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datetime: add ability to parse RFC 3339 dates and times #60077
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nagle commented
on Sep 6, 2012 naglemannequinMannequinAuthorThe datetime module has support for output to a string of dates and times in ISO 8601 format ("2012-09-09T18:00:00-07:00"), with the object method "isoformat([sep])". But there's no support for parsing such strings. A string to datetime class method should be provided, one capable of parsing at least the RFC 3339 subset of ISO 8601.
The problem is parsing time zone information correctly.
The allowed formats for time zone are
empty - no TZ, date/time is "naive" in the datetime sense
Z - zero, or Zulu time, i.e. UTC.
[+-]nn.nn - offset from UTC"strptime" does not understand timezone offsets. The "datetime" documentation suggests that the "z" format directive handles time zone info, but that's not actually implemented for input.
Pypi has four modules for parsing ISO 8601 dates. Each has least one major
problem in time zone handling:iso8601 0.1.4
Abandonware. Mishandles time zone when time zone is "Z" and
the default time zone is specified.
iso8601.py 0.1dev
Always returns a "naive" datetime object, even if zone specified.
iso8601plus 0.1.6
Fork of abandonware version above. Same bug.
zc.iso8601 0.2.0
Zope version. Imports the pytz module with the full Olsen time zone
database, but doesn't actually use that database.Thus, nothing in Pypi provides a good alternative.
It would be appropriate to handle this in the datetime module. One small, correct, tested function would be better than the existing five bad alternatives.
AlexanderBelopolsky commented
on Sep 6, 2012 AlexanderBelopolskymannequinMannequin%z format is supported, but it cannot accept colon in TZ offset. It can parse offsets like -0600 just fine. What OP is looking for is the GNU date %:z format which datetime does not support.
For ISO 8601 compliance, however I think we need a way to specify a parser that will accept any valid 8601 format: with T or space separator and with or without : in time and timezone and with or without dashes in date.
I would very much like such promiscuous parser to be implemented in datetime.__new__. So that we can create datetime objects from strings the way we do it with numbers.
nagle commented
on Sep 7, 2012 naglemannequinMannequinAuthorRe: "%z format is supported".
That's platform-specific; the actual parsing is delegated to the C library. It's not in Python 2.7 / Win32:
ValueError: 'z' is a bad directive in format '%Y-%m-%dT%H:%M:%S%z'
It really shouldn't be platform-specific; the underlying platform is irrelevant to this task. That's more of a documentation error; the features not common to all supported Python platforms should not be mentioned in the documentation.
Re: "I would very much like such promiscuous parser to be implemented in datetime.__new__. "
For string input, it's probably better to do this conversion in a specific class-level function. Full ISO 8601 dates/times generally come from computer-generated data via a file or API. If invalid text shows up, it should be detected as an error, not be heuristically interpreted as a date. There's already "fromtimestamp" and "fromordinal",
and "isoformat" as an instance method, so "fromisoformat" seems reasonable.I'd also suggest providing a standard subclass of tzinfo in datetime for fixed offsets. That's needed to express the time zone information in an ISO 8601 date. The new "fromisoformat" would convert an ISO 8601 date/time would be convertible to a time-zone "aware" datetime object. If converted back to an ISO 8601 string with .isoformat(), the round trip should preserve the original data, including time zone offset.
(Several more implementations of this conversion have turned up. In addition to the four already mentioned, there was one in xml.util, and one in feedparser. There are probably more yet to be found.)
AlexanderBelopolsky commented
on Sep 7, 2012 AlexanderBelopolskymannequinMannequinOn Thu, Sep 6, 2012 at 9:51 PM, John Nagle <report@bugs.python.org> wrote:
It's not in Python 2.7 / Win32.
Python 2.x series is closed and cannot accept new features. Both %z
and fixed offset tzinfo subclass are implemented in 3.2.I am attaching a quick python only prototype for the proposed feature. My goal is to make date/time objects behave like numeric types for which constructors accept strings produced by str(). Since str() format is ISO 8601, it is natural to accept ISO 8601 formats in constructors.
roysmith commented
on Sep 9, 2012 roysmithmannequinMannequinWe need to define the scope of what input strings will be accepted. ISO-8601 defines a lot of stuff which we may not wish to accept.
Do we want to accept both basic format (YYYYMMDD) and extended format (YYYY-MM-DD)?
Do we want to accept things like "1985-W15-5", which is (if I understand this correctly(), the 5th day of the 15th week of 1985 [section 4.1.4.2].
Do we want to accept [section 4.2.2.4], "23:20,8", which is 23 hours, 20 minutes, 8 tenths of a minute.
I suspect most people who have been following the recent thread (https://groups.google.com/d/topic/comp.lang.python/Q2w4R89Nq1w/discussion) would say none of the above are needed. All that's needed is if you have an existing datetime object, d1, you can do:
s = str(d1) d2 = datetime.datetime(s) assert d1 == d2
for all values of d1.
But, let's at least agree on that. Or, in the alternative, agree on something else. Then we know what we're shooting for.
AlexanderBelopolsky commented
on Sep 9, 2012 AlexanderBelopolskymannequinMannequinOn Sep 9, 2012, at 8:15 AM, Roy Smith <report@bugs.python.org> wrote:
We need to define the scope of what input strings will be accepted.
Since it is easier to widen the domain of acceptable arguments than to narrow it in the future, I would say let's start by accepting str(x) only where x is date, time, timezone or datetime. I would leave out timedelta for now because it's str(x) does not resemble ISO at all.
Either that or full ISO 8601. Anything in between is just too hard to explain.
roysmith commented
on Sep 9, 2012 roysmithmannequinMannequinI see I mis-stated my example. When I wrote:
s = str(d1) d2 = datetime.datetime(s) assert d1 == d2
what I really meant was:
s = d1.isoformat() d2 = datetime.datetime(s) assert d1 == d2
But, now I realize that while that is certainly an absolute lower bound, it's almost certainly not sufficient. The most common use case I see on a daily basis is parsing strings that look like "2012-09-07T23:59:59+00:00". This is also John Nagle's original use case from the cited mailing list thread:
I want to parse standard ISO date/time strings such as
2012-09-09T18:00:00-07:00Datetime.isoformat() returns something that matches the beginning of that, but doesn't have the time zone offset. And it's the offset that makes strptime() not usable as a soluation, because "%z" isn't portable.
If we don't satisfy the "2012-09-07T23:59:59+00:00" case, then we won't have really done anything useful.
nagle commented
on Sep 9, 2012 naglemannequinMannequinAuthorFor what parts of ISO 8601 to accept, there's a standard: RFC3339, "Date and Time on the Internet: Timestamps". See section 5.6:
date-fullyear = 4DIGIT
date-month = 2DIGIT ; 01-12
date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on
; month/year
time-hour = 2DIGIT ; 00-23
time-minute = 2DIGIT ; 00-59
time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap second
; rules
time-secfrac = "." 1*DIGIT
time-numoffset = ("+" / "-") time-hour ":" time-minute
time-offset = "Z" / time-numoffsetpartial-time = time-hour ":" time-minute ":" time-second
[time-secfrac]
full-date = date-fullyear "-" date-month "-" date-mday
full-time = partial-time time-offsetdate-time = full-date "T" full-time
NOTE: Per [ABNF] and ISO8601, the "T" and "Z" characters in this
syntax may alternatively be lower case "t" or "z" respectively.ISO 8601 defines date and time separated by "T". Applications using this syntax may choose, for the sake of readability, to specify a full-date and full-time separated by (say) a space character.That's straightforward, and can be expressed as a regular expression.
I realize that while that is certainly an absolute lower bound,
it's almost certainly not sufficient. The most common use case
I see on a daily basis is parsing strings that look like
"2012-09-07T23:59:59+00:00".This is exactly what isoformat() of an aware datetime looks like:
>>> datetime.now(timezone.utc).isoformat() '2012-09-09T16:09:46.165886+00:00'
str() is the same up to T replaced by space:
>>> print(datetime.now(timezone.utc)) 2012-09-09 15:19:12.567692+00:00For what parts of ISO 8601 to accept, there's a standard: RFC3339
This is almost indistinguishable from the idea of accepting .isoformat() and str() results. From what I see the only difference is that 't' is accepted for date/time separator and 'z' is accepted as a timezone.
Let's start with this.
As an ultimate solution, I would like to see something like codec registry so that we can do things like datetime(.., format='rfc3339') or date(.., format='gnu') for GNU parse_datetime. I think this will look more pythonic than strptime(). Of course, strptime format can also be accepted as the value for the format keyword.
roysmith commented
on Sep 10, 2012 roysmithmannequinMannequinI've started collecting some test cases. I'll keep adding to the collection. I'm going to start trolling ISO 8601:2004(E) for more. Let me know if there are other sources I should be considering.
67 remaining items
+1 on what Paul said.
Mathieu, the goal for 3.7 will be to get Paul's PR merged. It will be great if you could help in reviewing it. We can return to the features in your PR during the 3.8 development cycle.
The other difference is Mattieu guarantees ValueError for invalid input strings, which I think is good.
I forgot to address this - but I don't think this is a difference in approaches. If you pass
Noneor an int or something, the problem is with the type, not the value, so at a minimum you're looking at TypeError and ValueError - and those are the only exceptions raised in my patch.(I'll note that my patch does not accept bytes, though this is something of an artificial limitation, since the patch makes use of the fact that all valid isoformat() strings will contain at most exactly 1 non-ascii character in position 10, so we could easily work around this, but I think the trend for CPython is to avoid blurring the lines between bytes and str rather than encouraging their interchangeable use.)
deronnax commented
on Dec 13, 2017 deronnaxmannequinMannequinI finally released my work. It looks like Paul's work is more comprehensive, but if you want to pick one thing or two in mine, feel free.
Regarding Matthieu’s RFC 3339 parser, Victor wanted to use the round-half-to-even rule to get a whole number of microseconds. But considering the “time” class cannot represent 24:00, how do you round up in the extreme case past 23:59?
time.fromisoformat("23:59:59.9999995")
Perhaps it is better to always truncate to zero, only support 6 digits (rejecting fractions of a microsecond), or add Anders’s truncate_microseconds=True option.
@martin.panter I don't see the problem here? Wouldn't 23:59.9999995 round up to 00:00?
Not if the time is associated with a particular day. Imagine implementing datetime.fromisoformat by separately calling date.fromisoformat and time.fromisoformat. The date will be off by one day if you naively rounded 2017-12-18 23:59 “up” to 2017-12-18 00:00.
Not if the time is associated with a particular day. Imagine implementing datetime.fromisoformat by separately calling date.fromisoformat and time.fromisoformat. The date will be off by one day if you naively rounded 2017-12-18 23:59 “up” to 2017-12-18 00:00.
Yes, I suppose this is a problem if you implement it that way. Seems like a somewhat moot point, but I think any decision about rounding should probably be driven by what people are expecting more than by how it is implemented.
That said, I can see a good case for truncation *and* rounding up for something like '2016-12-31T23:59:59.999999999'. Rounding up to '2017-01-01' is certainly the closest whole millisecond to round to, *but* often people expressing a "23:59:59.9999999" are trying to actually express "the last possible moment *before* 00:00".
jaitaiwan commented
on Dec 19, 2017 jaitaiwanmannequinMannequinI wanted to note here... I've been trying to get strptime to work with the types of dates specified in this request and came across a documentation bug here: https://docs.python.org/3.5/library/time.html#time.strptime
You can see that the %z attribute's examples given have colons in them while the format specified is +HHMM rather than +HH:MM which the examples illude to.
deronnax commented
on Dec 29, 2017 deronnaxmannequinMannequinmaybe it's worth adding an entry in python 3.7 "what's new" ? I think it was a very long awaited issue.
The opposite of isoformat() is a very frequent question from python newcomersCorrect, a new feature should always get a what's new entry. You could submit a PR for it :)
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