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Post details: Python Exception Weirdness ?

2005-08-12

18:35:47, Categories: Semantic Web, Software Development

Python Exception Weirdness ?

Why Python does not recognize RDF.RedlandError exception in the following code:

   try:
      parser = RDF.Parser('raptor')

      for s in parser.parse_string_as_stream(data, "http://something"):
         if (s.predicate == seeAlso and s.object.is_resource()):
            result.append(str(s.object.uri))

   except (KeyboardInterrupt, MemoryError):
      raise
   except RDF.RedlandError:
      print " : Caught Redland Exception : " , sys.exc_type
   except Exception:
      print " : Caught Exception :" , sys.exc_type

What is weird - the exception gets caught in "except Exception" clause and is shown as being RDF.RedlandError in the output. Why does Python behave that strange?

 : Caught Exception : RDF.RedlandError

6 comments

Comments, Pingbacks:

Comment from: Christopher Schmidt [Visitor]

Huh. I seem to remember trying to catch RedlandErrors in the past, but I don't know if I ever managed to succeed or what. I do agree that the code sample you have here doesn't work though.

Permalink 2005-08-12 @ 21:57

Comment from: captsolo [Member]

Hi, Chris!

Yes, it does not. Thanks for confirmation. Had I not had XML errors in test data, I'd have thought it catches RedlandErrors all right.

It is disturbing that there is nothing in the source code [that I can think of] that could be causing this error. Which means I'll leave it as is, unless someone can suggest what's causing this.

I have seen other weirdness in Python before - e.g. - I was using it as a simple calculator until I found out that even in simple calculations like 1+1 (don't remember what I was calculating then, but something simple) it will give a wrong result like 2.0001.

Permalink 2005-08-12 @ 23:21

Comment from: amk [Visitor] · http://www.amk.ca

When I've seen this kind of thing in the past, it's because the underlying module was imported twice through different paths, and there are therefore *two* copies of the class. To check this, print RDF.RedlandError and sys.exc_type; do the 'at 0x...' portions of the output match? I'll wager they don't.

A common cause of this is importing a module once in absolute form (from Redland import RDF) and once in relative form (import RDF).

The 1+1=2.001 (a good example is 2/5.0, which is 0.4000002) problem is just the nature of floating point numbers; there's not much to be done about it. You could use the Decimal type, but that's annoying when using it as a calculator.

Permalink 2005-08-13 @ 15:01

Comment from: Christopher Schmidt [Visitor]

Printing out repr(RDF.RedlandError) and sys.exc_type do return different things - however, I can't find anything in a small (10 line code sample) that would seem to cause this.

import sys
import RDF
try:  
      parser = RDF.Parser('raptor')
      for s in parser.parse_as_stream(RDF.Uri("http://crschmidt.net"), 
         RDF.Uri("http://something")):
         if (s.predicate == seeAlso and s.object.is_resource()):
            result.append(str(s.object.uri))

except RDF.RedlandError:
      print " : Caught Redland Exception : " , sys.exc_type
except Exception, E:
      print repr(RDF.RedlandError)
      print repr(sys.exc_type)
      print " : Caught Exception : %s, %s, %s" % (sys.exc_type, type(E), E)

Thoughts?

Permalink 2005-08-22 @ 00:11

Comment from: amk [Visitor] · http://www.amk.ca

What version of Redland? For the version I have installed (0.9.14, I think), the script prints:



Use of deprecated SAXv1 function internalSubset

/usr/lib/python2.3/site-packages/RDF.py:1445: RuntimeWarning: URI http://something:3 - Raptor error -Document element rdf:RDF missing.

  uri._reduri, base_uri._reduri)

URI http://crschmidt.net raptor www error - Failed writing body

Redland may have rearranged its internal imports and caused this problem. (If my theory of relative vs. absolute imports is correct, then this *must* be a Redland bug.)

Permalink 2005-08-22 @ 17:29

Comment from: Nick. [Visitor] · http://bashregion.com/blog/

Thank for the input!

Permalink 2006-10-11 @ 15:22

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