SQLAlchemy DateTime timezone

Question

SQLAlchemy's DateTime type allows for a timezone=True argument to save a non-naive datetime object to the database, and to return it as such. Is there any way to modify the timezone of the tzinfo that SQLAlchemy passes in so it could be, for instance, UTC? I realize that I could just use default=datetime.datetime.utcnow; however, this is a naive time that would happily accept someone passing in a naive localtime-based datetime, even if I used timezone=True with it, because it makes local or UTC time non-naive without having a base timezone to normalize it with. I have tried (using pytz) to make the datetime object non-naive, but when I save this to the DB it comes back as naive.

Note how datetime.datetime.utcnow does not work with timezone=True so well:

import sqlalchemy as sa
from sqlalchemy.sql import select
import datetime

metadata = sa.MetaData('postgres://user:pass@machine/db')

data_table = sa.Table('data', metadata,
    sa.Column('id',   sa.types.Integer, primary_key=True),
    sa.Column('date', sa.types.DateTime(timezone=True), default=datetime.datetime.utcnow)
)

metadata.create_all()

engine = metadata.bind
conn = engine.connect()
result = conn.execute(data_table.insert().values(id=1))

s = select([data_table])
result = conn.execute(s)
row = result.fetchone()

(1, datetime.datetime(2009, 1, 6, 0, 9, 36, 891887))

row[1].utcoffset()

datetime.timedelta(-1, 64800) # that's my localtime offset!!

datetime.datetime.now(tz=pytz.timezone("US/Central"))

datetime.timedelta(-1, 64800)

datetime.datetime.now(tz=pytz.timezone("UTC"))

datetime.timedelta(0) #UTC

Even if I change it to explicitly use UTC:

...

data_table = sa.Table('data', metadata,
    sa.Column('id',   sa.types.Integer, primary_key=True),
    sa.Column('date', sa.types.DateTime(timezone=True), default=datetime.datetime.now(tz=pytz.timezone('UTC')))
)

row[1].utcoffset()

...

datetime.timedelta(-1, 64800) # it did not use the timezone I explicitly added

Or if I drop the timezone=True:

...

data_table = sa.Table('data', metadata,
    sa.Column('id',   sa.types.Integer, primary_key=True),
    sa.Column('date', sa.types.DateTime(), default=datetime.datetime.now(tz=pytz.timezone('UTC')))
)

row[1].utcoffset() is None

...

True # it didn't even save a timezone to the db this time

Answer 1

http://www.postgresql.org/docs/8.3/interactive/datatype-datetime.html#DATATYPE-TIMEZONES

All timezone-aware dates and times are stored internally in UTC. They are converted to local time in the zone specified by the timezone configuration parameter before being displayed to the client.

The only way to store it with postgresql is to store it separately.

Answer 2

a solution is given in this question’s answer:

you can circumvent that by storing all (date)time objects in your database in UTC, and converting the resulting naïve datetime objects to aware ones on retrieval.

the only drawback is that you lose timezone information, but it’s probably a good idea to store your datetime objects in utc, anyway.

if you care about the timezone information, i’d store it separately, and only convert the utc to local time in the last possible instance (e.g. right before displaying)

or maybe you don’t need to care after all, and can use local timezone information from the machine you run your progam on, or the user’s browser if it’s a webapp.