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#14766: Add correct algorithm for when a 'time' object is naive. 

This patch also clarifies the definition of Naive and Aware.

Original patch by Greg Weller, I modified the first hunk
somewhat to make the exposition even clearer (I hope).
This commit is contained in:
R David Murray 2012-05-14 22:32:44 -04:00
parent b90252ed17
commit 089d4d4f7c
1 changed files with 25 additions and 10 deletions
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Doc/library/datetime.rst
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@ -17,16 +17,23 @@ focus of the implementation is on efficient attribute extraction for output
formatting and manipulation. For related
functionality, see also the :mod:`time` and :mod:`calendar` modules.
There are two kinds of date and time objects: "naive" and "aware". This
distinction refers to whether the object has any notion of time zone, daylight
saving time, or other kind of algorithmic or political time adjustment. Whether
a naive :class:`.datetime` object represents Coordinated Universal Time (UTC),
There are two kinds of date and time objects: "naive" and "aware".
An aware object has sufficient knowledge of applicable algorithmic and
political time adjustments, such as time zone and daylight saving time
information, to locate itself relative to other aware objects. An aware object
is used to represent a specific moment in time that is not open to
interpretation [#]_.
+A naive object does not contain enough information to unambiguously locate
+itself relative to other date/time objects. Whether
a naive object represents Coordinated Universal Time (UTC),
local time, or time in some other timezone is purely up to the program, just
like it's up to the program whether a particular number represents metres,
miles, or mass. Naive :class:`.datetime` objects are easy to understand and to
miles, or mass. Naive objects are easy to understand and to
work with, at the cost of ignoring some aspects of reality.
For applications requiring more, :class:`.datetime` and :class:`.time` objects
For applications requiring aware objects, :class:`.datetime` and :class:`.time` objects
have an optional time zone information attribute, :attr:`tzinfo`, that can be
set to an instance of a subclass of the abstract :class:`tzinfo` class. These
:class:`tzinfo` objects capture information about the offset from UTC time, the
@ -105,10 +112,13 @@ Objects of these types are immutable.
Objects of the :class:`date` type are always naive.
An object *d* of type :class:`.time` or :class:`.datetime` may be naive or aware.
*d* is aware if ``d.tzinfo`` is not ``None`` and ``d.tzinfo.utcoffset(d)`` does
not return ``None``. If ``d.tzinfo`` is ``None``, or if ``d.tzinfo`` is not
``None`` but ``d.tzinfo.utcoffset(d)`` returns ``None``, *d* is naive.
An object of type :class:`.time` or :class:`.datetime` may be naive or aware.
A :class:`.datetime` object *d* is aware if ``d.tzinfo`` is not ``None`` and
``d.tzinfo.utcoffset(d)`` does not return ``None``. If ``d.tzinfo`` is
``None``, or if ``d.tzinfo`` is not ``None`` but ``d.tzinfo.utcoffset(d)``
returns ``None``, *d* is naive. A :class:`.time` object *t* is aware
if ``t.tzinfo`` is not ``None`` and ``t.tzinfo.utcoffset(None)`` does not return
``None``. Otherwise, *t* is naive.
The distinction between naive and aware doesn't apply to :class:`timedelta`
objects.
@ -1707,3 +1717,8 @@ Notes:
(5)
For example, if :meth:`utcoffset` returns ``timedelta(hours=-3, minutes=-30)``,
``%z`` is replaced with the string ``'-0330'``.
.. rubric:: Footnotes
.. [#] If, that is, we ignore the effects of Relativity