Musings: Using decorators to reduce duplicate exception handling

Gabriel Genellina gagsl-py2 at
Wed Feb 18 02:01:36 EST 2009

En Tue, 17 Feb 2009 21:12:57 -0200, J Kenneth King <james at>  

> I recently started a project called TracShell
> ( where I make heavy use of the
> xmlrpclib core module.
> When the number of RPC calls was small, wrapping each call in try/except
> was acceptable. However, this obviously will duplicate code all over the
> place. There are only ever two exceptions that the module itself will
> throw: xmlrpclib.ProtocolError and xmlrpclib.Fault -- both very useful,
> but not show stoppers.
> To combat the duplication, my clever idea was to use a function
> decorator to wrap any function that used xmlrpclib calls:
> def catch_errors(fn):
>     """
>     A decorator to catch typical xmlrpclib exceptions
>     """
>     def wrapped(*args, **kwargs):
>         try:
>             return fn(*args, **kwargs)
>         except xmlrpclib.ProtocolError, e:
>             print "There was a problem communicating with the server."
>             print "URL: %s" % e.url
>             print "Headers: %s" % e.headers
>             print "Error code: %d" % e.errcode
>             print "Error message: %s" % e.errmsg
>             print "Please file a report with the TracShell developers."
>             pass
>         except xmlrpclib.Fault, e:
>             print "A fault ocurred"
>             print "Fault code: %d" % e.faultCode
>             print "Fault string: %s" % e.faultString
>             print "If you think this message is the result of an error,"
>             print "please file a report with the TracShell developers."
>             pass
>     return wrapped

I don't like the idea of "hiding" an exception. The caller code doesn't  
know an exception occurred, and just continue doing its work, with bogus  
results... is this what you want? Also, you don't include the stack trace  
- and usually it contains very valuable information. When your users start  
"filing a report with the TracShell developers" and you feel clueless, a  
stack trace is important (you don't have to show it on screen - a log file  
is even better).

If all you want is to customize the error message, use sys.except_hook

Looking into the code, those "pass" statement are useless; and error  
messages are usually written to stderr instead of stdout.

Gabriel Genellina

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