Built-in Exceptions

Docs: https://docs.python.org/3.10/library/exceptions.html#built-in-exceptions

The built-in exception classes can be subclassed to define new exceptions; programmers are encouraged to derive new exceptions from the Exception class or one of its subclasses, and not from BaseException.

More information on defining exceptions is available in the Python Tutorial under User-defined Exceptions.

Exception context

When raising (or re-raising) an exception in an except or finally clause __context__ is automatically set to the last exception caught; if the new exception is not handled the traceback that is eventually displayed will include the originating exception(s) and the final exception.

When raising a new exception (rather than using a bare raise to re-raise the exception currently being handled), the implicit exception context can be supplemented with an explicit cause by using from with raise.

nested_exceptions.py

def outer():
    try:
        inner()
    except Exception as err:
        err_name = f'{err.__class__.__module__}.{err.__class__.__name__}'
        print(f'Catched in outer: {err_name}; {str(err)}')
        raise ValueError from err
    finally:
        print('outer finally')


def inner():
    try:
        1 / 0
    except ZeroDivisionError as err:
        err_name = f'{err.__class__.__module__}.{err.__class__.__name__}'
        print(f'Catched in inner: {err_name}; {str(err)}')
        raise err
    finally:
        print('inner finally')

def main():
    outer()

if __name__ == '__main__':
    main()

output

Traceback (most recent call last):
  File "exceptions.py", line 3, in outer
    inner()
  File "exceptions.py", line 16, in inner
    raise err
  File "exceptions.py", line 13, in inner
    1 / 0
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "exceptions.py", line 24, in <module>
    main()
  File "exceptions.py", line 21, in main
    outer()
  File "exceptions.py", line 6, in outer
    raise ValueError from err
ValueError
Catched in inner: builtins.ZeroDivisionError; division by zero
inner finally
Catched in outer: builtins.ZeroDivisionError; division by zero
outer finally

Note

Passing arguments of the wrong type (e.g. passing a list when an int is expected) should result in a TypeError, but passing arguments with the wrong value (e.g. a number outside expected boundaries) should result in a ValueError.

Exception hierarchy

The class hierarchy for built-in exceptions is:

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- StopAsyncIteration
      +-- 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
      +-- SyntaxError
      |    +-- IndentationError
      |         +-- TabError
      +-- SystemError
      +-- TypeError
      +-- ValueError
      |    +-- UnicodeError
      |         +-- UnicodeDecodeError
      |         +-- UnicodeEncodeError
      |         +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
           +-- ImportWarning
           +-- UnicodeWarning
           +-- BytesWarning
           +-- EncodingWarning
           +-- ResourceWarning