How to convert local time to UTC, considering daylight saving time?

I have datetime values in local time zone, and I need to convert them into UTC. How can I do this conversion for historical records, considering daylight saving time in the past?

Local               UTC
2018/07/20 09:00    ???
2018/12/31 11:00    ???
2019/01/17 13:00    ???
2020/08/15 18:00    ???

This is what I have so far:

import pytz
without_timezone = datetime(2018, 7, 20, 9, 0, 0, 0)
timezone = pytz.timezone("Europe/Vienna")
with_timezone = timezone.localize(without_timezone)
with_timezone

So, I assigned Europe/Vienna to all records (I assume that this considers daylight saving time, right?)

Now I need to convert it into UTC...

Solution

Assuming Local contains date/time as observed locally, i.e. including DST active/inactive, you would convert to datetime object, set time zone, and convert to UTC.

Ex:

from datetime import datetime, timezone
from zoneinfo import ZoneInfo # Python 3.9

Local = ["2018/07/20 09:00", "2018/12/31 11:00", "2019/01/17 13:00", "2020/08/15 18:00"]

# to datetime object and set time zone
LocalZone = ZoneInfo("Europe/Vienna")
Local = [datetime.strptime(s, "%Y/%m/%d %H:%M").replace(tzinfo=LocalZone) for s in Local]

for dt in Local:
    print(dt.isoformat(" "))
# 2018-07-20 09:00:00+02:00
# 2018-12-31 11:00:00+01:00
# 2019-01-17 13:00:00+01:00
# 2020-08-15 18:00:00+02:00

# to UTC
UTC = [dt.astimezone(timezone.utc) for dt in Local]

for dt in UTC:
    print(dt.isoformat(" "))
# 2018-07-20 07:00:00+00:00
# 2018-12-31 10:00:00+00:00
# 2019-01-17 12:00:00+00:00
# 2020-08-15 16:00:00+00:00

Note: with Python 3.9, you don't need third party libraries for time zone handling in Python anymore. There is a deprecation shim for pytz.