docs@ list report by Daniel Dieterle:

in the documentation (http://docs.python.org/library/subprocess.html#subprocess.Popen.send_signal) is a bug.

CTRL_C_EVENT can not be sent to processes started with a creationflags parameter which includes CREATE_NEW_PROCESS_GROUP. Why can be read in the msdn documentation http://msdn.microsoft.com/en-us/library/windows/desktop/ms683155%28v=vs.85%29.aspx .

A workaround using CTRL_C_EVENT nevertheless is described here:
http://stackoverflow.com/questions/7085604/sending-c-to-python-subprocess-objects-on-windows/7980368#7980368 

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I do not know why the subprocess.CREATE_NEW_PROCESS_GROUP parameter was introduced. But it is useless for terminating a process with os.kill() in combination with signal.SIGTERM, which corresponds to a CTRL-C-EVENT.
A CTRL-C-EVENT is only forwarded to the process if the process group is zero. Therefore the Note in the documentation on Popen.send_signal() is wrong.