I am working with a large dataset of emails and loading one of them resulted in an exception: "TypeError: unorderable types: ValueTerminal() < CFWSList()"I have attached the (anonymised and minimised) email source of the email that triggered the exception.$ pythonPython 3.4.2 (default, Nov 12 2014, 18:23:59) [GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.54)] on darwinType "help", "copyright", "credits" or "license" for more information.>>> import email>>> from email import parser, policy>>> >>> f = open("testmail.eml",'rb')>>> src = f.read()>>> f.close()>>> >>> msg = email.parser.BytesParser(_class=email.message.EmailMessage, policy=email.policy.default).parsebytes(src)Traceback (most recent call last):  File "<stdin>", line 1, in <module>  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/parser.py", line 124, in parsebytes    return self.parser.parsestr(text, headersonly)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/parser.py", line 68, in parsestr    return self.parse(StringIO(text), headersonly=headersonly)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/parser.py", line 57, in parse    feedparser.feed(data)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/feedparser.py", line 178, in feed    self._call_parse()  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/feedparser.py", line 182, in _call_parse    self._parse()  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/feedparser.py", line 384, in _parsegen    for retval in self._parsegen():  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/feedparser.py", line 255, in _parsegen    if self._cur.get_content_type() == 'message/delivery-status':  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/message.py", line 579, in get_content_type    value = self.get('content-type', missing)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/message.py", line 472, in get    return self.policy.header_fetch_parse(k, v)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/policy.py", line 145, in header_fetch_parse    return self.header_factory(name, ''.join(value.splitlines()))  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/headerregistry.py", line 583, in __call__    return self[name](name, value)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/headerregistry.py", line 194, in __new__    cls.parse(value, kwds)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/headerregistry.py", line 441, in parse    kwds['decoded'] = str(parse_tree)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/_header_value_parser.py", line 195, in __str__    return ''.join(str(x) for x in self)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/_header_value_parser.py", line 195, in <genexpr>    return ''.join(str(x) for x in self)  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/_header_value_parser.py", line 1136, in __str__    for name, value in self.params:  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/email/_header_value_parser.py", line 1101, in params    parts = sorted(parts)TypeError: unorderable types: ValueTerminal() < CFWSList()

New changesetdc10c52c6539 by R David Murray in branch '3.4':#23745: handle duplicate MIME parameter names in new parser.https://hg.python.org/cpython/rev/dc10c52c6539New changesetfe9a578d5f38 by R David Murray in branch 'default':Merge:#23745: handle duplicate MIME parameter names in new parser.https://hg.python.org/cpython/rev/fe9a578d5f38

The issue arose from the duplicated parameter name.  I fixed it by (mostly) copying the error recovery used by the older api (get_param).Note that you don't need to specify both policy and _class.  If you use the new policies (such as default), it automatically uses EmailMessage for the _class.