While working for toaster, I came across an incident. A variable which was used for private use was having value as subprocess.Popen. The problem was detected in function: getAllKeysWithFlags from file: bitbake/lib/bb/cooker.py. The error shown was:

ERROR: <Fault 1: "<class 'TypeError'>:cannot marshal <class '_thread.lock'> objects">
Traceback (most recent call last):
  File "/home/sujith/MEL/toaster-latest/poky/bitbake/lib/bb/ui/toasterui.py", line 256, in main
    buildinfohelper.save_build_layers_and_variables()
  File "/home/sujith/MEL/toaster-latest/poky/bitbake/lib/bb/ui/buildinfohelper.py", line 1151, in save_build_layers_and_variables
    data = self.server.runCommand(["getAllKeysWithFlags", ["doc", "func"]])[0]
  File "/usr/lib/python3.4/xmlrpc/client.py", line 1098, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python3.4/xmlrpc/client.py", line 1437, in __request
    verbose=self.__verbose
  File "/usr/lib/python3.4/xmlrpc/client.py", line 1140, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python3.4/xmlrpc/client.py", line 1156, in single_request
    return self.parse_response(resp)
  File "/usr/lib/python3.4/xmlrpc/client.py", line 1327, in parse_response
    return u.close()
  File "/usr/lib/python3.4/xmlrpc/client.py", line 657, in close
    raise Fault(**self._stack[0])
xmlrpc.client.Fault: <Fault 1: "<class 'TypeError'>:cannot marshal <class '_thread.lock'> objects">


So if we handle the types which come from the data store as marshallable or at least if we try to have a mechanism to convert non marshallable type objects to str, then this issue could have been suppressed.