attribute 'tzinfo' of 'datetime.datetime' objects is not writable

Question

How do I set the timezone of a datetime instance that just came out of the datastore?

When it first comes out it is in UTC. I want to change it to EST.

I'm trying, for example:

class Book( db.Model ):
    creationTime = db.DateTimeProperty()

When a Book is retrieved, I want to set its tzinfo immediately:

book.creationTime.tzinfo = EST

Where I use this example for my EST object

However I get:

attribute 'tzinfo' of 'datetime.datetime' objects is not writable

I've seen a number of answers that recommend pytz and python-dateutil, but I really want an answer to this question.

Answer 1

What you want is right there in the docs.

from datetime import tzinfo, timedelta, datetime

ZERO = timedelta(0)
HOUR = timedelta(hours=1)
DSTSTART = datetime(1, 4, 1, 2)
DSTEND = datetime(1, 10, 25, 1)

def first_sunday_on_or_after(dt):
    days_to_go = 6 - dt.weekday()
    if days_to_go:
        dt += timedelta(days_to_go)
    return dt

class USTimeZone(tzinfo):

    def __init__(self, hours, reprname, stdname, dstname):
        self.stdoffset = timedelta(hours=hours)
        self.reprname = reprname
        self.stdname = stdname
        self.dstname = dstname

    def __repr__(self):
        return self.reprname

    def tzname(self, dt):
        if self.dst(dt):
            return self.dstname
        else:
            return self.stdname

    def utcoffset(self, dt):
        return self.stdoffset + self.dst(dt)

    def dst(self, dt):
        if dt is None or dt.tzinfo is None:
            # An exception may be sensible here, in one or both cases.
            # It depends on how you want to treat them.  The default
            # fromutc() implementation (called by the default astimezone()
            # implementation) passes a datetime with dt.tzinfo is self.
            return ZERO
        assert dt.tzinfo is self

        # Find first Sunday in April & the last in October.
        start = first_sunday_on_or_after(DSTSTART.replace(year=dt.year))
        end = first_sunday_on_or_after(DSTEND.replace(year=dt.year))

        # Can't compare naive to aware objects, so strip the timezone from
        # dt first.
        if start <= dt.replace(tzinfo=None) < end:
            return HOUR
        else:
            return ZERO

now = datetime.now()
print now
print now.tzinfo

Eastern = USTimeZone(-5, 'Eastern', 'EST', 'EDT')
now_tz_aware = now.replace(tzinfo=Eastern)
print now_tz_aware

output:

2010-01-18 17:08:02.741482
None
2010-01-18 17:08:02.741482-05:00

Answer 2

If you're receiving a datetime that's in EST, but doesn't have its tzinfo field set, use dt.replace(tzinfo=tz) to assign a tzinfo without modifying the time. (Your database should be doing this for you.)

If you're receiving a datetime that's in UDT, and you want it in EST, then you need astimezone. http://docs.python.org/library/datetime.html#datetime.datetime.astimezone

In the vast majority of cases, your database should be storing and returning data in UDT, and you shouldn't need to use replace (except possibly to assign a UDT tzinfo).

Answer 3

datetime's objects are immutable, so you never change any of their attributes -- you make a new object with some attributes the same, and some different, and assign it to whatever you need to assign it to.

I.e., in your case, instead of

book.creationTime.tzinfo = EST

you have to code

book.creationTime = book.creationTime.replace(tzinfo=EST)