Python exception chaining [duplicate]

by

Exception chaining is only available in Python 3, where you can write:

try:
    v = {}['a']
except KeyError as e:
    raise ValueError('failed') from e

which yields an output like

Traceback (most recent call last):
  File "t.py", line 2, in <module>
    v = {}['a']
KeyError: 'a'

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

Traceback (most recent call last):
  File "t.py", line 4, in <module>
    raise ValueError('failed') from e
ValueError: failed

In most cases, you don’t even need the from; Python 3 will by default show all exceptions that occured during exception handling, like this:

Traceback (most recent call last):
  File "t.py", line 2, in <module>
    v = {}['a']
KeyError: 'a'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "t.py", line 4, in <module>
    raise ValueError('failed')
ValueError: failed

What you can do in Python 2 is adding custom attributes to your exception class, like:

class MyError(Exception):
    def __init__(self, message, cause):
        super(MyError, self).__init__(message + u', caused by ' + repr(cause))
        self.cause = cause

try:
    v = {}['a']
except KeyError as e:
    raise MyError('failed', e)

More Related Contents:

  1. I’m getting an IndentationError. How do I fix it?
  2. What is the difference between range and xrange functions in Python 2.X?
  3. Is it ever useful to use Python’s input over raw_input?
  4. Python – ‘ascii’ codec can’t decode byte
  5. Dangers of sys.setdefaultencoding(‘utf-8’)
  6. Forced naming of parameters in Python
  7. Python import coding style
  8. How to break a line of chained methods in Python?
  9. How to handle exceptions in a list comprehensions?
  10. threading ignores KeyboardInterrupt exception
  11. How can I make silent exceptions louder in tkinter?
  12. How do you use subprocess.check_output() in Python?
  13. Is there a way to prevent a SystemExit exception raised from sys.exit() from being caught?
  14. What is the difference between isinstance(‘aaa’, basestring) and isinstance(‘aaa’, str)?
  15. How do convert unicode escape sequences to unicode characters in a python string
  16. How to accomplish relative import in python
  17. What is a “callable” in Python?
  18. In Python, when should I use a function instead of a method?
  19. Uninstall python built from source?
  20. Run interactive Bash with popen and a dedicated TTY Python
  21. hasattr() vs try-except block to deal with non-existent attributes
  22. How to re-raise an exception in nested try/except blocks?
  23. Python: Is it bad form to raise exceptions within __init__?
  24. SQLite, python, unicode, and non-utf data
  25. raise statement on a conditional expression
  26. IndentationError: unindent does not match any outer indentation level, although the indentation looks correct
  27. Python long filename support broken in Windows
  28. Length of longest word in a list
  29. How can I denote unused function arguments?
  30. Can Syntax Errors be handled?

Leave a Comment