[docs] [issue30803] Truth value of sets not properly documented

[Peter Thomassen]

New submission from Peter Thomassen:

The truth value of sets is not properly documented, in particular regarding whether an empty set is considered false or not.

Ignoring primitive (such as numerals) as well as user-defined types, https://docs.python.org/3/library/stdtypes.html#truth says:

> The following values are considered false:
> 
> - [...]
> - any empty sequence, for example, '', (), [].
> - any empty mapping, for example, {}.
> - [...]
> 
> All other values are considered true

According to https://docs.python.org/3/library/stdtypes.html#sequence-types-list-tuple-range, a set is not a sequence (it is unordered, its elements do not have indices, etc.):

> There are three basic sequence types: lists, tuples, and range objects.

And, according to https://docs.python.org/3/library/stdtypes.html#mapping-types-dict,

> There is currently only one standard mapping type, the dictionary.

So, as per the documentation, the set type is not a type that can ever be False. However, when I try, bool(set()) evaluates to False.

When I asked this on Stack Overflow, someone checked in the CPython code and judged that this is most likely a mere documentation issue: https://stackoverflow.com/a/44813565/6867099

----------
assignee: docs at python
components: Documentation
messages: 297268
nosy: docs at python, thomassen
priority: normal
severity: normal
status: open
title: Truth value of sets not properly documented
type: enhancement
versions: Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30803>
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