[Tutor] perplexing error with shelve

[Orest Kozyar]

I have a program which queries an online database (Medline) for XML data.
It caches all data using shelve to avoid hitting the database too many
times.  For some reason, I keep getting a RuntimeError: maximum recursion
depth exceeded when attempting to add a certain record.  This program has
successfully handled over 40,000 records from Medline, but for whatever
reason, this particular record (PMID: 16842422) produces this error.  If I
set maximum recursion depth to a larger value such as 5,000, I get a
segfault.  

Below is a script that should reproduce the problem I am having.  Any
guidance in solving this problem would greatly be appreciated.  

Thank you,
Orest

####### START SCRIPT #######
import urllib, shelve
from xml.dom import minidom

baseurl = 'http://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?'

params = {
        'db':       'pubmed',
        'retmode':  'xml',
        'rettype':  'medline'
        }

badkey = '16842422'
goodkey = '16842423' #or just about any other ID

data = shelve.open('data.tmp', writeback=True)

try:
    params['id'] = goodkey
    url = baseurl + urllib.urlencode(params)
    doc = minidom.parseString(urllib.urlopen(url).read())
    print 'Successfully retrieved and parsed XML document with ID %s' %
goodkey
    data[goodkey] = doc
    print 'Successfully shelved XML document with ID %s' % goodkey
except RuntimeError:
    print 'Should not see this error message!'

try:
    params['id'] = badkey
    url = baseurl + urllib.urlencode(params)
    doc = minidom.parseString(urllib.urlopen(url).read())
    print 'Successfully retrieved and parsed XML document with ID %s' %
badkey
    data[badkey] = doc
    #Should not get this message!
    print 'Successfully shelved XML document with ID %s' % badkey
except RuntimeError, e:
    print 'Error shelving XML document with ID %s' % badkey
    print e
####### END SCRIPT #######