bpo-32118: Document that, in sequence comparisons, reflexivity means otherwise u…#4514
bpo-32118: Document that, in sequence comparisons, reflexivity means otherwise u…#4514dubslow wants to merge 2 commits into
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…norderable objects are ignored In sequence comparisons, the enforcement of reflexivity of elements means that only non-identical elements are actually compared. The docs then note, with example, that non-reflexive elements thus always "compare" equal inside the sequence. This patch adds a second corollary, that non-orderable singletons (e.g. None) will also not break sequence comparison. Yes, the consequence is logically derivable from the statement "element identity is compared first, and element comparison is performed only for distinct elements", but the first example is given because "For non-reflexive elements, the result is different than for strict element comparison, and may be surprising", which also holds for the example I add here: different from strict element comparison, which may lead to otherwise surprising results (it sure was surprising to me when I expected a list with Nones to fail to compare, hence why I went trawling through the docs). In the manner of the first example, explicit is better than implicit, and (I believe) it will be helpful for readers to have this second consequence demonstrated.
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rhettinger
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rhettinger
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See the comment in the issue tracker. I prefer not to make this patch or anything else that looks like an exception to an exception with a list of examples are all atypical and weird). It makes Python look like a minefield of special cases and doesn't facilitate building a clean mental model that allows people to make successful predictions.
terryjreedy
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I agree that the current doc might be improved.
However, the phenomenon noted is not about reflexivity nor about singletons. The example does not need the 'is' and '==' parts. I am not sure this is the best place for an addition. I think the patch as is should be rejected.
I will look at the current doc and try to suggest something that I think is better.
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@terryjreedy I like your version (on bpo) better. I'm closing this PR in favor of TJR's suggestion (other than perhaps the one word grammar patch in 5d5fe0d) |
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…norderable objects are ignored
In sequence comparisons, the enforcement of reflexivity of elements means that only non-identical elements are actually compared. The docs then note, with example, that non-reflexive elements thus always "compare" equal inside the sequence.
This patch adds a second corollary, that non-orderable singletons (e.g. None) will also not break sequence comparison.
Yes, the consequence is logically derivable from the statement "element identity is compared first, and element comparison is performed only for distinct elements", but the first example is given because "For non-reflexive elements, the result is different than for strict element comparison, and may be surprising", which also holds for the example I add here: different from strict element comparison, which may lead to otherwise surprising results (it sure was surprising to me when I expected a list with Nones to fail to compare, hence why I went trawling through the docs). In the manner of the first example, explicit is better than implicit, and (I believe) it will be helpful for readers to have this second consequence demonstrated.
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Where: [X.Y] is the branch name, e.g. [3.6].
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https://bugs.python.org/issue32118