How to tell if Daylight Savings Time is in effect using Python

dst.py

from datetime import datetime
import pytz

def is_dst ():
    """Determine whether or not Daylight Savings Time (DST)
    is currently in effect"""

    x = datetime(datetime.now().year, 1, 1, 0, 0, 0, tzinfo=pytz.timezone('US/Eastern')) # Jan 1 of this year
    y = datetime.now(pytz.timezone('US/Eastern'))

    # if DST is in effect, their offsets will be different
    return not (y.utcoffset() == x.utcoffset())

The script below should work. TL;DR, you should pass in the timezone to evaluate as a parameter.

Sorry @oqamar but you are mistaken and have dived into a classic Northern hemisphere bias trap. In the southern Hemisphere, this premise:

Jan 1 of this year, when all tz assumed to not be in dst

is erroneous as we have DST on Jan 1. Your test is also broken, (confirmation bias at play), because in 2020 in Sydney Oct 5 was in DST and Oct 4 was NOT:

https://www.timeanddate.com/time/change/australia/sydney?year=2020

Once you've localized a datetime in fact you can simply check it's .dst() method to see if it's zero.

Your script is easily fixed and made simpler as follows:

from datetime import datetime, timedelta
import pytz


def is_dst(when):
    '''Given the name of Timezone will attempt determine if that timezone is in Daylight Saving Time now (DST)'''
    return when.dst() != timedelta(0)

#################################################
# Test cases
#################################################


# DST in Eastern ends Nov 1, 2020 at 2 am
timezone_eastern = 'US/Eastern'
datetime_dst_eastern = pytz.timezone(timezone_eastern).localize(datetime(year=2020, month=11, day=1))
datetime_non_dst_eastern = pytz.timezone(timezone_eastern).localize(datetime(year=2020, month=11, day=2))

print(timezone_eastern, datetime_dst_eastern, "is in dst:", is_dst(datetime_dst_eastern), "should be: True")
print(timezone_eastern, datetime_non_dst_eastern, "is in dst:", is_dst(datetime_non_dst_eastern), "should be: False")

# DST in Jerusalem ends Oct 25, 2020 at 2 am
timezone_jerusalem = 'Asia/Jerusalem'
datetime_dst_jerusalem = pytz.timezone(timezone_jerusalem).localize(datetime(year=2020, month=10, day=25))
datetime_non_dst_jerusalem = pytz.timezone(timezone_jerusalem).localize(datetime(year=2020, month=10, day=26))

print(timezone_jerusalem, datetime_dst_jerusalem, "is in dst:", is_dst(datetime_dst_jerusalem), "should be: True")
print(timezone_jerusalem, datetime_non_dst_jerusalem, "is in dst:", is_dst(datetime_non_dst_jerusalem), "should be: False")

# DST in Sydney starts Oct 4, 2020 at 2 am
timezone_sydney = 'Australia/Sydney'
datetime_dst_sydney = pytz.timezone(timezone_sydney).localize(datetime(year=2020, month=10, day=5))
datetime_non_dst_sydney = pytz.timezone(timezone_sydney).localize(datetime(year=2020, month=10, day=4))

print(timezone_sydney, datetime_dst_sydney, "is in dst:", is_dst(datetime_dst_sydney), "should be: True")
print(timezone_sydney, datetime_non_dst_sydney, "is in dst:", is_dst(datetime_non_dst_sydney), "should be: False")

This passes the Sydney test ;-), producing:

US/Eastern 2020-11-01 00:00:00-04:00 is in dst: True should be: True
US/Eastern 2020-11-02 00:00:00-05:00 is in dst: False should be: False
Asia/Jerusalem 2020-10-25 00:00:00+03:00 is in dst: True should be: True
Asia/Jerusalem 2020-10-26 00:00:00+02:00 is in dst: False should be: False
Australia/Sydney 2020-10-05 00:00:00+11:00 is in dst: True should be: True
Australia/Sydney 2020-10-04 00:00:00+10:00 is in dst: False should be: False