lxml.etree.iterparse closes input file handler?

Question

filterous is using iterparse to parse a simple XML StringIO object in a unit test. However, when trying to access the StringIO object afterwards, Python exits with a "ValueError: I/O operation on closed file" message. According to the iterparse documentation, "Starting with lxml 2.3, the .close() method will also be called in the error case," but I get no error message or Exception from iterparse. My IO-foo is obviously not up to speed, so does anyone have suggestions?

The command and (hopefully) relevant code:

$ python2.6 setup.py test

setup.py:

from setuptools import setup
from filterous import filterous as package

setup(
    ...
    test_suite = 'tests.tests',

tests/tests.py:

from cStringIO import StringIO
import unittest

from filterous import filterous

XML = '''<posts tag="" total="3" ...'''

class TestSearch(unittest.TestCase):
    def setUp(self):
        self.xml = StringIO(XML)
        self.result = StringIO()
    ...
    def test_empty_tag_not(self):
        """Empty tag; should get N results."""
        filterous.search(
            self.xml,
            self.result,
            {'ntag': [u'']},
            ['href'],
            False)
        self.assertEqual(
            len(self.result.getvalue().splitlines()),
            self.xml.getvalue().count('<post '))

filterous/filterous.py:

from lxml import etree
...
def search(file_pointer, out, terms, includes, human_readable = True):
    ...
    context = etree.iterparse(file_pointer, tag='posts')

Traceback:

ERROR: Empty tag; should get N results.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/victor/dev/filterous/tests/tests.py", line 149, in test_empty_tag_not
    self.xml.getvalue().count('<post '))
ValueError: I/O operation on closed file

PS: The tests all ran fine on 2010-07-27.

Answer 1

Seems to work fine with StringIO, try using that instead of cStringIO. No idea why it's getting closed.