Can't subtract offset-naive and offset-aware datetimes

问题:

I have a timezone aware timestamptz field in PostgreSQL. When I pull data from the table, I then want to subtract the time right now so I can get it's age.

The problem I'm having is that both datetime.datetime.now() and datetime.datetime.utcnow() seem to return timezone unaware timestamps, which results in me getting this error:

TypeError: can't subtract offset-naive and offset-aware datetimes  

Is there a way to avoid this (preferably without a third-party module being used).

EDIT: Thanks for the suggestions, however trying to adjust the timezone seems to give me errors.. so I'm just going to use timezone unaware timestamps in PG and always insert using:

NOW() AT TIME ZONE 'UTC' 

That way all my timestamps are UTC by default (even though it's more annoying to do this).

回答1:

have you tried to remove the timezone awareness?

from http://pytz.sourceforge.net/

naive = dt.replace(tzinfo=None) 

may have to add time zone conversion as well.

edit: Please be aware the age of this answer. A Python 3 answer is below.

回答2:

The correct solution is to add the timezone info e.g., to get the current time as an aware datetime object in Python 3:

from datetime import datetime, timezone  now = datetime.now(timezone.utc) 

On older Python versions, you could define the utc tzinfo object yourself (example from datetime docs):

from datetime import tzinfo, timedelta, datetime  ZERO = timedelta(0)  class UTC(tzinfo):   def utcoffset(self, dt):     return ZERO   def tzname(self, dt):     return "UTC"   def dst(self, dt):     return ZERO  utc = UTC() 

then:

now = datetime.now(utc) 

回答3:

I know some people use Django specifically as an interface to abstract this type of database interaction. Django provides utilities that can be used for this:

from django.utils import timezone now_aware = timezone.now() 

You do need to set up a basic Django settings infrastructure, even if you are just using this type of interface (in settings, you need to include USE_TZ=True to get an aware datetime).

By itself, this is probably nowhere near enough to motivate you to use Django as an interface, but there are many other perks. On the other hand, if you stumbled here because you were mangling your Django app (as I did), then perhaps this helps...

回答4:

This is a very simple and clear solution
Two lines of code

# First we obtain de timezone info o some datatime variable      tz_info = your_timezone_aware_variable.tzinfo  # Now we can subtract two variables using the same time zone info # For instance # Lets obtain the Now() datetime but for the tz_info we got before  diff = datetime.datetime.now(tz_info)-your_timezone_aware_variable 

You must mange your datetime variables with the same time info

回答5:

The psycopg2 module has its own timezone definitions, so I ended up writing my own wrapper around utcnow:

def pg_utcnow():     import psycopg2     return datetime.utcnow().replace(         tzinfo=psycopg2.tz.FixedOffsetTimezone(offset=0, name=None)) 

and just use pg_utcnow whenever you need the current time to compare against a PostgreSQL timestamptz

回答6:

I also faced the same problem. Then I found a solution after a lot of searching .

The problem was that when we get the datetime object from model or form it is offset aware and if we get the time by system it is offset naive.

So what I did is I got the current time using timezone.now() and import the timezone by from django.utils import timezone and put the USE_TZ = True in your project settings file.

回答7:

Is there some pressing reason why you can't handle the age calculation in PostgreSQL itself? Something like

select *, age(timeStampField) as timeStampAge from myTable 

回答8:

I know this is old, but just thought I would add my solution just in case someone finds it useful.

I wanted to compare the local naive datetime with an aware datetime from a timeserver. I basically created a new naive datetime object using the aware datetime object. It's a bit of a hack and doesn't look very pretty but gets the job done.

import ntplib import datetime from datetime import timezone  def utc_to_local(utc_dt):     return utc_dt.replace(tzinfo=timezone.utc).astimezone(tz=None)      try:     ntpt = ntplib.NTPClient()     response = ntpt.request('pool.ntp.org')     date = utc_to_local(datetime.datetime.utcfromtimestamp(response.tx_time))     sysdate = datetime.datetime.now() 

...here comes the fudge...

    temp_date = datetime.datetime(int(str(date)[:4]),int(str(date)[5:7]),int(str(date)[8:10]),int(str(date)[11:13]),int(str(date)[14:16]),int(str(date)[17:19]))     dt_delta = temp_date-sysdate except Exception:     print('Something went wrong :-(') 

回答9:

I've found timezone.make_aware(datetime.datetime.now()) is helpful in django (I'm on 1.9.1). Unfortunately you can't simply make a datetime object offset-aware, then timetz() it. You have to make a datetime and make comparisons based on that.

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