Python datetime strptime() and strftime(): how to

问题:

See the following code:

import datetime
import pytz

fmt = '%Y-%m-%d %H:%M:%S %Z'

d = datetime.datetime.now(pytz.timezone("America/New_York"))
d_string = d.strftime(fmt)
d2 = datetime.datetime.strptime(d_string, fmt)
print d_string 
print d2.strftime(fmt)

the output is

2013-02-07 17:42:31 EST
2013-02-07 17:42:31 

The timezone information simply got lost in the translation.

If I switch '%Z' to '%z', I get

ValueError: 'z' is a bad directive in format '%Y-%m-%d %H:%M:%S %z'

I know I can use python-dateutil, but I just found it bizzare that I can't achieve this simple feature in datetime and have to introduce more dependency?

回答1:

Part of the problem here is that the strings usually used to represent timezones are not actually unique. "EST" only means "America/New_York" to people in North America. This is a limitation in the C time API, and the Python solution is… to add full tz features in some future version any day now, if anyone is willing to write the PEP.

You can format and parse a timezone as an offset, but that loses daylight savings/summer time information (e.g., you can't distinguish "America/Phoenix" from "America/Los_Angeles" in the summer). You can format a timezone as a 3-letter abbreviation, but you can't parse it back from that.

If you want something that's fuzzy and ambiguous but usually what you want, you need a third-party library like dateutil.

If you want something that's actually unambiguous, just append the actual tz name to the local datetime string yourself, and split it back off on the other end:

d = datetime.datetime.now(pytz.timezone("America/New_York"))
dtz_string = d.strftime(fmt) + ' ' + "America/New_York"

d_string, tz_string = dtz_string.rsplit(' ', 1)
d2 = datetime.datetime.strptime(d_string, fmt)
tz2 = pytz.timezone(tz_string)

print dtz_string 
print d2.strftime(fmt) + ' ' + tz_string

Or… halfway between those two, you're already using the pytz library, which can parse (according to some arbitrary but well-defined disambiguation rules) formats like "EST". So, if you really want to, you can leave the %Z in on the formatting side, then pull it off and parse it with pytz.timezone() before passing the rest to strptime.

回答2:

Unfortunately, strptime() can only handle the timezone configured by your OS, and then only as a time offset, really. From the documentation:

Support for the %Z directive is based on the values contained in tzname and whether daylight is true. Because of this, it is platform-specific except for recognizing UTC and GMT which are always known (and are considered to be non-daylight savings timezones).

strftime() doesn't officially support %z.

You are stuck with python-dateutil to support timezone parsing, I am afraid.

回答3:

Here is my answer in Python 2.7

Print current time with timezone

from datetime import datetime
import tzlocal  # pip install tzlocal

print datetime.now(tzlocal.get_localzone()).strftime("%Y-%m-%d %H:%M:%S %z")

Print current time with specific timezone

from datetime import datetime
import pytz # pip install pytz

print datetime.now(pytz.timezone('Asia/Taipei')).strftime("%Y-%m-%d %H:%M:%S %z")

It will print something like

2017-08-10 20:46:24 +0800