🚨 Python Custom Exceptions – Create Meaningful Errors with Clarity

🧲 Introduction – Why Create Custom Exceptions?

Python provides built-in exceptions like ValueError, TypeError, and KeyError, but sometimes your application needs domain-specific error handling.

Custom exceptions help you:

• Raise meaningful and readable errors
• Handle errors at different layers of the codebase
• Provide cleaner debugging and logging
• Separate your business logic from system errors

🎯 In this guide, you’ll learn:

• What custom exceptions are
• How to define and raise your own exception classes
• Real-world use cases
• Best practices for naming and structure

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✅ What Is a Custom Exception?

A custom exception is a user-defined class that inherits from Python’s built-in Exception class (or its subclasses like ValueError, RuntimeError).

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🔧 How to Create a Custom Exception

✅ Basic Custom Exception

class MyError(Exception):
    pass

raise MyError("Something went wrong")

✅ Output:

Traceback (most recent call last):
  ...
MyError: Something went wrong

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🧩 Adding Custom Behavior

class InvalidAgeError(Exception):
    def __init__(self, age, message="Age must be at least 18"):
        self.age = age
        self.message = message
        super().__init__(self.message)

    def __str__(self):
        return f"{self.message}. Given: {self.age}"

✅ Usage:

age = 16
if age < 18:
    raise InvalidAgeError(age)

✅ Output:

InvalidAgeError: Age must be at least 18. Given: 16

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🎯 Real-World Use Case – Banking App

class InsufficientFundsError(Exception):
    pass

def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientFundsError("Withdrawal exceeds balance")
    return balance - amount

try:
    withdraw(100, 150)
except InsufficientFundsError as e:
    print("Error:", e)

✅ Output:

Error: Withdrawal exceeds balance

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🧱 Custom Exception Hierarchy

class AppError(Exception):
    """Base exception for the app"""

class LoginError(AppError):
    pass

class TokenExpiredError(LoginError):
    pass

class PermissionDeniedError(AppError):
    pass

💡 This allows group catching:

try:
    raise TokenExpiredError("Session expired")
except AppError as e:
    print("App error:", e)

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🚀 Raising Exceptions with raise from

Preserve original traceback:

try:
    int("abc")
except ValueError as e:
    raise MyError("Custom message") from e

✅ Output includes both errors in traceback.

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📘 Best Practices

✅ Do This	❌ Avoid This
Inherit from Exception or a subclass	Inheriting from BaseException directly
Use meaningful names and docstrings	Using vague names like Error1, Bug
Create a base class for your app/library	Duplicating logic in multiple exceptions
Catch specific exceptions when possible	Catching broad except: unless needed

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📌 Summary – Recap & Next Steps

Python custom exceptions give you fine-grained control over error handling. They’re a best practice for clean, maintainable, and readable code.

🔍 Key Takeaways:

• ✅ Create custom exceptions by inheriting from Exception
• ✅ Use meaningful messages and attributes
• ✅ Build a hierarchy for better organization
• ✅ Use raise from to preserve traceback

⚙️ Real-World Relevance:
Used in web apps, data pipelines, API design, and business logic validation.

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❓ FAQ – Python Custom Exceptions

❓ What class should I inherit from?

✅ Use Exception or its subclasses (ValueError, RuntimeError, etc.)

❓ Can I create a hierarchy of exceptions?

✅ Yes. It’s good practice to create a base class like AppError and subclass from it.

❓ Is raise from required?

❌ No. But it’s helpful when chaining exceptions to maintain full traceback.

❓ Can I add custom attributes to exceptions?

✅ Yes. Define them in __init__() and override __str__() if needed.

❓ Should I catch all exceptions with except:?

❌ Avoid unless you’re logging or cleaning up. Always prefer catching specific exceptions.

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