[Python-Dev] Adding the 'path' module (was Re: Some RFE forreview)

[Guido van Rossum]

[Anders J. Munch]
> > > Alas datetime objects do not unambiguously identify a point in time.
> > > datetime objects are not timestamps: They represent the related but
> > > different concept of _local time_, which can be good for
> > presentation,
> > > but shouldn't be allowed anywhere near a persistent store.

[GvR]
> > You misunderstand the datetime module! You can have a datetime object
> > whose timezone is UTC; or you can have a convention in your API that
> > datetime objects without timezone represent UTC.

[Anders]
> I can do a lot of things in my own code, and I'm sure the datetime
> module provides tools that I can build on to do so, but that doesn't
> help in interfacing with other people's code -- such as the standard
> library.
> 
> Try saving a pickle of datetime.now() and reading it on a computer set
> to a different time zone.  The repr will then show the local time on
> the _originating_ computer, and figuring out the absolute time this
> corresponds to requires knowledge of time zone and DST settings on the
> originating computer.
> 
> How about datetime.utcnow(), then?  Just use UTC for datetime
> timestamps?  But that goes against the grain of the datetime module:

Against the grain? There's just a bug in your example; stop assigning
intentions to datetime that it doesn't have.

> Python 2.4 (#60, Nov 30 2004, 11:49:19) [MSC v.1310 32 bit (Intel)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
> >>> import datetime, time
> >>> datetime.datetime.fromtimestamp(time.time()) - datetime.datetime.now()
> datetime.timedelta(0)
> >>> datetime.datetime.fromtimestamp(time.time()) - datetime.datetime.utcnow()
> datetime.timedelta(0, 7200)
> 
> It seems when I subtract the present time from the present time,
> there's a two hour difference.

No, you're mixing intentions. I can't tell if you're doing this
intentionally to make the design look bad or if you don't understand
the design; I'm going to assume the latter (if the former there's no
point to this discussion at all) and explain what you should have
done:

>>> datetime.datetime.utcfromtimestamp(time.time()) - datetime.datetime.utcnow()
datetime.timedelta(0)
>>>

Your bug is similar to comparing centimeters to inches, or speed to
acceleration, or any number of similar mistakes.

What I give you, however, is that a timezone object representing UTC
should be part of the standard library, so that you wouldn't have an
excuse for using tz-less datetime objects to represent UTC.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)