[Datetime-SIG] Matching +-HH:MM in strptime

Sat Oct 21 08:55:36 EDT 2017

On 21 October 2017 at 13:18, Oren Tirosh <orent at hishome.net> wrote:

>
> On Sat, 21 Oct 2017 at 13:24, Mario Corchero <mariocj89 at gmail.com> wrote:
>
>> My opinion (as a user, I have no authority here whatsoever)
>>
>> *1) About parsing colons in offsets with strptime*
>>
>> I think having %z support both +-HH:MM and +-HHMM would be the best
>> choice, as it seems the simplest for me as a user.
>> I'd go even further, making %z support ':' and 'Z', *a la glibc*.
>> This effectively means that %z can now parse: Z, ±hh:mm, ±hhmm, or ±hh
>>
>
> That is fine for parsing, but my issue with this is symmetry with
> strftime. If the same extensions are also implemented for formatting (I
> have a prototype) then you need some way to specify whether you want a :
> separator or not. The %z will have to remain without colon on formatting
> for backward compatibility.
>
> So l agree that the parser can be safely made more liberal in what it
> accepts, but the formatter must be strict and specific in what it produces.
>
> I think this gives the best experience to the strptime user. It basically
>> makes the time-offset rfc3339 <https://tools.ietf.org/html/rfc3339>
>> compatible.
>>
>
> Yes, that's the goal.
>
> *2) Adding a handy function to build a datetime from a string serialized
>> with isoformat*
>> Absolutely agree on having an isoparse. That would be amazing, we can
>> even build it on top of 1).
>>
>
> ...and building it on top of 1 requires several extensions and variants.
> People here seem to be a bit taken aback by the scope of these extensions.
> I understand this reaction, but I maintain that most or all this complexity
> is necessary if you want to implement this on to of strptime rather than a
> custom isoparse().
>
> *Side note:*
>> I am not totally in favour with "%?:z" (probably because I am leaning on
>> %z doing the parsing for both and ?z will have no place on strftime).
>> I think this starts to add way too much complexity to just say "parse a
>> time-offset".
>>
>
> Again, what is the alternative? If you want a parser that accepts the
> output of isoformat() for all possible datetime values (except custom
> tzinfo) then it needs to support a missing tz offset as indicating a naive
> timestamp.
>
> You can say that the real source of the asymmetry here is not with my
> proposal but rather in the underlying strftime/strptime: on formatting, %z
> yields an empty string for a naive timestamp rather that producing an
> error. But on parsing, it refuses to parse a timestamp with no offset. A
> truly symmetric implementation would have accepted it as an naive
> timestamp.
>
> Too late for %z because it must remain backward compatible, but perhaps
> %:z can be made to accept a missing offset as a naive timestamp. The user
> can then check for naive timestamp and reject them if they are unacceptable
> in that context, rather than specifying whether a missing timestamp is
> acceptable or not in the format string. I have no problem with either
> solution.
>
>>
>> *Implementation:*
>> I am happy to work with PaulG in the isoparse implementation if we decide
>> to go with it and if he wants to get involved :)
>>
>
> I have a working strptime:
>  https://github.com/orent/cpython/tree/strptime_extensions
>
> isoparse() on top of this strptime is a trivial one-liner.
>
> Oren
>
>>
>>
>> *Thanks:*
>> Thanks for dedicating time to this, I think that even if minor this would
>> be a killer addition to 3.7 if we manage to get it through.
>>
>> On 21 October 2017 at 07:34, Oren Tirosh <orent at hishome.net> wrote:
>>
>>> ok, let's try to separate the issues and choices on each one:
>>>
>>> 1. Extending strptime to support time zone offset with : separator:
>>> Should a single directive accepts either hhmm or by:mm or use two
>>> separate directives?
>>>
>>> 2. Round tripping of isoformat() back to datetime value:
>>> Implement custom isoparse() function or extend strptime so isoparse
>>> simply calls strptime with a default format?
>>> Support all variations produced by isoformat or just a subset?
>>> (Variations include with/without fraction, with/without tz and separator
>>> choice)
>>>
>>> I suggest 1 separate directives 2a extend strptime and 2b support all
>>> variations. Do you have different preferences on any of these questions?
>>>
>>> I understand that the number of extensions to support this seems
>>> excessive to you.
>>>
>>> Technically, my proposed "%.f" is not really necessary. I added it for
>>> completeness. We can keep using ".%f" for non-optional fraction and define
>>> "%?f" to implicitly include the dot.
>>>
>>> The distinction between "%z",  "%:z" and "%?:z"" can also be narrowed
>>> down. This can be done, for example, by making "%z" and "%?s" always accept
>>> hhmm with or without the : separator.
>>>
>>> On Fri, 20 Oct 2017 at 17:16, Paul G <paul at ganssle.io> wrote:
>>>
>>>> I think this would be a much bigger change to the strptime interface
>>>> than is actually warranted, and probably would add in additional,
>>>> unnecessary complexity by introducing the concept of optional matches.
>>>> Adding the capability to match HH:MM offsets is a reasonable extension
>>>> partially because that is a standard representation that is currently *not*
>>>> covered by strptime, and the fact that that's how isoformat() represents
>>>> the offset just makes this lack all the more acute.
>>>>
>>>> I think it should be uncontroversial to add *one* of these two %z
>>>> extensions to Python 3 without getting bogged down in allowing a single
>>>> strptime string to match any output from `.isoformat`.
>>>>
>>>> That said, I'm also very much in favor of a `.isoparse` or
>>>> `.fromisoformat` constructor that *is* the inverse of `isoformat`, which
>>>> should solve the issue without sweeping changes to how `strptime` works.
>>>>
>>>> On 10/19/2017 04:07 PM, Oren Tirosh wrote:
>>>> > https://github.com/orent/cpython/tree/strptime_extensions
>>>> >
>>>> > %:z  - matches +HH:MM
>>>> > %?:z - optional %:z
>>>> > %.f  - equivalent to .%f
>>>> > %?.f - optional %.f
>>>> > %?t  - matches ' ' or 'T'
>>>> >
>>>> > What they all have in common is that together they make it possible to
>>>> > write a strptime format that matches all possible output variations of
>>>> > datetime.__str__/ datetime.isoformat.
>>>> >
>>>> > The time zone not only supports the : separator but also allows
>>>> making the
>>>> > entire component optional, as isoformat() will add it only for aware
>>>> > datetime objects. The seconds fraction is dropped from the default
>>>> string
>>>> > representation if the datetime represents a whole second. Since it is
>>>> > dropped along with the decimal dot, I first made "%.f" that includes
>>>> the
>>>> > dot and then created the optional variant. Finally, "%?t" can be used
>>>> to
>>>> > accept a timestamp with either of the separators defined in iso8601.
>>>> >
>>>> > It is quite absurd that datetime cannot parse its own string
>>>> > representation. Using these extensions an .isoparse() method may be
>>>> added
>>>> > that calls strptime('%Y-%m-%d%?t%H:%M:%S%?.f%?:z') and supports full
>>>> > round-tripping of all possible datetime values that do not not use a
>>>> custom
>>>> > tzinfo.
>>>> >
>>>> > Oren
>>>> >
>>>> >
>>>> >
>>>> > On Thu, 19 Oct 2017 at 17:06, Paul G <paul at ganssle.io> wrote:
>>>> >>
>>>> >> There is a new issue about the %z directive in strptime on the issue
>>>> > tracker: https://bugs.python.org/issue31800 (linked to a few related
>>>> > issues), and a linked PR expanding the definition of %z to match
>>>> HH:MM:
>>>> > https://github.com/python/cpython/pull/4015
>>>> >>
>>>> >> I think either adding a %:z directive or expanding the definition of
>>>> %z
>>>> > would be pretty important, and I think there's a good case to be made
>>>> for
>>>> > either one. To summarize the arguments for people on the mailing list:
>>>> >>
>>>> >> The argument for expanding the definition of %z that I find
>>>> strongest is
>>>> > that according to the linux man pages (
>>>> > http://man7.org/linux/man-pages/man3/strptime.3.html ), while %z
>>>> generates
>>>> > +-HHMM in strftime, strptime is supposed to match "An RFC-822/ISO 8601
>>>> > standard timezone specification",and ISO 8601 uses +-HH:MM, so if
>>>> we're
>>>> > following those linux pages, we should be accepting the version with
>>>> the
>>>> > colon.
>>>> >>
>>>> >> The argument that I find most compelling for adding a %:z directive
>>>> are:
>>>> >>
>>>> >>     1. maintains the symmetry between strftime and strptime
>>>> >>     2. allows users to be stricter about their datetime format
>>>> >>     3. has precedent in that GNU's `date` command accepts %z, %:z and
>>>> > %::z formats
>>>> >>
>>>> >> Can we establish some consensus on which should be done so that it
>>>> can be
>>>> > implemented?
>>>> >>
>>>> >> Best,
>>>> >>
>>>> >> Paul
>>>> >>
>>>> >> _______________________________________________
>>>> >> Datetime-SIG mailing list
>>>> >> Datetime-SIG at python.org
>>>> >> https://mail.python.org/mailman/listinfo/datetime-sig
>>>> >> The PSF Code of Conduct applies to this mailing list:
>>>> > https://www.python.org/psf/codeofconduct/
>>>> >
>>>> >
>>>> >
>>>> > _______________________________________________
>>>> > Datetime-SIG mailing list
>>>> > Datetime-SIG at python.org
>>>> > https://mail.python.org/mailman/listinfo/datetime-sig
>>>> > The PSF Code of Conduct applies to this mailing list:
>>>> https://www.python.org/psf/codeofconduct/
>>>> >
>>>>
>>>> _______________________________________________
>>>> Datetime-SIG mailing list
>>>> Datetime-SIG at python.org
>>>> https://mail.python.org/mailman/listinfo/datetime-sig
>>>> The PSF Code of Conduct applies to this mailing list:
>>>> https://www.python.org/psf/codeofconduct/
>>>>
>>>
>>> _______________________________________________
>>> Datetime-SIG mailing list
>>> Datetime-SIG at python.org
>>> https://mail.python.org/mailman/listinfo/datetime-sig
>>> The PSF Code of Conduct applies to this mailing list:
>>> https://www.python.org/psf/codeofconduct/
>>>
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/datetime-sig/attachments/20171021/ee03478c/attachment-0001.html>