This question already has answers here:
pytz localize vs datetime replace
(4 answers)
Closed 8 years ago.
Assume that I have a timezone-less datetime object:
import datetime
import pytz
fmt = "%Y-%m-%d %H:%M:%S %Z%z"
dtUnaware = datetime.datetime(1979,2,20,6)
print(dtUnaware.strftime(fmt))
This yields:
1979-02-20 06:00:00
So far, so good. Now, I want to assign a timezone to this object. It seems like I could use either datetime.replace or pytz.localize.
First:
dtAware1 = dtUnaware.replace(tzinfo=pytz.timezone('Asia/Jerusalem'))
print(dtAware1.strftime(fmt))
returns: 1979-02-20 06:00:00 LMT+0221. Secondly:
dtAware2 = pytz.timezone('Asia/Jerusalem').localize(dtUnaware, is_dst=None)
print(dtAware2.strftime(fmt))
returns 1979-02-20 06:00:00 IST+0200.
What's wrong with the first method? It seems to assign a wrong timezone. Am I doing something wrong?
From Pytz documentation : This library differs from the documented Python API for
tzinfo implementations; if you want to create local wallclock
times you need to use the localize() method documented in this
document... Unfortunately these
issues cannot be resolved without modifying the Python datetime
implementation (see PEP-431)
My reading of that is that a pytz timezone is not exactly the same thing as a standard timezone. If you had a genuine timezone, first method should be good, but you have not.
There is a flaw in the datetime API: when you assign a timezone to it, the timezone is not given the opportunity to know the date and time it should be configured for. The historical database that pytz uses goes back a long way, often to periods when the timezone name or offset were different than the ones in use today. Without being given the chance to adjust to a specific date period, the details of the timezone may be wrong.
Using localize is the way around this problem, since both the date and timezone are available within the function at the same time.
Related
This question already has answers here:
Python datetime object show wrong timezone offset
(2 answers)
Closed 5 years ago.
Converting a timezone naive date time to a specific timezone gives a completely incorrect result.
import dateutil as du
import pytz
du.parser.parse('2017-05-31T15:00:00').replace(tzinfo=pytz.timezone('Europe/London')).isoformat()
returns a one minute not one hour offset vs UTC
'2017-05-31T15:00:00-00:01'
I've seen a few datetime peculiarities before but this one is breathtaking.
The main problem here is that you are using a pytz time zone. pytz zones do not follow the tzinfo interface and cannot be simply attached to datetime objects (either through the constructor or through replace). If you would like to use pytz time zones, you should use pytz.timezone.localize with a naive datetime. If the datetime is already timezone-aware, you can use datetime.astimezone to convert it between zones.
from dateutil import parser
import pytz
LON = pytz.timezone('Europe/London')
dt = parser.parse('2017-05-31T15:00:00')
dt = LON.localize(dt)
print(dt) # 2017-05-31 15:00:00+01:00
This is because pytz's interface uses localize to attach a static time zone to a datetime. For the same reason, if you do arithmetic on the now-localized datetime object, it may give similar improper results and you'll have to use pytz.timezone.normalize to fix it. The reason this is done this way is that, historically, it has not been possible to handle ambiguous datetimes using a Pythonic tzinfo interface, which changed with PEP 495 in Python 3.6, making pytz's workaround less necessary.
If you would like to pass a tzinfo to a datetime using replace or the constructor, or you would prefer to use the pythonic interface, dateutil's time zone suite implements a PEP 495-compliant tzinfo interface. The equivalent using a dateutil zone is:
from dateutil import parser
from dateutil import tz
LON = tz.gettz('Europe/London')
dt = parser.parse('2017-05-31T15:00:00').replace(tzinfo=LON)
print(dt) # 2017-05-31 15:00:00+01:00
I have often had bad luck using replace() with tzinfo objects. I have however found this construct to be reliable:
Code:
def naive_to_aware(ts, tz):
return tz.localize(ts)
Update from Comments:
From the (pytz DOCS)
Unfortunately using the tzinfo argument of the standard datetime constructors ‘’does not work’’ with pytz for many timezones.
It is safe for timezones without daylight saving transitions though, such as UTC
So it is not just bad luck, it is problematic for pytz objects with timezones having DST.
Test Code:
import dateutil as du
import pytz
print(naive_to_aware(du.parser.parse('2017-05-31T15:00:00'),
pytz.timezone('Europe/London')).isoformat())
Results:
2017-05-31T15:00:00+01:00
I have been reading python's datetime documentation and it seems like there are few things which are not clearly mentioned there.
date = datetime.datetime(2014,10,1,11,45,30)
Which timezone will the above date be in? UTC?
If I have to make sure that the above date remains in EST what could be done. I am not clear on tzinfo object here?
If I have to convert these datetimes to some other time zones based on latitude and longitude what should I do?
Your code would create a "timezone naive" datetime object. That means - no timezone. It'll be interpreted as local time based on where it is used.
If you want to set a timezone, try using the pytz library.
import pytz # 3rd party: $ pip install pytz
u = datetime.utcnow()
u = u.replace(tzinfo=pytz.utc) # NOTE: it works only with a fixed utc offset
# Then you can change timezones, e.g. http://www.timezoneconverter.com/cgi-bin/zoneinfo?tz=America/New_York
print u.astimezone(pytz.timezone("America/New_York"))
As for the lat/lon conversion to a timezone, this isn't a simple task. Here's a question that discusses possible solutions.
This question already has answers here:
How to make a timezone aware datetime object
(15 answers)
Closed 6 years ago.
I've got a datetime which has no timezone information. I'm now getting the timezone info and would like to add the timezone into the existed datetime instance, how can I do?
d = datetime.datetime.now()
tz = pytz.timezone('Asia/Taipei')
How to add the timezone info tz into datetime a
Use tz.localize(d) to localize the instance. From the documentation:
The first is to use the localize() method provided by the pytz library. This is used to localize a naive datetime (datetime with no timezone information):
>>> loc_dt = eastern.localize(datetime(2002, 10, 27, 6, 0, 0))
>>> print(loc_dt.strftime(fmt))
2002-10-27 06:00:00 EST-0500
If you don't use tz.localize(), but use datetime.replace(), chances are that a historical offset is used instead; tz.localize() will pick the right offset in effect for the given date. The US Eastern timezone DST start and end dates have changed over time, for example.
When you try to localize a datetime value that is ambiguous because it straddles the transition period from summer to winter time or vice-versa, the timezone will be consulted to see if the resulting datetime object should have .dst() return True or False. You can override the default for the timezone with the is_dst keyword argument for .localize():
dt = tz.localize(naive, is_dst=True)
or even switch off the choice altogether by setting is_dst=None. In that case, or in the rare cases there is no default set for a timezone, an ambiguous datetime value would lead to a AmbiguousTimeError exception being raised. The is_dst flag is only consulted for datetime values that are ambiguous and is ignored otherwise.
To go back the other way, turn a timezone-aware object back to a naive object, use .replace(tzinfo=None):
naivedt = awaredt.replace(tzinfo=None)
If you know that your original datetime was "measured" in the time zone you are trying to add to it, you could (but probably shouldn't) use replace rather than localize.
# d = datetime.datetime.now()
# tz = pytz.timezone('Asia/Taipei')
d = d.replace(tzinfo=tz)
I can imagine 2 times when this might make sense (the second one happened to me):
Your server locale is set to the incorrect time zone and you are trying to correct a datetime instance by making it aware of this incorrect timezone (and presumably later localizing it to the "correct" time zone so the values of now() match up to other times you are comparing it to (your watch, perhaps)
You want to "tag" a time instance (NOT a datetime) with a time zone (tzinfo) attribute so that attribute can be used later to form a full datetime instance.
This question already has answers here:
How to make a timezone aware datetime object
(15 answers)
Closed 6 years ago.
I've got a datetime which has no timezone information. I'm now getting the timezone info and would like to add the timezone into the existed datetime instance, how can I do?
d = datetime.datetime.now()
tz = pytz.timezone('Asia/Taipei')
How to add the timezone info tz into datetime a
Use tz.localize(d) to localize the instance. From the documentation:
The first is to use the localize() method provided by the pytz library. This is used to localize a naive datetime (datetime with no timezone information):
>>> loc_dt = eastern.localize(datetime(2002, 10, 27, 6, 0, 0))
>>> print(loc_dt.strftime(fmt))
2002-10-27 06:00:00 EST-0500
If you don't use tz.localize(), but use datetime.replace(), chances are that a historical offset is used instead; tz.localize() will pick the right offset in effect for the given date. The US Eastern timezone DST start and end dates have changed over time, for example.
When you try to localize a datetime value that is ambiguous because it straddles the transition period from summer to winter time or vice-versa, the timezone will be consulted to see if the resulting datetime object should have .dst() return True or False. You can override the default for the timezone with the is_dst keyword argument for .localize():
dt = tz.localize(naive, is_dst=True)
or even switch off the choice altogether by setting is_dst=None. In that case, or in the rare cases there is no default set for a timezone, an ambiguous datetime value would lead to a AmbiguousTimeError exception being raised. The is_dst flag is only consulted for datetime values that are ambiguous and is ignored otherwise.
To go back the other way, turn a timezone-aware object back to a naive object, use .replace(tzinfo=None):
naivedt = awaredt.replace(tzinfo=None)
If you know that your original datetime was "measured" in the time zone you are trying to add to it, you could (but probably shouldn't) use replace rather than localize.
# d = datetime.datetime.now()
# tz = pytz.timezone('Asia/Taipei')
d = d.replace(tzinfo=tz)
I can imagine 2 times when this might make sense (the second one happened to me):
Your server locale is set to the incorrect time zone and you are trying to correct a datetime instance by making it aware of this incorrect timezone (and presumably later localizing it to the "correct" time zone so the values of now() match up to other times you are comparing it to (your watch, perhaps)
You want to "tag" a time instance (NOT a datetime) with a time zone (tzinfo) attribute so that attribute can be used later to form a full datetime instance.
So, we have 'Europe/Moscow' TZ in our settings.
Currently this means daylight saving (this is going to change in the future, but at the moment it's UTC+03/04).
I understand that this TZ is used when saving dates to the DB, and when extracting them.
Now, I have to serialize the datetime object to ISO string, including the UTC offset. What is the correct way of doing this?
The dates don't contain the TZ info (i.e. d.strftime('%z') is empty)
I think I could convert them to UTC and serialize with +00:00, but how do I convert them to UTC if I don't know if the specific date is +03 (Moscow winter) or +04 (Moscow summer)
how do I convert them to UTC if I don't know if the specific date is +03 (Moscow winter) or +04 (Moscow summer)
There is no need for UTC conversion, pytz handles such thing for you.
Here's the code to convert from timezone-naive datetime to ISO with timezone offset:
from datetime import datetime
from pytz import timezone
server_timezone = timezone('Europe/Moscow')
server_timezone.localize(datetime(2011, 1, 1)).isoformat()
>>> '2011-01-01T00:00:00+03:00'
server_timezone.localize(datetime(2011, 7, 1)).isoformat()
>>> '2011-07-01T00:00:00+04:00'
First run new_dt = datetime.replace(tzinfo=tz) to create a new timezone-aware datetime. Then run your datetime.strftime() with %z.
Note that you can't then convert the date string back to a timezone-aware datetime directly -- datetime.strptime() doesn't support %z. So you need to instead create a naive datetime and a tzinfo then do datetime.replace(tzinfo=tz) as before.
Some useful external libraries:
http://pytz.sourceforge.net/
http://code.google.com/p/parsedatetime/
http://labix.org/python-dateutil
Also try searching right here on Stack Overflow for more questions on (Python OR django OR appengine) AND (datetime OR timezone OR date OR time OR tzinfo).
ISO-8601 has no notion of "Timezones", only dates and times, and as a convenience, times may be presented with an "offset".
To make matters even more annoying, datetime has only a half-hearted nod in acknowledgement of timezones; the tzinfo attribute on datetime objects is optional, and no implementation of that interface is provided by the main-line python.
The standard answer to this is to just always do everything in UTC; including having the server in UTC, not in local time. This is actually a pretty good idea; it's not the times that are different, only the way individual users prefer to read those times (which is similar to preferring '42' over '0b101010').
You can satisfy your users' (reasonable!) desire to view times in their local timezone by storing the preferred timezone (perhaps just a sitewide preference if everyone is in Moscow) separately from the actual times, and then you can use a robust timezone library (like pytz) to do the formatting to and from local time.