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

[Alexander Belopolsky]

On Sat, Sep 12, 2015 at 1:20 AM, Terry Reedy <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).


>   After closure,


Do you refer to "set closure" operation [1] here?  I am not sure why it is
relevant nor why it is "basic."


> associativity is the most 'basic' operation, but non-associative
> operations are studied.
>

I think you have missed the words "property of" before "operation" above.
 "Closure", "commutativity", "associativity", etc. are properties of
operations, not operations.


>
> The equality relation, mapping pairs of members of a set to True or False
> is a different matter.  Being an equivalence relation is fundamental to
> both normal logic, algebraic proofs, and the definition of sets.


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),


I assume you are using a word "member" to refer to class instances.  There
are no gaps in datetimes: there are instances that don't correspond to any
valid local time and (pre-PEP 495) there are local times that don't
correspond to any instances with a given tzinfo.  The unrepresentable times
can still be represented using a different tzinfo.   PEP 495 adds a way to
represent all times using instances with any tzinfo, but on the flip side
adds many more instances that are not "canonical" representations (e.g.
fold=1 instances for regular times.)


> a tz tag, and a 0/1 fold tag. The tz tags divide datetimes into
> equivalence classes.


That I don't understand.  Local t and u = t.astimezone(UTC) are equal (t ==
u evaluates to True), so u and t belong to the same equivalence class.


> The '-' operation is also unusual in being defined differently for pairs
> in the same or different equivalence classes.


I am not concerned about '-'.  My main concern is about order operations.
I am happy with the solution I have for ==, but I am still struggling with
the non-transitivity of <.

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.

[1]: http://mathworld.wolfram.com/SetClosure.html
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