User-Defined Exception in Python

We have learned about Exception in the article Errors and Exceptions in Python. Then, handling Exceptions was introduced in the article Handling exceptions in Python. Python also provides many built-in Exceptions to help us catch Exceptions. However, sometimes we also need to create Exceptions to serve our purposes. These Exceptions are called User-Defined Exception in Python.

1. Creating a User-Defined Exception in Python

We can create a new User-Defined Exception in the Python class by inheriting from the Exception class.

# class MyError inherits from class Exception
class MyError(Exception):
  # constructor method
  def __init__(self, message):
    self.message = message

  # __str__ display function
  def __str__(self):
    return self.message

try:
  # throw MyError Exception
  raise MyError("User-Defined Exception.")
except MyError as error:
  print('A New Exception occured:', error.message)

Result

A New Exception occured: User-Defined Exception.

When a User-Defined Exception inherits from the Exception class, it can redefine some of the methods of Exception that it inherits.

class MonthWrongValueError(Exception):
  """Exception raised for errors in the input month.

  Attributes:
      month -- input month which caused the error
      message -- explanation of the error
  """
  def __init__(self, month, message="Month is not in (1, 12) range"):
    self.month = month
    self.message = message
    super().__init__(self.message)

  def __str__(self):
    return f'{self.month} -> {self.message}'

month_input = int(input("Enter month: "))
if not 1 <= month_input <= 12:
  raise MonthWrongValueError(month_input)
else:
  print("month: ", month_input)

Result 1

month:  10

Result 2

Traceback (most recent call last):
  File "c:\python-examples\main.py", line 18, in <module>
    raise MonthWrongValueError(month_input)
__main__.MonthWrongValueError: 25 -> Month is not in (1, 12) range

In the above example, the MonthWrongValueError class inherits from the Exception class. The MonthWrongValueError class has redefined the __init__() and __str__() functions of the Exception class to meet its error message output requirements.

2. Inheriting a User-Defined Exception

Other User-Defined Exception classes can inherit a User-Defined Exception class we define ourselves.

# class MyError inherits from class Exception
class MyError(Exception):
  # constructor method
  def __init__(self, message):
    self.message = message

  # __str__ display function
  def __str__(self):
    return self.message

# class ValueTooSmallError inherits from class MyError
class ValueTooSmallError(MyError):
  """Raised when the input value is too small"""
  pass

# class ValueTooLargeError inherits from class MyError
class ValueTooLargeError(MyError):
  """Raised when the input value is too large"""
  pass

# you need to guess this number
number = 5

# user guesses a number until user gets it right
while True:
  try:
    i_num = int(input("Enter a number: "))
    if i_num < number:
      raise ValueTooSmallError("This value is too small, try again!")
    elif i_num > number:
      raise ValueTooLargeError("This value is too large, try again!")
    break
  except ValueTooSmallError as error:
    print(error.message)
  except ValueTooLargeError as error:
    print(error.message)

print("Congratulations! You guessed it correctly.")

Result

This value is too large, try again!
Enter a number: 7
This value is too large, try again!
Enter a number: 3
This value is too small, try again!
Enter a number: 5
Congratulations! You guessed it correctly.

In the above example, we have a User-Defined Exception class called MyError. MyError is also inherited by two other User-Defined Exception classes, ValueTooSmallError and ValueTooLargeError.

In conclusion, user-defined exceptions can make our code readable and error messages more informative. By creating exceptions specific to our program, we can provide more meaningful feedback to our users.