| msg161558 - (view) |
Author: Dirkjan Ochtman (djc) *  |
Date: 2012-05-25 08:53 |
| There's datetime.fromtimestamp() and datetime.timetuple(), but no datetime.timestamp(). It should be possible to round-trip a UNIX timestamp through a datetime.datetime. |
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| msg161568 - (view) |
Author: Alexander Belopolsky (Alexander.Belopolsky) |
Date: 2012-05-25 11:39 |
| See . |
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| msg162263 - (view) |
Author: Barry A. Warsaw (barry) * |
Date: 2012-06-04 15:15 |
| I completely agree. As easy (but obscure) as it is, it seems quite silly to have to go through time.mktime(dt.timetuple()) |
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| msg162267 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2012-06-04 15:45 |
| Barry, Did you review the long discussion under issue 2736? Specifically, please note the part about mxDateTime, ticks() and gmticks(). I am -0 on adding ticks() and gmticks() and as far as I can tell no one has implemented these methods for datetime without introducing some subtle bugs. I also suspect that those who ask for a timestamp() method expect that it will do more than EPOCH = datetime(1970, 1, 1) def timestamp(t): return (t - EPOCH).total_seconds() For example, account for leap seconds or magically guess the DST offset. If we introduce a method that implements the code above, we have to prepare to explain why it does not do those things. If instead we introduce a shortcut for time.mktime(t.timetuple()), we will have to deal with the fact that mktime() behaves differently on different systems. |
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| msg162268 - (view) |
Author: Barry A. Warsaw (barry) * |
Date: 2012-06-04 15:57 |
| On Jun 04, 2012, at 03:45 PM, Alexander Belopolsky wrote: >Did you review the long discussion under issue 2736? Specifically, please >note the part about mxDateTime, ticks() and gmticks(). I am -0 on adding >ticks() and gmticks() and as far as I can tell no one has implemented these >methods for datetime without introducing some subtle bugs. I skimmed it, but I don't have time to read the whole bug. I'll note that I've recently had to convert a bunch of code from using mxDateTime to datetime+time, for the only reason that we had to support Python 3 and mxDT isn't yet ported. >I also suspect that those who ask for a timestamp() method expect that it >will do more than > >EPOCH = datetime(1970, 1, 1) >def timestamp(t): > return (t - EPOCH).total_seconds() Not really. >For example, account for leap seconds or magically guess the DST offset. If >we introduce a method that implements the code above, we have to prepare to >explain why it does not do those things. That's fine, explain away! >If instead we introduce a shortcut for time.mktime(t.timetuple()), we will >have to deal with the fact that mktime() behaves differently on different >systems. Not really. If developers even care, they will know that the epoch is different for different systems. We don't have to deal with it, we just have to mention it. Let them use more verbose "correct" code if they care. Seriously, we should not over-engineer this. It's purely a more discoverable convenience and I think implementing the moral equivalent to time.mktime(t.timetuple()) is exactly the right thing to do. |
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| msg162269 - (view) |
Author: Alexander Belopolsky (Alexander.Belopolsky) |
Date: 2012-06-04 16:03 |
| On Mon, Jun 4, 2012 at 11:57 AM, Barry A. Warsaw <report@bugs.python.org> wrote: > I've recently had to convert a bunch of code from using mxDateTime to > datetime+time, That's a valuable experience. How big of a deal was the lack of .ticks() and .gmticks()? How did you work around it? |
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| msg162270 - (view) |
Author: Barry A. Warsaw (barry) * |
Date: 2012-06-04 16:12 |
| On Jun 04, 2012, at 04:03 PM, Alexander Belopolsky wrote: >That's a valuable experience. How big of a deal was the lack of >.ticks() and .gmticks()? How did you work around it? It was *much* less of an issue than all the magic date format parsing that mxDT supports. That's actually something that I think is a more serious deficiency, since .strptime() is pretty limited (e.g. how would you parse ISO 8601 dates both with and without the 'T'?). A Python 3 compatible time format parser would make for a very nice separate library (PyPI) and/or addition to the stdlib (eventually). FWIW, here's the changeset. http://bazaar.launchpad.net/~gwibber-committers/gwibber/trunk/revision/1354 |
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| msg162278 - (view) |
Author: Alexandre Zani (Alexandre.Zani) |
Date: 2012-06-04 17:33 |
| I think the easiest and most intuitive approach is to simply define timestamp() as being the reverse of fromtimestamp(). Don't worry about leap seconds and all that stuff. If non-1970 epochs are a concern, this could be renamed to posix_timestamp or some such with perhaps a generic timestamp function that takes both a time and epoch. But let's not let such a useful function not happen just because it won't solve everyone's problem. |
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| msg162280 - (view) |
Author: Alexander Belopolsky (Alexander.Belopolsky) |
Date: 2012-06-04 17:45 |
| On Mon, Jun 4, 2012 at 1:33 PM, Alexandre Zani <report@bugs.python.org> wrote: > I think the easiest and most intuitive approach is to simply define timestamp() > as being the reverse of fromtimestamp(). I would like to invite everyone to review the discussion leading to closing of issue 2736. We cannot implement "the reverse of fromtimestamp()" because fromtimestamp() is not reversible in presence of DST and because float cannot represent all values that datetime supports. Both issues can be resolved, but I did not see any solution that I would call intuitive. Is anyone motivated enough to port mxDT's ticks() method to datetime? I don't think we need to reinvent the wheel. |
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| msg162282 - (view) |
Author: Alexandre Zani (Alexandre.Zani) |
Date: 2012-06-04 19:21 |
| I'm still reading through the issue you mentioned. (It's a painful read I have to admit) One major obstacle seems to be that during the DST switch over, an hour gets repeated and so the datetime object is ambiguous. (are you on the first or second hour?) I would argue that it isn't a problem that this function needs to solve. The ambiguity isn't tied to the conversion. It's tied to the datetime object itself. Let's add an optional parameter to specify the DST status, doc the pitfall and not worry overmuch about it. Side note: Let me know if I misunderstood Alexander, but if I didn't this should be documented with the datetime object. Based upon my understanding, the datetime object is a bad choice if you care about that ambiguity. That's not really clear. |
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| msg162288 - (view) |
Author: Alexander Belopolsky (Alexander.Belopolsky) |
Date: 2012-06-04 20:20 |
| On Mon, Jun 4, 2012 at 3:21 PM, Alexandre Zani <report@bugs.python.org> wrote: > Let me know if I misunderstood Alexander, but if I didn't this should be documented with > the datetime object. Based upon my understanding, the datetime object is a bad choice > if you care about that ambiguity. That's not really clear. The datetime object is neither good nor bad if you want to deal with the DST ambiguity. If you want to store times as local time, the best practice is to include the UTC offset. The datetime module now provides the facilities to do so. If you neglect the timezone and deal with naive datetime objects instead, you should probably avoid mixing it with POSIX timestamps in the same application. Here is an example where naive datetime is quite appropriate: a timekeeping application for a small business with a single location. If you need to generate opening hours for a given week, [datetime(y,m,d, 9, 0) + datetime(i) for i in range(5)] is an adequate solution, but if you compare it to [datetime.fromtimestamp(x + 24*3600*i) for i in range(5)], you may be off by one hour if your week spans the DST change. In most applications, however, keeping local time without UTC offset is a bad choice. POSIX's timestamp only advantage is that it does no allow keeping local time at all. |
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| msg162316 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2012-06-05 01:28 |
| Barry, I looked at your changeset at http://bazaar.launchpad.net/~gwibber-committers/gwibber/trunk/revision/1354 and at the specification at http://developers.facebook.com/docs/reference/api/event/. Am I correct that you are parsing "string containing an ISO-8601 formatted date/time or a UNIX timestamp; if it contains a time zone (not recommended), it will be converted to Pacific time before being stored and displayed"? If so, it looks like converting input to POSIX timestamps is the wrong thing to do because users who specify naive time expect it to be displayed without conversion. The way I read the specs, a better implementation would store input in naive datetime objects, converting UNIX timestamps or TZ-aware strings to Pacific timezone. |
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| msg162452 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2012-06-07 03:29 |
| I think the attached patch implements what was agreed to on python-dev. We need more tests, particularly around DST change. I tested the patch on OSX, but I am interested to hear feedback from users of other OSes. |
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| msg162483 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2012-06-07 17:29 |
| Updated patch adds a few more tests and improves error handling in C implementation. |
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| msg162531 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2012-06-08 15:36 |
| Added ReST documentation. Will commit soon. Merging nosy list from issue 2736. |
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| msg162534 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2012-06-08 16:38 |
| I associated my commit with issue 2736 by mistake, but it is probably a good thing because that issue contains a longer history. Closing this as a duplicate. |
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