Responding here to Peter's PR comment.  Peter opened this issue with the claim that the doc failed a specific test case-- document the truth value of set().  Since mappings are (or can be viewed as) a specialized type of set, I always considered that the empty mapping line implicitly covered sets.  But I acknowledge that this is not clear for everyone.  The simplest fix to make the test pass would be: "any unordered collection, for example, set(), {}".  This should also cover frozenset and a possible frozendict.

Raymond noted that 'user-defined' in the last bullet point is wrong (it implies that built-in functions are different) and should be deleted.  He then combined the corrected rule for false with the default rule in the next sentence to produce a succinct statement of the actual rule.  (In CPython, 'default' is literally true. Class 'object' has neither __bool__ nor __len__; ditto for all subclasses that do not add one.)

With a minor change, I like this statement and agree that it should be moved above the examples.  But I think the bullet points should be reduced to just 3, rewritten, and single spaced, but not smashed into running text.  I suggest replacing everything between the first sentence (ending with 'below.') and the last two (beginning with 'Operations') with:

"By default, an object is considered true unless its class defines either a __bool__ method that returns False or __len__ method that returns zero, when called with the object.  Here are most of the built-in objects considered false.

* constants defined to be false: None and False.
* numeric 0 of any type: 0, 0.0, Decimal(0), Fractions(0, 1)
* empty sequences and collections: '', (), [], {}, set(), range(0)
"

Before writing the above, I checked that an instance attribute __bool__ = lambda: False is not consulted by bool().