Explain DST fold ambiguity with two explicit instants

Explain DST fold ambiguity with two explicit instants

During the 2020 autumn DST transition in Los Angeles, the local clock displayed 01:30 twice. Starting with explicit UTC instants removes any ambiguity: 08:30 UTC converts to 2020-11-01T01:30:00-07:00, while 09:30 UTC converts to 2020-11-01T01:30:00-08:00. They share the same wall-clock fields but have different offsets and fold values. fold=0 identifies the earlier occurrence and fold=1 the later occurrence.

The example derives both local values from UTC, then asserts their wall-clock portions match, their folds differ, and the source instants are one hour apart. That last assertion is made on the UTC values: comparisons or subtraction between aware datetimes that share a tzinfo object have rules that can be surprising around a fold. Treat the UTC instant, or another unambiguous stored representation, as the value used to order events.

ZoneInfo supplies IANA timezone rules, so this result depends on the available timezone database retaining those historical rules. It is not a promise about future political time changes, and a naive datetime(2020, 11, 1, 1, 30) alone cannot select either instant. zoneinfo was added in Python 3.9; on systems without system timezone data, its documented data-source configuration may require the tzdata package. Official zoneinfo documentation and fold behavior in datetime describe the model.

AI-assistance disclosure: AI helped draft this educational example; the assertions establish only these two named historical instants.

from datetime import datetime, timedelta, timezone
from zoneinfo import ZoneInfo

los_angeles = ZoneInfo("America/Los_Angeles")
first_utc = datetime(2020, 11, 1, 8, 30, tzinfo=timezone.utc)
second_utc = datetime(2020, 11, 1, 9, 30, tzinfo=timezone.utc)

first_local = first_utc.astimezone(los_angeles)
second_local = second_utc.astimezone(los_angeles)

assert first_local.replace(tzinfo=None) == second_local.replace(tzinfo=None)
assert (first_local.fold, second_local.fold) == (0, 1)
assert second_utc - first_utc == timedelta(hours=1)

print(first_local.isoformat(), f"fold={first_local.fold}")
print(second_local.isoformat(), f"fold={second_local.fold}")
2020-11-01T01:30:00-07:00 fold=0
2020-11-01T01:30:00-08:00 fold=1

Comments

Post a Comment

Popular posts from this blog

Compare Two Text Files in Python Without Modifying Them

Batu Lab Notes · Batu When comparing two versions of a configuration or notes file, “are they the same?” is only the first question. The bytes may be identical, the visible text may be equal despite different line endings, or the content may contain a real change. For small UTF-8 text files, Python’s standard library can show these distinctions without writing to either input. Quick answer For small, stable UTF-8 files, read bounded bytes, compare them first, then decode and produce a bounded unified diff. Decide explicitly whether newline differences matter. Keep both inputs read-only and review the diff for private data before sharing it. Set a limit before reading Loading an input of unknown size directly into memory is a poor default. This example checks both sizes with Path.stat() , then performs a bounded read and applies a 256 KiB limit per file. It rejects directories and other non-regular paths. If reading raises OSError , the result includes the exception ty...
Read more

Seven CSV Quality Checks to Run Before Importing Data

Batu Lab Notes · Batu A CSV file is not ready for import merely because it opens. The wrong delimiter can collapse an entire row into one column. An unexpected encoding can damage non-ASCII characters. A repeated identifier can overwrite an existing record in the destination. A short, read-only check makes these problems visible before data reaches a spreadsheet or application. Quick answer Before importing a CSV, set its encoding and delimiter, validate the exact header and every row width, distinguish empty IDs, duplicate IDs, and duplicate rows, and keep the check read-only. 1. Specify the encoding Python may open a text file with the platform’s default encoding, which can differ across machines. If the expected encoding is UTF-8, encoding="utf-8-sig" is a practical choice: it reads ordinary UTF-8 and skips an optional UTF-8 byte order mark at the start. If decoding fails, verify the source application’s export settings instead of silently trying unrelat...
Read more

Explain why SQLite transactions cannot make an HTTP call atomic

Direct answer A SQLite transaction can atomically commit database statements handled by that connection. It cannot include an HTTP service in the same commit protocol. The example commits an outbox row first, then simulates a network failure. The durable row remains pending , which is exactly the information a later delivery worker needs instead of pretending both actions succeeded or failed together. The edge case is failure after the database commit but before remote acknowledgement. Retrying then requires an idempotency design owned by the remote endpoint or a stable request key; it is not supplied by SQLite’s COMMIT . Conversely, sending the request before storing a durable intent can leave no local evidence when the process stops. This demonstration intentionally does not make an HTTP request, implement an outbox dispatcher, or prove delivery exactly once. It shows the boundary that must be designed around. Use transactions to protect the local state transition, and test remote...
Read more