Custom Exceptions

Python provides many built-in exceptions like ValueError, TypeError, and FileNotFoundError. But what if your application needs a very specific error type that Python does not have?

For example, in a banking app, you might want an InsufficientBalanceError. Python does not have this — so you create it yourself.

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How to Create a Custom Exception

A custom exception is simply a Python class that inherits from the built-in Exception class:

class InsufficientBalanceError(Exception):
    pass

That is it! Now you can raise it just like any built-in exception:

class InsufficientBalanceError(Exception):
    pass

def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientBalanceError(
            f"Cannot withdraw ₹{amount}. Available balance: ₹{balance}"
        )
    return balance - amount

try:
    remaining = withdraw(500, 1000)
except InsufficientBalanceError as e:
    print("Transaction denied:", e)
# Output: Transaction denied: Cannot withdraw ₹1000. Available balance: ₹500

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When to Use Custom Exceptions

Use custom exceptions when:

• Built-in exceptions are too generic for your use case
• You want the caller to handle your specific error differently from general errors
• You are building a library or API that other developers will use

Keep It Simple

For most beginner projects, built-in exceptions like ValueError are sufficient. Create custom exceptions only when your application genuinely needs a distinct error type.

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Quick Summary

• Custom Exception Creation: Inherit from Python's base Exception class: class CustomError(Exception): pass.
• Domain Modeling: Custom exceptions make application code expressive and allow callers to handle domain failures specifically (e.g., InvalidAuthTokenError).
• Attributes & Messages: Custom exception classes can accept and store extra contextual details like error codes or user IDs.

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What's Next?

Congratulations on mastering error handling! Next up is Module 15: File Handling, where you will learn how to read and write permanent files on disk!

Module 15: File Handling

Learn how to open, read, write, and process text and CSV files in Python