fix to_datetime() when given timezone naive datetime input

[Christian Boos]

While looking at some specifics of the trac.util.datefmt.to_datetime function, I discovered that a particular usage pattern is problematic with pytz timezones:

>>> from trac.util.datefmt import *
>>> from datetime import datetime
>>> t = to_datetime(datetime(2012,1,1), timezone("Europe/Paris"))
>>> t.utcoffset()
datetime.timedelta(0, 540)

(from ​gmessage:trac-dev:V0HeDyfa_pk/5r33UatF3nEJ)

The utcoffset() plays a role when converting a time to the number of ms (to_utimestamp()). Here, it has a value of 9 minutes, which is quite wrong for our purpose.

The workaround is to always specify a timezone aware datetime object to to_datetime, but this is not documented, and shouldn't be necessary either (assuming a timezone naive datetime should actually be the same as a datetime in utc).

The following should

trac/util/datefmt.py

@@ -66 +66 @@
     elif isinstance(t, datetime):
         if tzinfo is not None:
             if t.tzinfo is None:
-                t = t.replace(tzinfo=tzinfo)
+                t = tzinfo.localize(t)
             else:
                 t = tzinfo.normalize(t.astimezone(tzinfo))
         return t

With this change, the t.utcoffset() for the example above would give 3600 seconds, i.e. the one hour we would expect.

[Christian Boos]

I think it's OK for 0.12-stable. Feedback welcome!

[Christian Boos]

To be complete, it seems that datetime.fromtimestamp() doesn't have the problem that the datetime constructor has:

>>> datetime.fromtimestamp(1325376000, timezone("Europe/Paris")).utcoffset()
datetime.timedelta(0, 3600)

So we don't need to change the other parts of to_datetime().

[Christian Boos]

Same for datetime.now(), it's OK as well:

>>> datetime.now(timezone("Europe/Paris")).utcoffset()
datetime.timedelta(0, 7200)

(see also r8857)

[Jun Omae]

It looks good except datetime between DST boundaries. tzinfo.localize() generates invalid datetime between DST boundaries. I think we should use tzinfo.normalize().

>>> tz = timezone('Europe/Paris')
>>> t = datetime(2012, 3, 25, 2, 15)
>>> tz.localize(t)
datetime.datetime(2012, 3, 25, 2, 15, tzinfo=<DstTzInfo 'Europe/Paris' CET+1:00:00 STD>)
>>> tz.localize(t).utcoffset()
datetime.timedelta(0, 3600)
>>> tz.normalize(tz.localize(t))
datetime.datetime(2012, 3, 25, 3, 15, tzinfo=<DstTzInfo 'Europe/Paris' CEST+2:00:00 DST>)
>>> tz.normalize(tz.localize(t)).utcoffset()
datetime.timedelta(0, 7200)

trac/util/datefmt.py

@@ -66 +66 @@
     elif isinstance(t, datetime):
         if tzinfo is not None:
             if t.tzinfo is None:
-                t = t.replace(tzinfo=tzinfo)
+                t = tzinfo.localize(t)
+                t = tzinfo.normalize(t)
             else:
                 t = tzinfo.normalize(t.astimezone(tzinfo))
         return t

[Christian Boos]

Ah, yes! Thanks for the example, I'll integrate it into an unit-test.

[Christian Boos]

I've tried a more "radical" approach, by making to_datetime(t, tz) always:

• map the given input time to the given timezone (or localtz if no tz is specified)
• return a normalized datetime (also going in the direction of #10864)

I think it's a more consistent API and it's easier to use and to remember what it does. All tests still pass.

[Christian Boos]

rework to_datetime

[Jun Omae]

I tried the latest patch and it works good!

[Christian Boos]

Applied in r11397. Thanks for the review!