Terry J. Reedy added the comment:
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().
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