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There's 'execfile' in Lib/idlelib/PyShell.py...I'm looking for others.
There is also a class of functions 'exec*' (and a related class, 
'spawn*') in Lib/os.py that claims to do just what you want, but 
replacing the current process (or, in spawn*'s case, in a new thread).
That said, I agree that exec should handle streams, and if you could 
give me a heads up as to where the code is for exec(), I could try my 
hand at a patch.