Question

I want to set the .tzinfo of every datetime instance automatically as soon as it comes out of the datastore.

So if I have

class Message( db.Model ):
    creationTime = db.DateTimeProperty()
    someOtherTime = db.DateTimeProperty()

    ## I really want to define a method like this,
    ## that runs immediately AFTER an instance has
    ## been refreshed from the datastore
    def wakeup( self ): 
        self.creationTime.tzinfo = self.creationTime.replace( tzinfo=EST )
        self.someOtherTime.tzinfo = self.creationTime.replace( tzinfo=EST )

Is it possible to do this in GAE?

Answer 1

I think the best approach is for you to subclass the DateTimeProperty class and override its method make_value_from_datastore:

class EstDateTimeProperty(db.DateTimeProperty):
  def make_value_from_datastore(self, value):
    naive_utc = db.DateTimeProperty(self, value)
    aware_utc = naive_utc.replace(tzinfo=UTC)
    return aware_utc.astimezone(EST)

given suitable tzinfo objects UTC and EST of course (built by pytz or whatever). Of course you could also build a more generic “smart datetime” property class and let it set the timezone of interest from a keyword argument in its __init__, for example, if you need several different datetime attributes to use different timezones.