Difference between using pytz.localize and datetime.replace

date_time_tz_mess.py

from datetime import date, time, datetime
import pytz

pacific = pytz.timezone('US/Pacific')
eastern = pytz.timezone('US/Eastern')


def get_naive_date_time():
    date_start = date(2016, 9, 16)
    time_start = time(10, 0)
    return datetime.combine(date_start, time_start)


def print_all_times(date_time):
    print date_time
    print "UTC:", date_time.astimezone(pytz.utc)
    print "Eastern:", date_time.astimezone(eastern)


if __name__ == '__main__':
    naive_date_time = get_naive_date_time()

    print "Using tz replace"
    print_all_times(naive_date_time.replace(tzinfo=pacific))

    print "Using localize"
    print_all_times(pacific.localize(naive_date_time))

Using tz replace

2016-09-16 10:00:00-07:53
UTC: 2016-09-16 17:53:00+00:00
Eastern: 2016-09-16 13:53:00-04:00

Using localize

2016-09-16 10:00:00-07:00
UTC: 2016-09-16 17:00:00+00:00
Eastern: 2016-09-16 13:00:00-04:00

for those finding this while researching a problem read this
http://pytz.sourceforge.net/#localized-times-and-date-arithmetic