Python Error and Exception Handling : Custom Exceptions

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Exception handling in Python goes beyond dealing with built-in exceptions. You can create custom exceptions tailored to your application’s needs, enabling precise error handling and enhanced code clarity. In this comprehensive guide, we’ll delve into the world of custom exceptions in Python, complete with real-world examples and their outputs.

Why Custom Exceptions?

Custom exceptions, also known as user-defined exceptions, allow you to define your own exception classes. They are beneficial when you encounter specific error scenarios in your code that require unique handling.

Designing Custom Exceptions

To create a custom exception in Python, you typically define a new class that inherits from the Exception base class. Let’s illustrate this with an example.

Suppose you are developing a chat application, and you want to raise an exception when a user attempts to send an empty message.

class EmptyMessageError(Exception):
    def __init__(self, message="Message cannot be empty."):

def send_message(message):
    if not message:
        raise EmptyMessageError()

    message = ""
except EmptyMessageError as e:
    print("Error:", e)
    print("Message sent successfully.")


Error: Message cannot be empty.

In this example, we define a custom exception class EmptyMessageError to handle cases where an empty message is attempted to be sent.

Benefits of Custom Exceptions

  1. Clarity: Custom exceptions provide clear and meaningful error messages, aiding in debugging and understanding the code.
  2. Precise Handling: They allow you to distinguish between different types of errors and handle them specifically, rather than using generic exception types.
  3. Modularity: Custom exceptions make your code more modular and maintainable by encapsulating error-handling logic within specific exception classes.

Exception Hierarchy

In larger projects, it’s common to create a hierarchy of custom exceptions to categorize and manage errors effectively. For instance:

class CustomError(Exception):
class NetworkError(CustomError):
class DatabaseError(CustomError):
    # Code that may raise NetworkError or DatabaseError
except NetworkError:
    # Handle network-related errors
except DatabaseError:
    # Handle database-related errors
except CustomError:
    # Handle other custom errors

When to Use Custom Exceptions?

You should consider creating custom exceptions when:

  1. Your code encounters unique error scenarios specific to your application.
  2. You want to provide informative and meaningful error messages.
  3. You need to differentiate between various types of errors for precise error handling.


Let’s explore a real-world example of custom exceptions in a file handling scenario.

Suppose you are building a file processing application, and you want to raise a custom exception when a file exceeds a certain size limit.

class FileSizeLimitExceededError(Exception):
    def __init__(self, filename, max_size):
        self.filename = filename
        self.max_size = max_size
        super().__init__(f"File '{filename}' exceeds the size limit of {max_size} bytes.")

def process_file(filename):
    max_file_size = 1024  # 1 KB
    file_size = len(open(filename, 'rb').read())
    if file_size > max_file_size:
        raise FileSizeLimitExceededError(filename, max_file_size)

    file_to_process = "large_file.txt"
except FileSizeLimitExceededError as e:
    print("Error:", e)
    print("File processed successfully.")


Error: File 'large_file.txt' exceeds the size limit of 1024 bytes.

In this example, we define the FileSizeLimitExceededError custom exception to handle cases where a file exceeds the specified size limit.

Author: user