PEP 495: Addressed several (not all yet) mailing list commments.
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Alexander Belopolsky
parent
104dc4b901
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273
pep-0495.txt
273
pep-0495.txt
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@ -21,15 +21,15 @@ between two moments in time for which local times are the same.
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.. sidebar:: US public service advertisement
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.. image:: pep-0495-daylightsavings.png
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:align: right
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:width: 15%
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:align: center
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:width: 95%
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Rationale
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=========
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In the most world locations there have been and will be times when
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local clocks are moved back. In those times, intervals are introduced
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local clocks are moved back. [#]_ In those times, intervals are introduced
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in which local clocks show the same time twice in the same day. In
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these situations, the information displayed on a local clock (or
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stored in a Python datetime instance) is insufficient to identify a
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@ -37,14 +37,27 @@ particular moment in time. The proposed solution is to add a boolean
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flag to the ``datetime`` instances that will distinguish between the
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two ambiguous times.
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.. [#] People who live in locations observing the Daylight Saving
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Time (DST) move their clocks back (usually one hour) every Fall.
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It is less common, but occasionally clocks can be moved back for
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other reasons. For example, Ukraine skipped the spring-forward
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transition in March 1990 and instead, moved their clocks back on
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July 1, 1990, switching from Moscow Time to Eastern European Time.
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In that case, standard (winter) time was in effect before and after
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the transition.
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Both DST and standard time changes may result in time shifts other
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than an hour.
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Terminology
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===========
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When clocks are moved back, we say that a *fold* is created in the
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fabric of time. When the clock are moved forward, a *gap* is created.
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A local time that falls in the fold is called *ambiguous*. A local
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time that falls in the gap is called *missing*.
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When clocks are moved back, we say that a *fold* is created in time.
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When the clocks are moved forward, a *gap* is created. A local time
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that falls in the fold is called *ambiguous*. A local time that falls
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in the gap is called *missing*.
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Proposal
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@ -55,7 +68,7 @@ The "first" flag
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We propose adding a boolean member called ``first`` to the instances
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of ``datetime.time`` and ``datetime.datetime`` classes. This member
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should have the value True for all instances except those that
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should have the value ``True`` for all instances except those that
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represent the second (chronologically) moment in time in an ambiguous
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case. [#]_
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@ -143,6 +156,18 @@ second, microsecond and first.
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Affected Behaviors
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------------------
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What time is it?
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................
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The ``datetime.now()`` method called with no arguments, will set
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``first=False`` when returning the second of the two ambiguous times
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in a fold. When called with a ``tzinfo`` argument, the value of the
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``first`` will be determined by the ``tzinfo.fromutc()``
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implementation. If an instance of the built-in ``datetime.timezone``
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is passed as ``tzinfo``, the returned datetime instance will always
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have ``first=True``.
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Conversion from naive to aware
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..............................
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@ -159,16 +184,31 @@ For example, on a system set to US/Eastern timezone::
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>>> dt.replace(first=False).astimezone().strftime('%D %T %Z%z')
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'11/02/14 01:30:00 EST-0500'
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Conversion from POSIX seconds from EPOCH
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........................................
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The ``fromtimestamp()`` static method of ``datetime.datetime`` will
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set the ``first`` attribute appropriately in the returned object.
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For example, on a system set to US/Eastern timezone::
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>>> datetime.fromtimestamp(1414906200)
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datetime.datetime(2014, 11, 2, 1, 30)
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>>> datetime.fromtimestamp(1414906200 + 3600)
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datetime.datetime(2014, 11, 2, 1, 30, first=False)
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Conversion to POSIX seconds from EPOCH
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......................................
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The ``timestamp()`` method of ``datetime.datetime`` will return different
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values for ``datetime.datetime`` instances that differ only by the value
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of their ``first`` attribute if and only if these instances represent an
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ambiguous or a non-existent time.
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ambiguous or a missing time.
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When a ``datetime.datetime`` instance ``dt`` represents an ambiguous
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(repeated) time, there are two values ``s0`` and ``s1`` such that::
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time, there are two values ``s0`` and ``s1`` such that::
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datetime.fromtimestamp(s0) == datetime.fromtimestamp(s1) == dt
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@ -208,21 +248,6 @@ For example, on a system set to US/Eastern timezone::
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1425796200.0
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Conversion from POSIX seconds from EPOCH
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........................................
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The ``fromtimestamp()`` static method of ``datetime.datetime`` will
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set the ``first`` attribute appropriately in the returned object.
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For example, on a system set to US/Eastern timezone::
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>>> datetime.fromtimestamp(1414906200)
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datetime.datetime(2014, 11, 2, 1, 30)
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>>> datetime.fromtimestamp(1414906200 + 3600)
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datetime.datetime(2014, 11, 2, 1, 30, first=False)
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Combining and splitting date and time
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.....................................
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@ -233,11 +258,27 @@ The ``datetime.datetime.time()`` method will copy the value of the
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``first`` attribute to the resulting ``datetime.time`` instance.
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Pickles
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.......
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Pickle sizes for the ``datetime.datetime`` and ``datetime.time``
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objects will not change. The ``first`` flag will be encoded in the
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first bit of the 5th byte of the ``datetime.datetime`` pickle payload
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or the 2nd byte of the datetime.time. In the `current implementation`_
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these bytes are used to store minute value (0-59) and the first bit is
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always 0. Note that ``first=True`` will be encoded as 0 in the first
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bit and ``first=False`` as 1. (This change only affects pickle
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format. In the C implementation, the "first" member will get a full byte
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to store the actual boolean value.)
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.. _current implementation: https://hg.python.org/cpython/file/d3b20bff9c5d/Include/datetime.h#l17
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Implementations of tzinfo in stdlib
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...................................
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===================================
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No new implementations of ``datetime.tzinfo`` abstract class are
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introduced in this PEP. The existing (fixed offset) timezones do
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proposed in this PEP. The existing (fixed offset) timezones do
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not introduce ambiguous local times and their ``utcoffset()``
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implementation will return the same constant value as they do now
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regardless of the value of ``first``.
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@ -248,27 +289,32 @@ used anywhere in the stdlib because the only included ``tzinfo``
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implementation (the ``datetime.timzeone`` class implementing fixed
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offset timezones) override ``fromutc()``.
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New guidelines will be published for implementing concrete timezones
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with variable UTC offset.
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Guidelines for new tzinfo implementations
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-----------------------------------------
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Guidelines for New tzinfo Implementations
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=========================================
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Implementors of concrete ``datetime.tzinfo`` subclasses who want to
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support variable UTC offsets (due to DST and other causes) must follow
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support variable UTC offsets (due to DST and other causes) should follow
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these guidelines.
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New subclasses must override the base-class ``fromutc()`` method and
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Ignorance is Bliss
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------------------
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New implementations of ``utcoffset()``, ``tzname()`` and ``dst()``
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methods should ignore the value of ``first`` unless they are called on
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the ambiguous or missing times.
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In the DST Fold
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---------------
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New subclasses should override the base-class ``fromutc()`` method and
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implement it so that in all cases where two UTC times ``u1`` and
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``u2`` (``u1`` <``u2``) correspond to the same local time
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``fromutc(u1)`` will return an instance with ``first=True`` and
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``fromutc(u2)`` will return an instance with ``first=False``. In all
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other cases the returned instance must have ``first=True``.
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New implementations of ``utcoffset()`` and ``dst()`` methods should
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ignore the value of ``first`` unless they are called on the ambiguous
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or missing times.
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other cases the returned instance should have ``first=True``.
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On an ambiguous time introduced at the end of DST, the values returned
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by ``utcoffset()`` and ``dst()`` methods should be as follows
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@ -276,30 +322,37 @@ by ``utcoffset()`` and ``dst()`` methods should be as follows
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+-----------------+----------------+------------------+
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| | first=True | first=False |
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+=================+================+==================+
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| utcoff() | stdoff + hour | stdoff |
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| utcoffset() | stdoff + dstoff| stdoff |
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+-----------------+----------------+------------------+
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| dst() | hour | zero |
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| dst() | dstoff | zero |
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+-----------------+----------------+------------------+
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where ``stdoff`` is the standard (non-DST) offset,
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``hour = timedelta(hours=1)`` and ``zero = timedelta(0)``.
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where ``stdoff`` is the standard (non-DST) offset, ``dstoff`` is the
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DST correction (typically ``dstoff = timedelta(hours=1)``) and ``zero
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= timedelta(0)``.
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Mind the DST Gap
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----------------
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On a missing time introduced at the start of DST, the values returned
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by ``utcoffset()`` and ``dst()`` methods should be as follows
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+-----------------+----------------+------------------+
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| | first=True | first=False |
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+=================+================+==================+
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| utcoff() | stdoff | stdoff + hour |
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| utcoffset() | stdoff | stdoff + dstoff |
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+-----------------+----------------+------------------+
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| dst() | zero | hour |
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| dst() | zero | dstoff |
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+-----------------+----------------+------------------+
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Non-DST Folds and Gaps
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----------------------
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On ambiguous/missing times introduced by the change in the standard time
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offset, the ``dst()`` method should return the same value regardless of
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the value of ``first`` and the ``utcoff()`` should return values
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the value of ``first`` and the ``utcoffset()`` should return values
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according to the following table:
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+-----------------+----------------+-----------------------------+
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@ -310,28 +363,11 @@ according to the following table:
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| missing | oldoff | newoff = oldoff + delta |
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+-----------------+----------------+-----------------------------+
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where ``delta`` is the size of the fold or the gap.
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Pickle size
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-----------
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Pickle sizes for the ``datetime.datetime`` and ``datetime.time``
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objects will not change. The ``first`` flag will be encoded in the
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first bit of the 5th byte of the ``datetime.datetime`` pickle payload
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or the 2nd byte of the datetime.time. In the `current implementation`_
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these bytes are used to store minute value (0-59) and the first bit is
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always 0. Note that ``first=True`` will be encoded as 0 in the first
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bit and ``first=False`` as 1. (This change only affects pickle
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format. In C implementation, the "first" member will get a full byte
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to store the actual boolean value.)
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We chose the minute byte to store the the "first" bit because this
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choice preserves the natural ordering.
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.. _current implementation: https://hg.python.org/cpython/file/d3b20bff9c5d/Include/datetime.h#l17
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Temporal Arithmetics
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--------------------
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Temporal Arithmetic
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===================
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The value of "first" will be ignored in all operations except those
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that involve conversion between timezones. [#]_ As a consequence,
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@ -345,15 +381,16 @@ The result of addition (subtraction) of a timedelta to (from) a
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datetime will always have ``first`` set to ``True`` even if the
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original datetime instance had ``first=False``.
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.. [#] As of Python 3.5, ``tzinfo`` is ignored whenever timedelta is
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added or subtracted from a ``datetime.datetime`` instance or when
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one ``datetime.datetime`` instance is subtracted from another with
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the same (even not-None) ``tzinfo``. This may change in the future,
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but such changes are outside of the scope of this PEP.
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.. [#] Computing a difference between two aware datetime instances
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with different values of ``tzinfo`` involves an implicit timezone
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conversion. In this case, the result may depend on the value of
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the ``first`` flag in either of the instances, but only if the
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instance has ``tzinfo`` that accounts for the value of ``first``
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in its ``utcoffset()`` method.
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Backward and Forward Compatibility
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----------------------------------
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==================================
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This proposal will have little effect on the programs that do not read
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the ``first`` flag explicitly or use tzinfo implementations that do.
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@ -368,16 +405,19 @@ in the new versions will become unreadable by the older Python
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versions. Pickles of instances with ``first=True`` (which is the
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default) will remain unchanged.
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Questions and Answers
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=====================
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1. Why not call the new flag "isdst"?
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Why not call the new flag "isdst"?
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----------------------------------
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-------
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A non-technical answer
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......................
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* Alice: Bob - let's have a stargazing party at 01:30 AM tomorrow!
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* Bob: Should I presume initially that summer time (for example, Daylight
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Saving Time) is or is not (respectively) in effect for the specified time?
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* Bob: Should I presume initially that Daylight Saving Time is or is
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not in effect for the specified time?
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* Alice: Huh?
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-------
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@ -386,32 +426,77 @@ Questions and Answers
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* Alice: You know, Bob, 01:30 AM will happen twice tomorrow. Which time do you have in mind?
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* Bob: I did not think about it, but let's pick the first.
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-------
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2. Why "first"?
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A technical reason
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..................
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* Rejections:
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While the ``tm_isdst`` field of the ``time.struct_time`` object can be
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used to disambiguate local times in the fold, the semantics of such
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disambiguation are completely different from the proposal in this PEP.
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**second**
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rejected because "second" is already there.
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The main problem with the ``tm_isdst`` field is that it is impossible
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to know what value is appropriate for ``tm_isdst`` without knowing the
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details about the time zone that are only available to the ``tzinfo``
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implementation. Thus while ``tm_isdst`` is useful in the *output* of
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methods such as ``time.localtime``, it is cumbursome as an *input* of
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methods such as ``time.mktime``.
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If the programmer misspecifies a non-negative value of ``tm_isdst`` to
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``time.mktime``, the result will be time that is 1 hour off and since
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there is rarely a way to know anything about DST *before* a call to
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``time.mktime`` is made, the only sane choice is usually
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``tm_isdst=-1``.
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Unlike ``tm_isdst``, the proposed ``first`` flag has no effect on the
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interpretation of the datetime instance unless without that flag two
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(or no) interpretations are possible.
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Since it would be very confusing to have something called ``isdst``
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that does not have the same semantics as ``tm_isdst``, we need a
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different name. Moreover, the ``datetime.datetime`` class already has
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a method called ``dst()`` and if we called ``first`` "isdst", we would
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necessarily have situations when "isdst" and ``bool(dst())`` values
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are different.
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Why "first"?
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------------
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This is a working name chosen initially because the obvious
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alternative ("second") conflicts with the existing attribute. It has
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since became clear that it is desirable to have a flag with the
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default value ``False`` and such that chronological ordering of
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disambiguated (datetime, flag) pairs would match their lexicographical
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order.
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The following alternative names have been proposed:
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**fold**
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Suggested by Guido van Rossum and favored by the author. Has
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correct connotations and easy mnemonic rules, but at the same
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time does not invite unbased assumptions.
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**later**
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rejected because "later" is confusable with "latter".
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**earlier**
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rejected because "earlier" has the same issue as "first" (requires
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default to be True) but is two characters longer.
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* Remaining possibilities:
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A close contender to "fold". The author dislikes it because
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it is confusable with equally fitting "latter," but in the age
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of autocompletion everywhere this is a small consideration. A
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stronger objection may be that in the case of missing time, we
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will have ``later=True`` instance converted to an earlier time by
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``.astimezone(timezone.utc)`` that that with ``later=False``.
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Yet again, this can be interpreted as a desirable indication that
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the original time is invalid.
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**repeated**
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this is a strong candidate
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**is_first**
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arguably more grammatically correct than "first"
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Did not receive any support on the mailing list.
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**ltdf**
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(Local Time Disambiguation Flag) - short and no-one will
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attempt to guess what it means without reading the docs.
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(Local Time Disambiguation Flag) - short and no-one will attempt
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to guess what it means without reading the docs. (Feel free to
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use it in discussions with the meaning ltdf=False is the
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earlier if you don't want to endorse any of the alternatives
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above.)
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Implementation
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==============
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