same issue appeared  for usage_helper.py when use py34.

(py34) evan@devstack:~/code/openstack/oslo.log$ python doc/source/examples/usage_helper.py 
DEBUG:__main__:Prepare Oslo Logging
INFO:__main__:Size of configuration options before 0
INFO:__main__:Size of configuration options after 19
INFO:__main__:List of Oslo Logging configuration options and current values
INFO:__main__:================================================================================
INFO:__main__:logging_debug_format_suffix = %(funcName)s %(pathname)s:%(lineno)d
INFO:__main__:log_file = None
INFO:__main__:use_syslog = False
INFO:__main__:logging_default_format_string = %(asctime)s.%(msecs)03d %(process)d %(levelname)s %(name)s [-] %(instance)s%(message)s
INFO:__main__:debug = False
INFO:__main__:log_config_append = None
INFO:__main__:logging_exception_prefix = %(asctime)s.%(msecs)03d %(process)d ERROR %(name)s %(instance)s
INFO:__main__:log_dir = None
INFO:__main__:default_log_levels = ['amqp=WARN', 'amqplib=WARN', 'boto=WARN', 'qpid=WARN', 'sqlalchemy=WARN', 'suds=INFO', 'oslo.messaging=INFO', 'iso8601=WARN', 'requests.packages.urllib3.connectionpool=WARN', 'urllib3.connectionpool=WARN', 'websocket=WARN', 'requests.packages.urllib3.util.retry=WARN', 'urllib3.util.retry=WARN', 'keystonemiddleware=WARN', 'routes.middleware=WARN', 'stevedore=WARN', 'taskflow=WARN', 'keystoneauth=WARN', 'oslo.cache=INFO', 'dogpile.core.dogpile=INFO', 'dogpile=INFO', 'routes=INFO']
INFO:__main__:syslog_log_facility = LOG_USER
INFO:__main__:instance_format = [instance: %(uuid)s] 
INFO:__main__:logging_user_identity_format = %(user)s %(tenant)s %(domain)s %(user_domain)s %(project_domain)s
INFO:__main__:publish_errors = False
INFO:__main__:use_stderr = True
INFO:__main__:verbose = True
INFO:__main__:log_date_format = %Y-%m-%d %H:%M:%S
INFO:__main__:watch_log_file = False
INFO:__main__:logging_context_format_string = %(asctime)s.%(msecs)03d %(process)d %(levelname)s %(name)s [%(request_id)s %(user_identity)s] %(instance)s%(message)s
INFO:__main__:instance_uuid_format = [instance: %(uuid)s] 
INFO:__main__:================================================================================
2016-05-04 15:32:29.510 30676 INFO __main__ [-] Welcome to Oslo Logging
2016-05-04 15:32:29.511 30676 WARNING __main__ [-] A warning occurred
2016-05-04 15:32:29.511 30676 ERROR __main__ [-] An error occurred
--- Logging error ---
Traceback (most recent call last):
  File "/usr/lib/python3.4/logging/__init__.py", line 978, in emit
    msg = self.format(record)
  File "/home/evan/code/openstack/oslo.log/oslo_log/handlers.py", line 73, in format
    return logging.StreamHandler.format(self, record)
  File "/usr/lib/python3.4/logging/__init__.py", line 828, in format
    return fmt.format(record)
  File "/home/evan/code/openstack/oslo.log/oslo_log/formatters.py", line 264, in format
    record.exc_text = self.formatException(record.exc_info, record)
  File "/home/evan/code/openstack/oslo.log/oslo_log/formatters.py", line 274, in formatException
    None, stringbuffer)
  File "/usr/lib/python3.4/traceback.py", line 169, in print_exception
    for line in _format_exception_iter(etype, value, tb, limit, chain):
  File "/usr/lib/python3.4/traceback.py", line 146, in _format_exception_iter
    for value, tb in values:
  File "/usr/lib/python3.4/traceback.py", line 125, in _iter_chain
    context = exc.__context__
AttributeError: 'NoneType' object has no attribute '__context__'
Call stack:
  File "doc/source/examples/usage_helper.py", line 100, in <module>
    LOG.exception("An Exception occurred")
Message: 'An Exception occurred'
Arguments: ()