Question

I am working with SQLAlchemy, and I’m not yet sure which database I’ll use under it, so I want to remain as DB-agnostic as possible. How can I store a timezone-aware datetime object in the DB without tying myself to a specific database? Right now, I’m making sure that times are UTC before I store them in the DB, and converting to localized at display-time, but that feels inelegant and brittle. Is there a DB-agnostic way to get a timezone-aware datetime out of SQLAlchemy instead of getting naive datatime objects out of the DB?

Answer 1

There is a timezone parameter to DateTime column time, so there is no problem with storing timezone-aware datetime objects. However I found convenient to convert stored datetime to UTC automatically with simple type decorator:

from sqlalchemy import types
from datetime import datetime, timezone

class UTCDateTime(types.TypeDecorator):

    impl = types.DateTime

    def process_bind_param(self, value, engine):
        if value is None:
            return
        if value.utcoffset() is None:
            raise ValueError(
                'Got naive datetime while timezone-aware is expected'
            )
        return value.astimezone(timezone.utc)

    def process_result_value(self, value, engine):
        if value is not None:
            return value.replace(tzinfo=timezone.utc)

Note, that is behaves nicely when you use naive datetime by accident (meaning it will raise a ValueError).