Re: UTC "timezone" causing brain explosions topic

On 01/29/2014 01:52 PM, Skip Montanaro wrote:
> Following up on my earlier note about UTC v. GMT, I am having some
> trouble grokking attempts to convert a datetime into UTC.
>
> Okay, let's see what GMT does for us:
>
>>>> GMT = pytz.timezone("GMT")
>>>> u = GMT.normalize(s)
>>>> u
> datetime.datetime(2014, 1, 29, 21, 39, 35, 263666, tzinfo=<StaticTzInfo 'GMT'>)
>>>> u.hour
> 21
>
> That looks correct, but I don't understand why I don't get hour==21
> out of the UTC.normalize call. It's like it's a no-op.

Having not examined the code, I can't tell you why UTC is different. I can say that .astimezone seems to work in all cases:

--> GMT
<StaticTzInfo 'GMT'>

--> UTC
<UTC>

--> LOCAL_TZ = pytz.timezone("America/Los_Angeles")

--> now = datetime.datetime.now()

--> s = LOCAL_TZ.localize(now)
--> s
datetime.datetime(2014, 1, 29, 14, 9, 56, 494967, tzinfo=<DstTzInfo 'America/Los_Angeles' PST-1 day, 16:00:00 STD>)

--> s.astimezone(UTC)
datetime.datetime(2014, 1, 29, 22, 9, 56, 494967, tzinfo=<UTC>)

--> s.astimezone(GMT)
datetime.datetime(2014, 1, 29, 22, 9, 56, 494967, tzinfo=<StaticTzInfo 'GMT'>)

--
~Ethan~

Re: UTC "timezone" causing brain explosions topic

On Wed, Jan 29, 2014 at 1:52 PM, Skip Montanaro <(E-Mail Removed)> wrote:
> Following up on my earlier note about UTC v. GMT, I am having some
> trouble grokking attempts to convert a datetime into UTC. Consider
> these three values:
>
>>>> import pytz
>>>> UTC = pytz.timezone("UTC")
>>>> LOCAL_TZ = pytz.timezone("America/Chicago")
>>>> LOCAL_TZ
> <DstTzInfo 'America/Chicago' CST-1 day, 18:00:00 STD>
>>>> now = datetime.datetime.now()
>>>> now
> datetime.datetime(2014, 1, 29, 15, 39, 35, 263666)
>
> All well and good, right? The variable "now" is a naive datetime
> object. I happen to be sitting in a chair in the city of Chicago, so
> let's call it what it is, a datetime in the America/Chicago timezone:
>
>>>> s = LOCAL_TZ.localize(now)
>>>> s
> datetime.datetime(2014, 1, 29, 15, 39, 35, 263666, tzinfo=<DstTzInfo
> 'America/Chicago' CST-1 day, 18:00:00 STD>)
>
> That looks good to me. Now, let's normalize it to UTC:

I don't think .normalize() doesn't do what you think it does; it's
related to timezones with DST.
I believe you want datetime.astimezone() instead.

>>>> t = UTC.normalize(s)
>>>> t
> datetime.datetime(2014, 1, 29, 15, 39, 35, 263666, tzinfo=<UTC>)
>>>> t.hour
> 15
>
> WTF? Why isn't the t.hour == 21?

Because you didn't actually perform a proper timezone conversion:
>>> t = s.astimezone(UTC)
>>> t
datetime.datetime(2014, 1, 29, 21, 39, 35, 263666, tzinfo=<UTC>)
>>> t.hour == 21
True

<snip>
> That looks correct, but I don't understand why I don't get hour==21
> out of the UTC.normalize call. It's like it's a no-op.

It is indeed a no-op:
"You can take shortcuts when dealing with the UTC side of timezone
conversions. normalize() and localize() are not really necessary when
there are no daylight savings time transitions to deal with."
-- http://pytz.sourceforge.net

Cheers,
Chris

UTC "timezone" causing brain explosions topic

Following up on my earlier note about UTC v. GMT, I am having some
trouble grokking attempts to convert a datetime into UTC. Consider
these three values:

>>> import pytz
>>> UTC = pytz.timezone("UTC")
>>> UTC
<UTC>
>>> LOCAL_TZ = pytz.timezone("America/Chicago")
>>> LOCAL_TZ
<DstTzInfo 'America/Chicago' CST-1 day, 18:00:00 STD>
>>> now = datetime.datetime.now()
>>> now
datetime.datetime(2014, 1, 29, 15, 39, 35, 263666)

All well and good, right? The variable "now" is a naive datetime
object. I happen to be sitting in a chair in the city of Chicago, so
let's call it what it is, a datetime in the America/Chicago timezone:

>>> s = LOCAL_TZ.localize(now)
>>> s
datetime.datetime(2014, 1, 29, 15, 39, 35, 263666, tzinfo=<DstTzInfo
'America/Chicago' CST-1 day, 18:00:00 STD>)

That looks good to me. Now, let's normalize it to UTC:

>>> t = UTC.normalize(s)
>>> t
datetime.datetime(2014, 1, 29, 15, 39, 35, 263666, tzinfo=<UTC>)
>>> t.hour
15

WTF? Why isn't the t.hour == 21?

Okay, let's see what GMT does for us:

>>> GMT = pytz.timezone("GMT")
>>> u = GMT.normalize(s)
>>> u
datetime.datetime(2014, 1, 29, 21, 39, 35, 263666, tzinfo=<StaticTzInfo 'GMT'>)
>>> u.hour
21

That looks correct, but I don't understand why I don't get hour==21
out of the UTC.normalize call. It's like it's a no-op.

Skip