Alexander Belopolsky writes:

On Tue, Aug 25, 2015 at 7:19 PM, Akira Li <4kir4.1i@gmail.com> wrote:

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Here's a distilled example:

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from datetime import datetime, timezone datetime(2013, 10, 28, tzinfo=timezone.utc).astimezone().strftime('%Z%z')

If you *disable tm_gmtoff attribute* then it produces UTC+04:00+0400. That differs from the expected output MSK+0400, like the same code demonstrates if you enable the attribute. Notice (direct quote): "if tm_gmtoff or tm_zone are not available" above.

Of course! That's why we exposed tm_gmtoff attribute in time.time_struct on *all platfoms* IIRC. It's been a long time, by I recall that we went to some great lengths to emulate tm_gmtoff by comparing the results of localtime calls to those of gmtime. Could it be that we missed some corner cases? Sure. But your "if tm_gmtoff or tm_zone are not available" sounds like complaining that after

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del datetime.timezone

the datetime module does not support even the UTC timezone!

Whether or not tm_gmtoff is available depends on C library. The time module documentation says [1]: .. versionchanged:: 3.3 :attr:`tm_gmtoff` and :attr:`tm_zone` attributes are available on platforms with C library supporting the corresponding fields in ``struct tm``. i.e., tm_gmtoff, tm_zone may be absent on some platforms. How is it similar to *del datetime.timezone* if whether or not the attributes are available depends on platform? I don't see how any system without a historical timezone database may support the attributes and as far as I know C library that is used by python.exe does not use the tz database on Windows. [1] https://docs.python.org/3.4/library/time.html#time.struct_time To make sure that we are talking about the same thing. Here's how I've temporarily disabled the attributes to perform the test. It *emulates* platforms with one or both attributes missing: diff --git a/Lib/datetime.py b/Lib/datetime.py --- a/Lib/datetime.py +++ b/Lib/datetime.py @@ -1507,6 +1507,7 @@ local = datetime(*localtm[:6]) try: # Extract TZ data if available + raise AttributeError gmtoff = localtm.tm_gmtoff zone = localtm.tm_zone except AttributeError: @@ -2129,7 +2130,7 @@ # pretty bizarre, and a tzinfo subclass can override fromutc() if it is. try: - from _datetime import * + from __datetime import * except ImportError: pass else: 1st line forces AttributeError exception handler to execute. 2nd line disables C code that looks identical to the pure Python code. And here's the code from the except AttributeError clause above that I've shown in earlier messages [2] (C code does the same [3]): # Compute UTC offset and compare with the value implied # by tm_isdst. If the values match, use the zone name # implied by tm_isdst. delta = local - datetime(*_time.gmtime(ts)[:6]) dst = _time.daylight and localtm.tm_isdst > 0 gmtoff = -(_time.altzone if dst else _time.timezone) if delta == timedelta(seconds=gmtoff): tz = timezone(delta, _time.tzname[dst]) else: tz = timezone(delta) note: as the example with datetime(2013, 10, 28, tzinfo=timezone.utc).astimezone().strftime('%Z%z') in TZ=Europe/Moscow demonstrates, this code [4] does not find the correct tz.tzname() and therefore it is not an appropriate replacement for *tm_zone*. The code [4] can't be fixed without data from the tz database. [2] https://mail.python.org/pipermail/datetime-sig/2015-August/000471.html [3] https://github.com/python/cpython/blob/fced0e12fc510e4a6158628695774ccfd0239... [4] https://github.com/python/cpython/blob/fced0e12fc510e4a6158628695774ccfd0239...