Python Custom Exceptions
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Creating a Custom Exception
Defining a new class that inherits from the built-in Exception class creates a custom exception type -- even an empty 'class InsufficientFundsError(Exception): pass' is immediately usable with raise and except, giving your errors a name specific to your application's domain.
Example: Creating a Custom Exception
class InsufficientFundsError(Exception):
pass
try:
raise InsufficientFundsError()
except InsufficientFundsError:
print("Custom exception caught")Adding Custom Messages
Overriding __init__ to accept a message and passing it to the parent class via super().__init__(message) lets your custom exception carry a description just like built-in exceptions do, so str(my_exception) returns something meaningful instead of being blank.
Example: Adding Custom Messages
class InsufficientFundsError(Exception):
def __init__(self, message):
super().__init__(message)
try:
raise InsufficientFundsError("balance too low")
except InsufficientFundsError as e:
print(str(e))Raising Custom Exceptions
You trigger a custom exception the same way as a built-in one -- raise InsufficientFundsError('balance too low') -- which immediately stops normal execution and looks for a matching except block up the call stack.
Example: Raising Custom Exceptions
class InsufficientFundsError(Exception):
pass
def withdraw(balance, amount):
if amount > balance:
raise InsufficientFundsError("balance too low")
try:
withdraw(100, 200)
except InsufficientFundsError as e:
print(e)Catching Custom Exceptions
Catching a custom exception works exactly like catching a built-in one: 'except InsufficientFundsError as e:' -- this lets calling code respond specifically to your application's own error conditions rather than generic ones.
Example: Catching Custom Exceptions
class InsufficientFundsError(Exception):
pass
try:
raise InsufficientFundsError("Not enough money")
except InsufficientFundsError as e:
print("Caught:", e)Adding Custom Attributes
Beyond the message, you can attach extra attributes to a custom exception in its __init__ (like self.error_code = code), letting except blocks that catch it access structured details about exactly what went wrong, not just a human-readable string.
Example: Adding Custom Attributes
class InsufficientFundsError(Exception):
def __init__(self, message, error_code):
super().__init__(message)
self.error_code = error_code
try:
raise InsufficientFundsError("balance too low", 402)
except InsufficientFundsError as e:
print(e, e.error_code)Chapter Quiz — Complete all 5 topics to unlock
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