Python Custom Exceptions

1. Why Use Custom Exceptions

Custom exceptions improve clarity, maintainability, and semantic accuracy by representing domain-specific error conditions.

class InvalidInputError(Exception):
    pass

Defines a new exception type tailored to your application logic.

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2. Basic Custom Exception Example

class NegativeValueError(Exception):
    pass

def process_value(value):
    if value < 0:
        raise NegativeValueError("Negative values are not allowed")

process_value(-10)

Raises a user-defined exception with a meaningful message.

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3. Extending Exception with Custom Message

class AgeLimitError(Exception):
    def __init__(self, age):
        super().__init__(f"Age {age} is below allowed limit")

def validate_age(age):
    if age < 18:
        raise AgeLimitError(age)

validate_age(15)

Adds dynamic context to error messages.

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4. Catching Custom Exceptions

class AuthenticationError(Exception):
    pass

try:
    raise AuthenticationError("Invalid credentials")
except AuthenticationError as e:
    print("Login failed:", e)

Handled like any built-in exception.

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5. Creating Hierarchy of Custom Exceptions

class ApplicationError(Exception):
    pass

class DatabaseError(ApplicationError):
    pass

class NetworkError(ApplicationError):
    pass

Facilitates organized error taxonomy and layered handling.

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6. Custom Exception with Additional Attributes

class TransactionError(Exception):
    def __init__(self, code, message):
        self.code = code
        self.message = message
        super().__init__(f"[{code}] {message}")

raise TransactionError(403, "Unauthorized action")

Carries structured metadata for better diagnostics.

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7. Using Custom Exceptions in Functions

class FileMissingError(Exception):
    pass

def load_config(filename):
    if not filename.endswith(".json"):
        raise FileMissingError("Only JSON configuration files supported")

load_config("config.txt")

Enforces logical validation constraints.

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8. Custom Exception with Logging

class DataValidationError(Exception):
    pass

try:
    raise DataValidationError("Invalid CSV column format")
except DataValidationError as e:
    print("Validation Error Logged:", e)

Useful for application-level logging and audit trails.

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9. Chaining Custom Exceptions

class InitialError(Exception):
    pass

class DerivedError(Exception):
    pass

try:
    try:
        raise InitialError("Initial failure")
    except InitialError as e:
        raise DerivedError("Follow-up failure") from e
except DerivedError as final_error:
    print(final_error)

Preserves original error context using exception chaining.

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10. Best Practice: Naming and Design Pattern

class InvalidOrderStateError(Exception):
    """Raised when an order is in an invalid processing state"""
    pass

def process_order(status):
    if status != "confirmed":
        raise InvalidOrderStateError("Order must be confirmed before processing")

process_order("draft")

Follow naming conventions:

• Use meaningful names

• Inherit from Exception

• Include descriptive docstrings

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