Python exception class

Python exception class is the base class from which exceptions inherit. The class Exception and its subclasses are a form of Throwable that indicates conditions that a reasonable application might want to catch.

The class Exception and any subclasses that are not also subclasses of RuntimeException are checked exceptions.

Creating Exception Class

class JustException(Exception):
    def __init__(self, message):
        print(message)

Python exception class

Simple example code of Creating User-Defined Exception in Python

# Base class
class PerError(Exception):
    pass


# Exception class for percentage > 100
class InvalidPerError(PerError):
    def __init__(self):
        super().__init__("Percentage is invalid")


# Exception class for percentage < 80
class LessPerError(PerError):
    def __init__(self):
        super().__init__("The Percentage is lesser than the Cut-off.")


# class to check percentage range
class checkPer(PerError):
    def __init__(self, per):
        if per < 80:
            raise LessPerError
        if per > 100:
            raise InvalidPerError
        print("Congrats you're Enrolled")


# different cases and output
try:
    print("For Percentage: 93")
    checkPer(93)
except Exception as e:
    print(e)

try:
    print("For Percentage: 102")
    checkPer(102)
except Exception as e:
    print(e)

try:
    print("For Percentage: 58")
    checkPer(58)
except Exception as e:
    print(e)

Output:

Which classes are exception classes in Python?

Answer: All exception classes are derived from the BaseException class. The BaseException is the base class of all other exceptions. The Python Exception Hierarchy is like this below.

• BaseException
• Exception
  • ArithmeticError
    • FloatingPointError
    • OverflowError
    • ZeroDivisionError
    • AssertionError
    • AttributeError
    • BufferError
    • EOFError
    • ImportError
      • ModuleNotFoundError
    • LookupError
      • IndexError
      • KeyError
    • MemoryError
    • NameError
      • UnboundLocalError
    • OSError
      • BlockingIOError
      • ChildProcessError
      • ConnectionError
        • BrokenPipeError
        • ConnectionAbortedError
        • ConnectionRefusedError
        • ConnectionResetError
    • FileExistsError
    • FileNotFoundError
    • InterruptedError
    • IsADirectoryError
    • NotADirectoryError
    • PermissionError
    • ProcessLookupError
    • TimeoutError
• ReferenceError
• RuntimeError
  • NotImplementedError
  • RecursionError
• StopIteration
• StopAsyncIteration
• SyntaxError
  • IndentationError
    • TabError
• SystemError
• TypeError
• ValueError
  • UnicodeError
    • UnicodeDecodeError
    • UnicodeEncodeError
    • UnicodeTranslateError
• Warning
  • BytesWarning
  • DeprecationWarning
  • FutureWarning
  • ImportWarning
  • PendingDeprecationWarning
  • ResourceWarning
  • RuntimeWarning
  • SyntaxWarning
  • UnicodeWarning
  • UserWarning
• GeneratorExit
• KeyboardInterrupt
• SystemExit

Do comment if you have any doubts or suggestions on this Python exception topic.

Note: IDE: PyCharm 2021.3.3 (Community Edition)

Windows 10

Python 3.10.1

All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.