This frequently-asked question appears, for example, in 
[[http://groups.google.com/group/comp.lang.python/msg/7edd2c4b0c7fe185|this Usenet posting]].  No single answer is canonical, for each has definite liabilities and limitations; there are, however, a number of expedients which apply in at least some situations:
 * "fire and forget", that is, put the [[http://docs.python.org/lib/module-subprocess.html|subprocess]] in the background and let it run freely;
 * put the subprocess in the background, return to the GUI's event loop, and poll to detect when the subprocess has terminated;
 * put the subprocess in its own thread, and rely on Python ThreadProgramming ...;
 * launch '''subprocess.Popen()''' in a conventional way, but interleave attention in a single thread of control between the GUI controller and the '''stdout''' from the subprocess.
 * the particular GUI framework involved--wxPython, TkInter, probably has specific mechanisms for managing concurrency in an event-based way.  The toolkit behind Tkinter, in fact, already has a Wiki page on [[http://wiki.tcl.tk/1526|the subject above]], many of whose hints can be adapted to other frameworks;
 * ...

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