> It seems like self.stdout.read() hangs even after the child process 
> has been killed.

This is an issue when the standard handles are inherited or duplicated to a grandchild process, and so on. In the case of Popen(sys.executable), the system is duplicating the standard handles implicitly because sys.executable is a console process (assuming it's python[_d].exe), among other criteria. An additional reference on the write side prevents the pipe from closing. fh.read() in the grandparent won't return as long as there's a writer that could potentially write more data.