[ADD] field.date.context_today: added function to use as _defaults for date fields

As discussed on bug 925361, in order to have
a consistent behavior everywhere we need default
values to be using the client's timezone, just
as if the user had set the value themselves
using their local clock.
Because fields.date values are timezone
agnostic by design, they must be correctly
initialized from the start, no conversion
happens later.
The new context_today method is mean to be
easily usable as _defaults initialize, i.e.:
 'date_field': fields.date.context_today
is all that is needed.
It also avoids the classical mistake of
forgetting to make the default value
dynamic and directly passing the result of
a time.strftime() call.

lp bug: https://launchpad.net/bugs/925361 fixed

bzr revid: odo@openerp.com-20120213180355-ruyw5j2w7r06kyue

openerp/osv/fields.py

@@ -35,14 +35,12 @@
 import base64
 import datetime as DT
 import logging
+import pytz
 import re
-import string
-import sys
 import xmlrpclib
 from psycopg2 import Binary
 
 import openerp
-import openerp.netsvc as netsvc
 import openerp.tools as tools
 from openerp.tools.translate import _
 from openerp.tools import float_round, float_repr
@@ -262,6 +260,7 @@ class float(_column):
 
 class date(_column):
     _type = 'date'
+
     @staticmethod
     def today(*args):
         """ Returns the current date in a format fit for being a
@@ -273,6 +272,28 @@ class date(_column):
         return DT.date.today().strftime(
             tools.DEFAULT_SERVER_DATE_FORMAT)
 
+    @staticmethod
+    def context_today(cr, uid, today=None, context=None):
+        """Returns the current date as seen in the client's timezone
+           in a format fit for date fields.
+           This method may be passed as value to initialize _defaults.
+
+           :param datetime today: optional date value to use instead of
+                                  the current date"""
+        today = DT.datetime.now()
+        context_today = None
+        if context and context.get('tz'):
+            try:
+                utc = pytz.timezone('UTC')
+                context_tz = pytz.timezone(context['tz'])
+                utc_today = utc.localize(today, is_dst=False)
+                context_today = utc_today.astimezone(context_tz)
+            except Exception:
+                _logger.debug("failed to compute context/client-specific today date, "
+                              "using the UTC value for `today`",
+                              exc_info=True)
+        return (context_today or today).strftime(tools.DEFAULT_SERVER_DATE_FORMAT)
+
 class datetime(_column):
     _type = 'datetime'
     @staticmethod