UTCDateTimeAttribute fails on Python's default `.toisoformat()` method

[boompig]

This is for the new changes in Pynamo 5.x which IMO makes the UTCDateTimeAttribute unusable:

https://pynamodb.readthedocs.io/en/stable/upgrading.html

No longer parsing date-time strings leniently¶
UTCDateTimeAttribute now strictly requires the date string format '%Y-%m-%dT%H:%M:%S.%f%z' to ensure proper ordering. PynamoDB has always written values with this format but previously would accept reading other formats. Items written using other formats must be rewritten before upgrading.

This means that strings generated using the default python from datetime import datetime, UTC; datetime.now(UTC).isoformat() will fail the validation check because the generated format looks like this:

2025-12-17T21:47:08.305464+00:00

And I have confirmed you get this error with pynamodb version 6.1.0:

ValueError: Datetime string '2025-12-17T21:47:08.305464+00:00' does not match format '%Y-%m-%dT%H:%M:%S.%f+0000'

Which means that values generated using standard python code will fail to parse with pynamo. It basically requires you to write to DynamoDB exclusively with Pynamo and never use boto3 + native python in any write applications. IMO this is a library-breaking change, with the suggested fix to just "update your values" in the database being clearly not feasible.

[ryancausey]

A workaround for the time being might be to use UnicodeDatetimeAttribute from pynamodb-attributes instead. It serializes datetimes to ISO8601 strings and so it is compatible with datetime.isoformat() and datetime.fromisoformat().

https://github.com/lyft/pynamodb-attributes