The doc [1] actually says:
object.__nonzero__(self)
    Called to implement truth value testing, and the built-in operation
bool(); should return False or True, or their integer equivalents 0 or
1. When this method is not defined, __len__() is called, if it is
defined (see below). If a class defines neither __len__() nor
__nonzero__(), all its instances are considered true.

I suggest to:
1) drop the comma after 'testing';
2) clarify what happens when __nonzero__ is defined and where 'below'
actually is (and possibly add a link).


[1]: http://docs.python.org/reference/datamodel.html#object.__nonzero__