UTCDateTime field for sqlalchemy

utc_datetime_field.py

from datetime import timezone

import sqlalchemy as sa


class UTCDateTime(sa.TypeDecorator):  # pylint:disable=W0223
    """By default if we provide datetime object without timezone Postgres applies
    his timezone to that datetime. To prevent unexpected behaviour we add utc timezone
    before every write. And convert back to utc, after every read.

    From PostgreSQL docs:
    For timestamp with time zone, the internally stored value is always in UTC.
    An input value that has an explicit time zone specified is converted to UTC
    using the appropriate offset for that time zone. If no time zone is stated
    in the input string, then it is assumed to be in the time zone indicated
    by the system's timezone parameter, and is converted to UTC using the offset
    for the timezone zone.
    When a timestamp with time zone value is output, it is always converted from UTC
    to the current timezone zone, and displayed as local time in that zone.
    """

    impl = sa.DateTime

    def process_bind_param(self, value, dialect):
        """Way into the database.
        Convert datetime to utc timezone, with tzinfo=timezone.utc, so
        Postgres doesnt apply his own timezone
        """
        if value is not None:
            if not value.tzinfo:
                value = value.replace(tzinfo=timezone.utc)
            return value.astimezone(timezone.utc)

        return None

    def process_result_value(self, value, dialect):
        """Way out of the database.
        Here we receive datetime with Postgres timezone and convert to UTC,
        then replace timezone with None
        """
        if value:
            return value.astimezone(timezone.utc).replace(tzinfo=None)
        return None