[Python-Dev] PEP 495 Was: PEP 498: Literal String Interpolation is ready for pronouncement (original) (raw)

Terry Reedy tjreedy at udel.edu
Sun Sep 13 02:49:12 CEST 2015

On 9/12/2015 1:04 PM, Alexander Belopolsky wrote:

On Sat, Sep 12, 2015 at 1:20 AM, Terry Reedy <tjreedy at udel.edu_ _<mailto:tjreedy at udel.edu>> wrote: A mathematician has no problem with 'a'+'b' != 'b'+'a'. I doubt it. A binary operation denoted + (and called addition) is almost universally a commutative operation. A non-commutative binary operation is usually denoted * (and called multiplication).

I am aware of the single-operation group theory convention, but the context was sequence concatenation and scalar multiplication, where '*' is repeated '+'. But these details are not directly relevant to DateTimes.

After closure,

I perhaps should have said 'completeness'. In any case, I was referring to 'a op b' existing for all a and b in the set.

Agree, and we have a solution for PEP 495 which preserves == as and equivalence (symmetric, reflexive and transitive) relationship.

Datetime members, are rather unusual beasts. They are triples consisting of a member of a discrete sequence (with some odd gaps),

Your correction, summarized, is that there are no gaps, so the set is simpler than I thought. Skipping on to the heart of the matter.

I am not concerned about '-'. My main concern is about order operations.

Is not '<' defined, in the obvious way, in terms of '-' and the sign of the resul?

I am happy with the solution I have for ==, but I am still struggling with the non-transitivity of <.

I am guessing that the 'struggle' is at least partly this: "Is non-transitivity of < necessary given other constraints, including back-compatibility, or is there another solution possible that would also be <-transitive?"

Comparison operations are special because they are used implicitly in other operations. The < operator is used implicitly in bisect. If it does not satisfy the (partial?) order properties, bisect may enter an infinite loop.

and, if we are stuck with <-intransitivity, what do we do? If back-compatibility allowed, I might suggest defining 'lt' or 'less' rather than 'lt' so that sort and bisect don't work with DateTimes. Then document that the function is not transitive.

-- Terry Jan Reedy

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