Updated PEP 495.

pep-0495.txt

@@ -72,28 +72,170 @@ Methods
 
 The ``replace()`` methods of the ``datetime.time`` and
 ``datetime.datetime`` classes will get a new keyword-only argument
-called ``first`` with the default value ``True``.  The value of the
+called ``first`` with the default value ``True``.  It will
-``first`` argument will be used to set the value of the ``first``
+becave similarly to the other ``replace()`` arguments: if the ``first``
-attribute in the returned instance.
+argument is specified and given a boolean value, the new instance
+returned by ``replace()`` will have its ``first`` attribute set
+to that value.  In CPython, a non-boolean value of ``first`` will
+raise a ``TypeError``, but other implementations may allow the value
+``None`` to behave the same as when ``first`` is not given.  If the
+``first`` argument is not specified, the original value of the ``first``
+attribute is copied to the result.   
 
 Affected Behaviors
 ------------------
 
-The ``timestamp()`` method of ``datetime.datetime`` will return value
+Conversion from naive to aware
-advanced by 3600 if ``self`` represents an ambiguous hour and
+..............................
-``first`` is False.
+
+The ``astimezone()`` method will now work for naive ``self``.  The
+system local timezone will be assumed in this case and the ``first``
+flag will be used to determine which local timezone is in effect
+in the ambiguous case.
+
+For example, on a system set to US/Eastern timezone::
+
+  >>> dt = datetime(2014, 11, 2, 1, 30)
+  >>> dt.astimezone().strftime('%D %T %Z%z')
+  '11/02/14 01:30:00 EDT-0400'
+  >>> dt.replace(first=False).astimezone().strftime('%D %T %Z%z')
+  '11/02/14 01:30:00 EST-0500'
+
+Conversion to POSIX seconds from EPOCH
+......................................
+
+The ``timestamp()`` method of ``datetime.datetime`` will return different
+values for ``datetime.datetime`` instances that differ only by the value
+of their ``first`` attribute if and only if these instances represent an
+ambiguous or a non-existent value.
+
+When a ``datetime.datetime`` instance ``dt`` represents an ambiguous
+(repeated) time, there are two values ``s0`` and ``s1`` such that::
+
+  datetime.fromtimestamp(s0) == datetime.fromtimestamp(s1) == dt
+
+In this case, ``dt.timestamp()`` will return the smaller of ``s0``
+and ``s1`` values if ``dt.first == True`` and the larger otherwise.
+
+
+For example, on a system set to US/Eastern timezone::
+
+  >>> datetime(2014, 11, 2, 1, 30, first=True).timestamp()
+  1414906200.0
+  >>> datetime(2014, 11, 2, 1, 30, first=False).timestamp()
+  1414909800.0
+
+
+When a ``datetime.datetime`` instance ``dt`` represents an invalid
+time, there is no value ``s`` for which::
+
+  datetime.fromtimestamp(s) == dt
+
+but we can form two "nice to know" values of ``s`` that differ
+by the size of the gap in seconds.  One is the value of ``s``
+that would correspond to ``dt`` in a timezone where the UTC offset
+is always the same as the offset right before the gap and the
+other is the similar value but in a timezone the  UTC offset
+is always the same as the offset right after the gap.
+
+The value returned by ``dt.timestamp()`` given the invalid
+``dt`` will be the larger of the two "nice to know" values
+if ``dt.first  == True`` and the larger otherwise.
+
+For example, on a system set to US/Eastern timezone::
+
+  >>> datetime(2015, 3, 8, 2, 30, first=True).timestamp()
+  1425799800.0
+  >>> datetime(2015, 3, 8, 2, 30, first=False).timestamp()
+  1425796200.0
+
+
+Conversion from POSIX seconds from EPOCH
+........................................
+
 
 The ``fromtimestamp()`` static method of ``datetime.datetime`` will
 set the ``first`` attribute appropriately in the returned object.
 
+For example, on a system set to US/Eastern timezone::
+
+  >>> datetime.fromtimestamp(1414906200)
+  datetime.datetime(2014, 11, 2, 1, 30)
+  >>> datetime.fromtimestamp(1414906200 + 3600)
+  datetime.datetime(2014, 11, 2, 1, 30, first=False)
+
+
+Implementations of tzinfo in stdlib
+...................................
+
+No new implementations of ``datetime.tzinfo`` abstract class are
+introduced in this PEP.  The existing (fixed offset) timezones do
+not introduce ambiguous local times and their ``utcoffset()``
+implementation will return the same constant value as they do now
+regardless of the value of ``first``. 
+
+New guidelines will be published for implementing concrete timezones
+with variable UTC offset.
+
+
+Guidelines for new tzinfo implementations
+-----------------------------------------
+
+Implementors of concrete ``datetime.tzinfo`` subclasses who want to
+support variable UTC offsets (due to DST and other causes) must follow
+these guidelines.
+
+New subclasses should override the baseclass ``fromutc()`` method so
+that in all cases where two UTC times ``u1`` and ``u2`` (``u1`` <``u2``)
+corespond to the same local time ``fromutc(u1)`` will return an instance
+with ``first=True`` and ``fromutc(u1)`` will return an instance
+with ``first=False``.
+
+New implementations of ``utcoffset()`` and ``dst()`` methods should
+ignore the value of ``first`` unless they are called on the ambiguous
+or invalid times.
+
+On an ambiguous time introduced at the end of DST, the values returned
+by ``utcoffset()`` and ``dst()`` methods should be as follows
+
++-----------------+----------------+------------------+
+|                 |   first=True   |    first=False   |
++-----------------+----------------+------------------+
+|   utcoff()      | stdoff + hour  |    stdoff        |
++-----------------+----------------+------------------+
+|    dst()        |     hour       |     zero         |
++-----------------+----------------+------------------+
+
+where ``stdoff`` is the standard (non-DST) offset,
+``hour = timedelta(hours=1)`` and ``zero = timedelta(0)``.
+
+On an invalid time introduced at the start of DST, the values returned
+by ``utcoffset()`` and ``dst()`` methods should	be as follows
+
+
++-----------------+----------------+------------------+
+|                 |   first=True   |    first=False   |
++-----------------+----------------+------------------+
+|   utcoff()      |     stdoff     |    stdoff + hour |
++-----------------+----------------+------------------+
+|    dst()        |     zero       |     hour         |
++-----------------+----------------+------------------+
+
+
+On ambiguous/invalid times introduced by the change in the standard time
+offset, the ``dst()`` method should return the same value regardless of
+the value of ``first`` and the ``utcoff()`` should return values
+according to the following table:
+
++-----------------+----------------+-----------------------------+
+|                 |   first=True   |    first=False              |
++-----------------+----------------+-----------------------------+
+|   ambiguous     |     oldoff     |     newoff = oldoff - delta |
++-----------------+----------------+-----------------------------+
+|   invalid       |     oldoff     |     newoff = oldoff + delta |
++-----------------+----------------+-----------------------------+
 
-Implementations of tzinfo
-.........................
 
-Subclasses of ``datetime.tzinfo`` will read the values of ``first`` in
-``utcoffset()`` and ``dst()`` methods and set it appropriately in the
-instances returned by the ``fromutc()`` method.  No change to the
-signatures of these methods is proposed.
 
 Pickle size
 -----------
@@ -117,7 +259,7 @@ Temporal Arithmetics
 --------------------
 
 The value of "first" will be ignored in all operations except those
-that involve conversion between timezones.
+that involve conversion between timezones. [#]_
 
 The result of addition (subtraction) of a timedelta to (from) a
 datetime will always have ``first`` set to ``True`` even if the
@@ -125,9 +267,17 @@ original datetime instance had ``first=False``.
 
 (The only methods that will be able to produce non-default value of
 "first" are ``__new__``, and ``replace()`` methods of the
-``datetime.datetime`` and ``datetime.time`` classes ``now()`` and
+``datetime.datetime`` and ``datetime.time`` classes ``now()``,
-``fromtimestamp()`` methods of the ``datetime.datetime`` class, and
+``astimezone()`` and ``fromtimestamp()`` methods of the
-``fromutc()`` method of some tzinfo implementations.)
+``datetime.datetime`` class, and ``fromutc()`` method of some tzinfo
+implementations.)
+
+
+.. [#] As of Python 3.5, ``tzinfo`` is ignored whenever timedelta is
+   added or subtracted from a ``datetime.datetime`` instance or when
+   one ``datetime.datetime`` instance is subtracted from another with
+   the same (even not-None) ``tzinfo``.  This may change in the future,
+   but such changes are outside of the scope of this PEP.
 
 Comparison
 ----------