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I've just broken this patch by changing the signature of the set_hook/
on_hook functions.

There seem to be some whitespace oddities (e.g. right at the start of the 
patch).

The docstrings should use the PyDoc_STRVAR macros.

I'm not sure about some of the details of the py_callback stuff (although 
the functionality seems desirable):

1) the name -- I think I'd find python_command more transparent
2) it would be nice to pass the count and key arguments to the Python 
callable
3) the comments around it's implementation don't make much sense to 
me.
4) the example in the docs -- which is what finally clued me in to what the 
function did :) -- isn't self-contained.

None of this should be very hard to deal with, but I don't know when/if I'd 
get round to it myself.