Bern:

I implemented pretty(self, format="%0.3f").

One thing I didn't expect is that "%0.3f"%True is 1.000.

What should pretty return if the value is None, or can't be represented using the specified 
format parameter? 

I implemented "" for None and str(self._value) for anything else.

Does that work?

If the format is invalid (e.g. '%0.3') it raises a ValueError and that should
get handled like any other error.

changeset:   6832:234fefd7568a