Method user_agent_parser::_lookup() generates a KeyError exception at line 237 in uap-python/ua_parser/user_agent_parser.py. Not all callers of this method can handle the exception.

It would be better if line 237 was this:

_PARSE_CACHE.pop(next(iter(_PARSE_CACHE)), "UNKNOWN")

I'll send a pull request. Please let me know if you need more info.