[issue30803] Truth value of sets not properly documented
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-ra..., 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@python components: Documentation messages: 297268 nosy: docs@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 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Raymond Hettinger added the comment: This case was supposed to be covered by the last bullet point, "instances of user-defined classes, if the class defines a __bool__() or __len__() method, when that method returns the integer zero or bool value False.". The word "user-defined" should be dropped. Also, the whole section can be simplified to something like: """ By default, objects are considered true unless they define either a __bool__ method that returns False or __len__ method that returns zero. Practically, this means that empty containers are false (such as [], (), {}, '', etc) and that numbers equal to zero are false (such as 0, 0.0, 0.0j, False, Decimal(0), Fractions(0, 1), etc). Also, *None* is a false value. """ ---------- assignee: docs@python -> rhettinger nosy: +rhettinger priority: normal -> low _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Changes by Roundup Robot <devnull@psf.upfronthosting.co.za>: ---------- pull_requests: +2579 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Peter Thomassen added the comment: I submitted a PR on github, and signed the CLA before doing so. (I double-checked my bpo username in the CLA, and my github username in the bpo profile.) Still, the bot says I need to sign the CLA. I'm not sure what to do? ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Terry J. Reedy added the comment: 1. You have to go to your profile page https://bugs.python.org/user26480 and add your GitHub name in the GitHub Name box. 2. A committer has to change the labels to trigger the robot to recheck. I did that but it did not work because of 1. As I said in my review, I strongly prefer leaving the bulleted list and making a minimal addition of 'set or' and 'set(), '. I would not merge the current patch. ---------- nosy: +terry.reedy _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
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(). ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Peter Thomassen added the comment: I like your most recent suggestion, and updated the PR after fixing a typo ('Fractions') and making it more complete (complex numbers). Let me know if anything else is needed. (A mapping is not a specialized set, at least as far as typing is concerned: `isinstance({}, set)` is false. Semantically, they may be related, but the question here is whether the types are actually related.) ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Terry J. Reedy added the comment: Raymond, if you either unassign yourself or approve the current PR, I will merge and backport to 3.6 (but not 3.5). ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Changes by Raymond Hettinger <raymond.hettinger@gmail.com>: ---------- assignee: rhettinger -> terry.reedy _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Terry J. Reedy added the comment: New changeset caa1280d1ee5f828f346b585169a7592371d3faa by Terry Jan Reedy (Peter Thomassen) in branch 'master': bpo-30803: clarify truth value testing documentation (#2508) https://github.com/python/cpython/commit/caa1280d1ee5f828f346b585169a7592371... ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Changes by Terry J. Reedy <tjreedy@udel.edu>: ---------- pull_requests: +2995 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Terry J. Reedy added the comment: New changeset 4c7b368de7bcabdd821059c023c46c9d85668d3f by Terry Jan Reedy in branch '3.6': [3.6] bpo-30803: clarify truth value testing documentation (GH-2508) (#2946) https://github.com/python/cpython/commit/4c7b368de7bcabdd821059c023c46c9d856... ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Changes by Terry J. Reedy <tjreedy@udel.edu>: ---------- resolution: -> fixed stage: -> resolved _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
Changes by Terry J. Reedy <tjreedy@udel.edu>: ---------- status: open -> closed _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue30803> _______________________________________
participants (4)
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Peter Thomassen -
Raymond Hettinger
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Roundup Robot
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Terry J. Reedy