Differences between revisions 2 and 18 (spanning 16 versions)

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'

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

-  ⇤ ← Revision 2 as of 2003-09-06 19:11:37 → 
  Size: 1108
  Editor: ip503dabc3
  Comment:
+   ← Revision 18 as of 2011-05-16 19:13:51 → ⇥
  Size: 2697
  Editor: VPN-18-101-8-113
  Comment: Remind readers that super() doesn't magically mean __init__.
-Deletions are marked like this.
+Additions are marked like this.
 Line 3:
-I (LionKimbro) don't know much about writing exception classes; Here's hoping someone rewrites this better.

Exception classes are not special; You just write an ordinary class:
+Exception classes are not special, you just derive them from Exception:
-Line 12:
+Line 10:
+        Exception.__init__(self, 'Host Not Found exception: missing %s' % host)
-Line 19:
+Line 18:
-    raise HostNotFound( "taoriver.net" )
except HostNotFound, X:
    print "Host Not Found:", X.host
+    raise HostNotFound("taoriver.net")
except HostNotFound, exc:
    # Handle exception.
    print exc  # -> 'Host Not Found exception: missing taoriver.net'
    print exc.host  # -> 'taoriver.net'
-Line 24:
+Line 26:
-== Overloading __str__ ==
+== See Also ==
-Line 26:
+Line 28:
-You can overload {{{__str__}}} to get the exception to explain itself:
+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 <<DateTime(2003-09-07T15:23:43Z)>>

  ''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:
-Line 30:
+Line 47:
-class HostNotFound:
    def __init__( self, host ):
        self.host = host
    def __str__( self ):
        return "Host Not Found exception: missing %s" % self.host
+class LocalNamesSyntaxError(Exception):
    def __init__(self, msg):
        self.msg=msg
        super(LocalNamesSyntaxError, self).__init__('Local Names v1.1 Syntax Error: %s' % msg)
-Line 37:
+Line 53:
-That way, you only need print the exception instance:
-Line 40:
+Line 54:
-#!python
try:
    raise HostNotFound( "taoriver.net" )
except HostNotFound, X:
    print X
+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
-Line 47:
+Line 66:
-I don't know if this is a good idea or not.
+Is this a Python2.4 v. Python2.3 thing? Or is there a simple error in my code? -- LionKimbro
-Line 49:
+Line 68:
-== Questions ==
+super() only works for new-style classes. Exception is still an old-style class: type 'classobj'. I've fixed the example. -- JohannesGijsbers
-Line 51:
+Line 70:
-  * How do you relay the traceback information?
  * What better exception-foo is out there?
+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

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Writing Exception Classes

See Also

Questions