields are marked *