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Can't compare naive and aware datetime.now() <= challenge.datetime_end

Posted by: admin November 30, 2017 Leave a comment

Questions:

I am trying to compare the current date and time with dates and times specified in models using comparison operators:

if challenge.datetime_start <= datetime.now() <= challenge.datetime_end:

The script errors out with:

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

The models look like this:

class Fundraising_Challenge(models.Model):
    name = models.CharField(max_length=100)
    datetime_start = models.DateTimeField()
    datetime_end = models.DateTimeField()

I also have django using locale date and times.

What I haven’t been able to find is the format django uses for DateTimeField(). Is it naive or aware? And how do I get datetime.now() to recognize locale datetime?

Answers:

datetime.datetime.now is not timezone aware.

Django comes with a helper for this, which requires pytz

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

You should be able to compare now to challenge.datetime_start

Questions:
Answers:

By default the datetime object is naive in Python, so you need to make both of them to either naive or aware datetime objects. This can be done using.

import datetime
import pytz

utc=pytz.UTC

challenge.datetime_start = utc.localize(challenge.datetime_start) 
challenge.datetime_end = utc.localize(challenge.datetime_end) 
# now both the datetime objects are aware, and you can compare them

Note: This would raise a ValueError if tzinfo is already set. If you are not sure about that, just use

start_time = challenge.datetime_start.replace(tzinfo=utc)
end_time = challenge.datetime_end.replace(tzinfo=utc)

BTW, you could format a UNIX timestamp in datetime.datetime object with timezone info as following

d = datetime.datetime.utcfromtimestamp(int(unix_timestamp))
d_with_tz = datetime.datetime(
    year=d.year,
    month=d.month,
    day=d.day,
    hour=d.hour,
    minute=d.minute,
    second=d.second,
    tzinfo=pytz.UTC)

Questions:
Answers:

One line of code solution

if timezone_aware_var <= datetime.datetime.now(timezone_aware_var.tzinfo):
    pass #some code

Explained version:

# Timezone info of your timezone aware variable
timezone = your_timezone_aware_variable.tzinfo

# Current datetime for the timezone of your variable
now_in_timezone = datetime.datetime.now(timezone)

# Now you can do a fair comparison, both datetime variables have the same time zone
if your_timezone_aware_variable <= now_in_timezone:
    pass #some code

Sumary:
You must add the timezone info to your now() datetime.
But you must add the same timezone of the reference variable; that’s why I first read the tzinfo attribute.

Questions:
Answers:

So the way I would solve this problem is to make sure the two datetimes are in the right timezone.

I can see that you are using datetime.now() which will return the systems current time, with no tzinfo set.

tzinfo is the information attached to a datetime to let it know what timezone it is in. If you are using naive datetime you need to be consistent through out your system. I would highly recommend only using datetime.utcnow()

seeing as somewhere your are creating datetime that have tzinfo associated with them, what you need to do is make sure those are localized (has tzinfo associated) to the correct timezone.

Take a look at Delorean, it makes dealing with this sort of thing much easier.