Python timedelta does not take DST into account

normalize_timedelta.py

from datetime import datetime, timedelta

from pytz import timezone as pytz_timezone


def normalize_timedelta(dt: datetime, delta: timedelta) -> datetime:
    """Normalize timedelta operation, to preserve DST properly."""
    if hasattr(dt.tzinfo, "zone"):
        timezone_ = dt.tzinfo.zone
        return pytz_timezone(timezone_).normalize(dt + delta)
    
    raise Exception("Datetime object does not include a pytz timezone.")

test_timedelta_dst.py

from datetime import datetime, timedelta

from pytz import timezone as pytz_timezone

from normalize_timedelta import normalize_timedelta


def test_dst_failing():
    before_dst = pytz_timezone("Europe/Stockholm").localize(
        datetime(
            year=2022,
            month=10,
            day=29,
            hour=3,
            minute=30,
        )
    )

    after_dst = pytz_timezone("Europe/Stockholm").localize(
        datetime(
            year=2022,
            month=10,
            day=30,
            hour=3,
            minute=30,
        )
    )

    before_dst_with_time_delta = before_dst + timedelta(days=1)

    assert before_dst.isoformat() == "2022-10-29T03:30:00+02:00"
    assert after_dst != before_dst_with_time_delta
    assert after_dst.isoformat() == "2022-10-30T03:30:00+01:00"
    assert before_dst_with_time_delta.isoformat() == "2022-10-30T03:30:00+02:00"


def test_dst_passing_():
    before_dst = pytz_timezone("Europe/Stockholm").localize(
        datetime(
            year=2022,
            month=10,
            day=29,
            hour=3,
            minute=30,
        )
    )

    after_dst = pytz_timezone("Europe/Stockholm").localize(
        datetime(
            year=2022,
            month=10,
            day=30,
            hour=3,
            minute=30,
        )
    )

    before_dst_with_time_delta = normalize_timedelta(before_dst, timedelta(days=1))

    assert before_dst.isoformat() == "2022-10-29T03:30:00+02:00"
    assert after_dst == before_dst_with_time_delta