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

Sat Oct 21 08:18:10 EDT 2017

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
>>> >>
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>>> >
>>> >
>>> >
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>>> >
>>>
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>>
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