Working with Time Zones in Python

Using the SQLite applications you have already developed you pull all the datetimes and book titles from the db.

Run python timezones.py. This will result in an AssertionError. There are two sections that must be fixed to pass the assertion.

In this first section, you must make the due dates aware datetime objects by adding time zone information to the datetime object. The time zone added should the time zone indicated for a book title.

You create a time zone object with pytz.

my_timezone = pytz.timezone(<time zone name>)

Then make the due date aware of that time zone. This is done by replacing the None time zone with the real time zone. This does not change the due date, just makes it aware of its time zone:

aware_due_date = due_date.replace(my_timezone)

We need to install pytz, so run pip3 install pytz. Now make those times aware of their time zone:

import pytz
from datetime import datetime

# due date data from db
title_due_dates = [
    ['Oh Python! My Python!', '2020-11-15 23:59:59'], 
    ['Fun with Django', '2020-06-23 23:59:59'], 
    ['When Bees Attack! The Horror!', '2020-12-10 23:59:59'], 
    ["Martin Buber's Philosophies", '2020-07-12 23:59:59'], 
    ['The Sun Also Orbits', '2020-10-31 23:59:59']
]

# dictionary matching the timezone value to book title
title_time_zones = {
    'Oh Python! My Python!': 'US/Central', 
    'Fun with Django': 'US/Pacific', 
    'When Bees Attack! The Horror!': 'Europe/London', 
    "Martin Buber's Philosophies": 'Australia/Melbourne', 
    'The Sun Also Orbits': 'Europe/Paris'
}

# when the db due_date string is converted to a datetime object it is naive
# it does not contain timezone info
# make the due_date timezone aware with `<due_date>.replace(timezone)`
# make the timezone the same as indicated in `title_time_zones`
# this makes the book due just before midnight local time
aware_title_due_dates = {}
for book in title_due_dates:
    book_timezone = pytz.timezone(title_time_zones[book[0]])
    naive_date_due = datetime.strptime(book[1], "%Y-%m-%d %H:%M:%S")
    aware_date_due = naive_date_due.replace(tzinfo=book_timezone)
    aware_title_due_dates[book[0]] =  aware_date_due

# remaining code omitted-used in step 2.

We now have a dictionary of aware datetimes by book author timezone. These aware datetimes are set for essentially midnight in the author's timezone.

When storing due dates, it is a standard to store the datetime in UTC and convert to the user's timezone on the fly. So now we need to change the timezone on the author's due date to UTC and make it a text string for storage in the database.

# code prior to this omitted

# aware_title_due_dates has the due date in the author's timezone
# following good db practice we will store the dates as UTC and
# only convert when necessary to time zone needed
# update `title_due_dates` with the due_date in UTC time
for book in title_due_dates:
    utc_due_date = aware_title_due_dates[book[0]].astimezone(pytz.utc)
    utc_due_date_string = utc_due_date.strftime("%Y-%m-%d %H:%M:%S %Z%z")
    book[1] = utc_due_date_string

# we can now use aware_title_due_dates for updating the db

Run python timezones.py.

Congrats! You have shown basic understanding of datetimes and timezones. This skill will be necessary if you work for a compant that has an application used worldwide.