attribute 'tzinfo' of 'datetime.datetime' objects is not writable
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 immedia开发者_运维百科tely:
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.
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)
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).
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
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