Python ElementTree parse xml with NoneType exception -

i meet problem werid. here code.

#!/usr/bin/python # filename: parse_dblp.py # author: ivanchou  import codecs, os import xml.etree.elementtree et paper_tag = ('article','inproceedings','proceedings','book',              'incollection','phdthesis','mastersthesis','www')  class allentities:     def __getitem__(self, key):         return key   print ('----------parse begin----------')  # parse result store authors result = codecs.open('authors','w','utf-8')  parser = et.xmlparser() parser.parser.useforeigndtd(true) parser.entity = allentities()  event, article in et.iterparse('dblp_part.xml', events=("start",   "end"), parser=parser):     author in article.findall('author'):         result.write(author.text + u'|')     if event == 'end' , article.tag in paper_tag:         result.write(os.linesep)     article.clear() print ('----------parse end----------') 

the file dblp_part.xml have create gist here: dblp_part.xml

it contains head 2336 line of dblp.xml, , last article element returns me nonetype error, , if exchange last 2 element, it's ok. bug of elementtree?

i newer of python, search of help.