pytz and astimezone() cannot be applied to a naive datetime

Question

I have a date and I need to make it time zone aware.

local_tz = timezone('Asia/Tokyo')
start_date = '2012-09-27'
start_date = datetime.strptime(start_date, "%Y-%m-%d")   
start_date = start_date.astimezone(local_tz)


now_utc = datetime.now(timezone('UTC'))
local_now = now_utc.astimezone(local_tz)

I need to find if this is true:

print start_date>local_now

But I get this error.

   start_date = start_date.astimezone(local_tz)
   ValueError: astimezone() cannot be applied to a naive datetime

I convert utc to tokyo with no issue. I need to make start_date timezone aware ad well in tokyo.

Thanks

Answer 1

For pytz timezones, use their .localize() method to turn a naive datetime object into one with a timezone:

start_date = local_tz.localize(start_date)

For timezones without a DST transition, the .replace() method to attach a timezone to a naive datetime object should normally also work:

start_date = start_date.replace(tzinfo=local_tz)

See the localized times and date arithmetic of the pytz documentation for more details.

Answer 2

You could use local_tz.localize(naive_dt, is_dst=None) to convert a naive datetime object to timezone-aware one.

from datetime import datetime
import pytz

local_tz = pytz.timezone('Asia/Tokyo')

start_date = local_tz.localize(datetime(2012, 9, 27), is_dst=None)
now_utc = datetime.utcnow().replace(tzinfo=pytz.utc)

print start_date > now_utc

is_dst=None forces .localize() to raise an exception if given local time is ambiguous.

Answer 3

If you are using Django Rest Framework you could override the DateTimeField class like:

class DateTimeFieldOverridden(serializers.DateTimeField):

def to_representation(self, value):
    local_tz = pytz.timezone(TIME_ZONE)
    value = local_tz.localize(value)
    return super(DateTimeFieldOverridden, self).to_representation(value)

And you use it like this in your serializer:

date_time = DateTimeFieldOverridden(format='%d-%b-%Y', read_only=True)

Hope this helps someone.