Python Localize Time

Last month I wrote a telegram bot: Choke(@getAqiBot. I need to create a datetime object from a string of hour and minute, then add time zone information to it and finally convert it to UTC time zone.

strftime() function converts datetime object to string. On the contrary, striptime() converts a string to a datetime object¹. Note that we need to add today’s year, month and day, otherwise Python will use January 1, 1900. And that will give us an inaccurate result when we convert time zones later.

from datetime import datetime

hour_minute = "12:00"
time_string = datetime.now().strftime('%Y%m%d') + hour_minute
datetime_obj = datetime.strptime(time_string, '%Y%m%d%H:%M')

>>> datetime.strptime("12:00", "%H:%M")
datetime.datetime(1900, 1, 1, 12, 0)
>>> print(time_string)
2018030212:00
>>> print(datetime_obj)
2018-03-02 12:00:00

We can use pytz’s localize() function to localize datetime object.

from pytz import timezone

tzinfo = "Europe/Paris" # UTC+1
localized_datetime = timezone(tzinfo).localize(datetime_obj)

>>> print(localized_datetime)
2018-03-02 12:00:00+01:00

Using astimezone() function to convert Paris time zone to UTC. Because I hosted the bot on Heroku, and Heroku uses UTC.

import pytz
utc_datetime = localized_datetime.astimezone(pytz.utc)

>>> print(utc_datetime)
2018-03-02 11:00:00+00:00

Finally, using time() function to get time object.

notification_time = utc_datetime.time()

>>> print(notification_time)
11:00:00

I didn’t notice that free Heroku dynos sleep after thrity minutes of inactivity², so most daily notifications will never send. You can upgrade to Hobby or try Kaffeine.

References