From 4be51959042405c809024ee76fc8f4650cdffbdd Mon Sep 17 00:00:00 2001
From: Simon Willison <swillison@gmail.com>
Date: Thu, 12 Dec 2024 14:58:13 -0800
Subject: [PATCH] Fixes for datetime UTC warnings in Python

Refs:
- https://github.com/simonw/llm/issues/672
---
 python/utc-warning-fix.md | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)
 create mode 100644 python/utc-warning-fix.md

diff --git a/python/utc-warning-fix.md b/python/utc-warning-fix.md
new file mode 100644
index 000000000..67a039623
--- /dev/null
+++ b/python/utc-warning-fix.md
@@ -0,0 +1,35 @@
+# Fixes for datetime UTC warnings in Python
+
+I was getting the following warning for one of my Python test suites:
+
+> `DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).`
+
+I also saw a deprecation warning elsewhere for my usage of `datetime.datetime.utcfromtimestamp(iso_timestamp_string)`.
+
+## datetime.datetime.now(datetime.timezone.utc)
+
+I tried switching to `datetime.UTC` for the first warning, but that caused my tests to fail on versions of Python prior to 3.11. I support all versions of Python that have not yet hit EOL for support, which is currently [Python 3.9 and higher](https://devguide.python.org/versions/).
+
+According to [the documentation](https://docs.python.org/3/library/datetime.html#datetime.UTC) `datetime.UTC` was added in Python 3.11 as an alias for `datetime.timezone.utc` - so I switched my usage of `datetime.utcnow()` to the following instead:
+
+```python
+import datetime
+
+utcnow = datetime.datetime.now(datetime.timezone.utc)
+```
+These tests now passed for Python 3.9+.
+
+## datetime.datetime.fromtimestamp(s, datetime.timezone.utc)
+
+I found another hint [in the docs](https://docs.python.org/3/library/datetime.html#datetime.datetime.utcfromtimestamp) for replacing the now deprecated `datetime.datetime.utcfromtimestamp`:
+
+> **Warning**: Because naive `datetime` objects are treated by many `datetime` methods as local times, it is preferred to use aware datetimes to represent times in UTC. As such, the recommended way to create an object representing a specific timestamp in UTC is by calling `datetime.fromtimestamp(timestamp, tz=timezone.utc)`.
+
+Again, `timezone.utc` was the right fix here to keep things working in pre-3.11 versions:
+
+```python
+import python
+
+utc_datetime = datetime.datetime.fromtimestamp(created_string, datetime.timezone.utc)
+```
+Here's [the commit](https://github.com/simonw/llm/commit/571f4b2a4da52ad127061b7fa953562f6ba6aeb0) where I fixed both of these warnings for my LLM project.