[Tutor] String Encoding problem

[Strax-Haber, Matthew (LARC-D320)]

________________________________
> From: spir <denis.spir at free.fr>
> Date: Mon, 20 Apr 2009 12:22:59 -0500
> To: Python Tutor <tutor at python.org>
> Subject: Re: [Tutor] String Encoding problem
>
> Le Mon, 20 Apr 2009 10:46:47 -0400,
> Matt <HellZFury+Python at gmail.com> s'exprima ainsi:
>
>> Hey everyone,
>>
>> I'm hoping someone here can help me solve an odd problem (bug?). I'm having
>> trouble with string encoding, object deletion, and the xml.etree library. If
>> this isn't the right list to be posting this question, please let me know.
>> I'm new to Python and don't know of any other "help me" Python mailing
>> lists. I have tried debugging this ad-infinitem. Anyway, at the bottom of
>> this e-mail you will find the code of a python file. This is a gross
>> over-simplification of my code, with little exception handling so that the
>> errors are obvious.
>>
>> Running this interactively, if you finish off with 'del db', it exits fine
>> and creates a skeleton xml file called 'db.xml' with text '<root />'.
>> However, if you instead CTRL-D, it throws at exception while quitting and
>> then leaves an empty 'db.xml' which won't work. Can anyone here help me
>> figure out why this is?
>>
>> Stuff I've done:
>> I've traced this down to the self.commit() call in __del__. The stacktrace
>> and a few print statements injected into xml.etree leads me to the call
>> 'root'.encode('us-ascii') throwing a LookupError on line 751 of
>> xml.etree.ElementTree. This makes no sense to me, since it works fine
>> normally.
>>
>> Thank you very much. Any and all help or pointers are appreciated.
>>
>> ~Matt
>>
>> #### db.py ###
>> from xml.etree import ElementTree as ET
>> import os
>>
>> class Database(object):
>>     def __init__(self, path):
>>         self.__dbpath = path    ## Path to the database
>>         self.load()
>>     def __del__(self):
>>         ## FIXME: Known bug:
>>         ##  del db at command line works properly
>>         ##  Ctrl-D, when there is no db file present, results in a
>> LookupError
>>         ##    and empty xml file
>>         from StringIO import StringIO
>>         from traceback import print_exc
>>         trace = StringIO()
>>         try:
>>             print 5
>>             self.commit()
>>             print 7
>>         except Exception:
>>             print_exc(100, trace)
>>             print trace.getvalue()
>>     def load(self):
>>         if os.path.exists(self.__dbpath):
>>             self.root = ET.parse(self.__dbpath).getroot()
>>         else:
>>             self.root = ET.Element("root")
>>     def commit(self):
>>         ET.ElementTree(self.root).write(self.__dbpath)
>> db = Database('db.xml')
>
> Actually, it all runs well for me -- after the following modification:
>
>     def __del__(self):
>         ## FIXME: Known bug:
>         ##  del db at command line works properly
>         ##  Ctrl-D, when there is no db file present, results in a LookupError
>         ##    and empty xml file
>         try:
>             print 5
>             self.commit()
>             print 7
>         except Exception:
>             raise

I must be missing something.... I run the following code (in DB.py) without any other files in the current directory:
from xml.etree import ElementTree as ET import os class Database(object):    def __init__(self, path):        self.dbpath = path    ## Path to the database        self.load()    def __del__(self):        try:            print 5            self.commit()            print 7        except Exception:            raise    def load(self):        if os.path.exists(self.dbpath):            self.root = ET.parse(self.dbpath).getroot()        else:            self.root = ET.Element("root")    def commit(self):        ET.ElementTree(self.root).write(self.dbpath) db = Database('db.xml')

Output:
5 Exception LookupError: LookupError('unknown encoding: us-ascii',) in <bound method Database.__del__ of <__main__.Database object at 0x87870>> ignored

If you're not getting the same output, please let me know what your environment is. Perhaps this is an implementation difference across platforms.


>
> Notes:
> * I don't know for what reason you needed such a complicated traceback
> construct.

That was only to demonstrate the error. Without that, you see a LookupError without any trace.

> * Before I did this modif, I indeed had a weird exception about stringIO.
Top-level imports are not consistently available in __del__. That shouldn't be necessary with the code I have above.

> * __del__() seems to do the contrary: it writes back to file through commit()???
Yes, I know. In my actual code, there is a flag that is set when certain run-time conditions are met or when the user wants the DB to be saved on quit. Most of the time, however, modifications to the database need to be done in memory because they are not intended to be saved.

> * "del db" works fine, anyway
> * When I run without any bd.xml, it properly creates one with text "<root />".
> * When I run with an ampty db.xml, I have the following exception message:
>
> Traceback (most recent call last):
>   File "xmlTree.py", line 29, in <module>
>     db = Database('db.xml')
>   File "xmlTree.py", line 10, in __init__
>     self.load()
>   File "xmlTree.py", line 24, in load
>     self.root = ET.parse(self.__dbpath).getroot()
>   File "/usr/lib/python2.5/xml/etree/ElementTree.py", line 862, in parse
>     tree.parse(source, parser)
>   File "/usr/lib/python2.5/xml/etree/ElementTree.py", line 587, in parse
>     self._root = parser.close()
>   File "/usr/lib/python2.5/xml/etree/ElementTree.py", line 1254, in close
>     self._parser.Parse("", 1) # end of data
> xml.parsers.expat.ExpatError: no element found: line 2, column 0
> 5
> Exception exceptions.AttributeError: AttributeError("'Database' object has no
> attribute 'root'",) in <bound method Database.__del__ of <__main__.Database
> object at 0xb7e78fec>> ignored
>
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--
~Matthew Strax-Haber
National Aeronautics and Space Administration
Langley Research Center (LaRC)
Co-op, Safety-Critical Avionics Systems Branch
Matthew.Strax-Haber at nasa.gov
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