All the error constants in `xml.parsers.expat.errors` are strings.
However, when expat raises an ExpatError exception, ExpatError.code
attribute is a number. There seems to be no way of associating
ExpatError with a corresponding error code from `xml.parsers.expat.errors.

Following code snippet should print "Ignore empty file" but in Python
2.6 it raises ExpatError.

    from xml.etree import ElementTree
    from xml.parsers import expat
    
    try:
        ElementTree.parse('')
    except expat.ExpatError as e:
        if e.code == expat.errors.XML_ERROR_NO_ELEMENTS:
            print "Ignore empty file"
        else:
            raise

In the snippet `ElementTree.parse('')` should be replaced by
`ElementTree.fromstring('')`.

After talking to Fred, I'll add a "codes" attribute mapping the error constant strings to their codes.  Changing the constants to integers would be very bad for b/w compatibility.

Fixed in r85526.

Please, add a tiny unit test for the presence of this feature.
This is the only way for vm implementers to follow CPython development.

You're completely correct, added one in r85528. Thanks!