Generate RFC 3339 timestamp in Python

问题:

I'm trying to generate an RFC 3339 UTC timestamp in Python. So far I've been able to do the following:

>>> d = datetime.datetime.now()
>>> print d.isoformat('T')
2011-12-18T20:46:00.392227

My problem is with setting the UTC offset.

According to the docs, the classmethod datetime.now([tz]), takes an optional tz argument where tz must be an instance of a class tzinfo subclass, and datetime.tzinfo is an abstract base class for time zone information objects.

This is where I get lost- How come tzinfo is an abstract class, and how am I supposed to implement it?

---

(NOTE: In PHP it's as simple as timestamp = date(DATE_RFC3339);, which is why I can't understand why Python's approach is so convoluted...)

回答1:

Further down in the same doc that you linked to, it explains how to implement it, giving some examples, including full code for a UTC class (representing UTC), a FixedOffset class (representing a timezone with a fixed offset from UTC, as opposed to a timezone with DST and whatnot), and a few others.

回答2:

Timezones are a pain, which is probably why they chose not to include them in the datetime library.

try pytz, it has the tzinfo your looking for: http://pytz.sourceforge.net/

You need to first create the datetime object, then apply the timezone like as below, and then your .isoformat() output will include the UTC offset as desired:

d = datetime.datetime.utcnow()
d_with_timezone = d.replace(tzinfo=pytz.UTC)
d_with_timezone.isoformat()

'2017-04-13T14:34:23.111142+00:00'

Or, just use UTC, and throw a "Z" (for Zulu timezone) on the end to mark the "timezone" as UTC.

d = datetime.datetime.utcnow() # <-- get time in UTC
print d.isoformat("T") + "Z"

'2017-04-13T14:34:23.111142Z'

回答3:

In Python 3.3+:

>>> from datetime import datetime, timezone                                
>>> local_time = datetime.now(timezone.utc).astimezone()
>>> local_time.isoformat()
'2015-01-16T16:52:58.547366+01:00'

On older Python versions, if all you need is an aware datetime object representing the current time in UTC then you could define a simple tzinfo subclass as shown in the docs to represent UTC timezone:

from datetime import datetime

utc_now = datetime.now(utc)
print(utc_now.isoformat('T'))
# -> 2015-05-19T20:32:12.610841+00:00

You could also use tzlocal module to get pytz timezone representing your local timezone:

#!/usr/bin/env python
from datetime import datetime
from tzlocal import get_localzone # $ pip install tzlocal

now = datetime.now(get_localzone())
print(now.isoformat('T'))

It works on both Python 2 and 3.

回答4:

I struggled with RFC3339 datetime format a lot, but I found a suitable solution to convert date_string <=> datetime_object in both directions.

You need two different external modules, because one of them is is only able to do the conversion in one direction (unfortunately):

first install:

sudo pip install rfc3339
sudo pip install iso8601

then include:

import datetime     # for general datetime object handling
import rfc3339      # for date object -> date string
import iso8601      # for date string -> date object

For not needing to remember which module is for which direction, I wrote two simple helper functions:

def get_date_object(date_string):
  return iso8601.parse_date(date_string)

def get_date_string(date_object):
  return rfc3339.rfc3339(date_object)

which inside your code you can easily use like this:

input_string = '1989-01-01T00:18:07-05:00'
test_date = get_date_object(input_string)
# >>> datetime.datetime(1989, 1, 1, 0, 18, 7, tzinfo=<FixedOffset '-05:00' datetime.timedelta(-1, 68400)>)

test_string = get_date_string(test_date)
# >>> '1989-01-01T00:18:07-05:00'

test_string is input_string # >>> True

Heureka! Now you can easily (haha) use your date strings and date strings in a useable format.

回答5:

The pytz package is available for Python 2.X and 3.X. It implements concrete subclasses of tzinfo, among other services, so you don't have to.

To add a UTC offset: import datetime import pytz

dt = datetime.datetime(2011, 12, 18, 20, 46, 00, 392227)
utc_dt = pytz.UTC.localize(dt)

And now this:

print utc_dt.isoformat()

would print:

2011-12-18T20:46:00.392227+00:00

回答6:

Another useful utility I just started working with: dateutil library for timezone handling and date parsing. Recommended around SO, including this answer

回答7:

You can indeed use the built-in datetime module. As @ruakh mentions, there are examples in the page that show how. If you look into the section on tzinfo you'll see a long example showing many different use cases. Here's the code for the one you're looking for, which is to generate an RFC 3339 UTC timestamp.

from datetime import tzinfo, timedelta, datetime
import time as _time

ZERO = timedelta(0)
STDOFFSET = timedelta(seconds=-_time.timezone)
if _time.daylight:
    DSTOFFSET = timedelta(seconds=-_time.altzone)
else:
    DSTOFFSET = STDOFFSET

DSTDIFF = DSTOFFSET - STDOFFSET


class LocalTimezone(tzinfo):

    def utcoffset(self, dt):
        if self._isdst(dt):
            return DSTOFFSET
        else:
            return STDOFFSET

    def dst(self, dt):
        if self._isdst(dt):
            return DSTDIFF
        else:
            return ZERO

    def tzname(self, dt):
        return _time.tzname[self._isdst(dt)]

    def _isdst(self, dt):
        tt = (dt.year, dt.month, dt.day,
              dt.hour, dt.minute, dt.second,
              dt.weekday(), 0, 0)
        stamp = _time.mktime(tt)
        tt = _time.localtime(stamp)
        return tt.tm_isdst > 0

Local = LocalTimezone()

d = datetime.now(Local)
print d.isoformat('T')

# which returns
# 2014-04-28T15:44:45.758506-07:00