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After searching through tkinter.py for '_default_root', the patch looks better. The point I missed before is that Tk().__init__(self,...) normally calls _loadtk(self) which installs self as _default_tk if it was None before. So the new function will return the same Tk object each call after the first (unless _default_tk is somehow reset to None.) This also means that the assignment in BaseWidget.setup, "_default_root = Tk()", which the patch deletes, is redundant and misleading, and should go.
I think 'getmaster' or 'get_master' would be a better name than 'setup_master' as setting up a new master is the 2nd backup choice and will happen only once in a session. Most call will get an existing master -- either the one passed in or the once stored as _default_root 
The patch needs to be updated for 3.4 and run with current tests.