WritingExceptionClasses

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Writing Exception Classes

Exception classes are not special, you just derive them from Exception:

   1 class HostNotFound(Exception):
   2     def __init__( self, host ):
   3         self.host = host
   4         Exception.__init__(self, 'Host Not Found exception: missing %s' % host)

You may later write:

   1 try:
   2     raise HostNotFound("taoriver.net")
   3 except HostNotFound, exc:
   4     # Handle exception.
   5     print exc  # -> 'Host Not Found exception: missing taoriver.net'
   6     print exc.host  # -> 'taoriver.net'

See Also

HandlingExceptions, TracebackModule

Questions

• How do you relay the traceback information? Relay the traceback information? Moving it higher up the call-stack? Could you try to explain your question?

  • When you’re logging exceptions, you want access to the traceback information to. After some research, I believe what you use is extract_tb or extract_stack from the traceback module. – LionKimbro 2003-09-07 15:23:43

  Look at cgitb for how to do detailed TB introspection, and as an example of why mixing logic and (HTML) layout is a very bad thing.

• What better exception-foo is out there?

  AlexMartelli’s “Dos and Don’ts”.

• The new text involving super doesn’t seem to work for me.

Using Python 2.3:

   1 class LocalNamesSyntaxError(Exception):
   2     def __init__(self, msg):
   3         self.msg=msg
   4         super(LocalNamesSyntaxError, self).__init__('Local Names v1.1 Syntax Error: %s' % msg)

Traceback (most recent call last):
  File "parser.py", line 92, in ?
    pprint.pprint(parse_text(test_string))
  File "parser.py", line 69, in parse_text
    cursor=parse_record_type(cursor,line,results)
  File "parser.py", line 43, in parse_record_type
    raise LocalNamesSyntaxError("unrecognized v1.1 record type- require LN, NS, X, or PATTERN")
  File "parser.py", line 17, in __init__
    super(LocalNamesSyntaxError, self)('Local Names v1.1 Syntax Error: %s' % msg)
TypeError: super() argument 1 must be type, not classobj

Is this a Python2.4 v. Python2.3 thing? Or is there a simple error in my code? – LionKimbro

super() only works for new-style classes. Exception is still an old-style class: type ‘classobj’. I’ve fixed the example. – JohannesGijsbers

Thanks a lot, I was having hardtime figureing this out, as exactly the same construct worked in other places. – HariDara

It might become a 2.4 vs 2.5 thing though, as 2.5 should allow new-style classes.

Don’t forget the method name after super; it doesn’t magically assume __init__. (fixed above) -kcarnold