Commit d91dc623 authored by R David Murray's avatar R David Murray

Merge #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).
parents a79e7fed 539f239e
......@@ -12,28 +12,34 @@
The :mod:`datetime` module supplies classes for manipulating dates and times in
both simple and complex ways. While date and time arithmetic is supported, the
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),
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
work with, at the cost of ignoring some aspects of reality.
For applications requiring more, :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
time zone name, and whether Daylight Saving Time is in effect. Note that only
one concrete :class:`tzinfo` class, the :class:`timezone` class, is supplied by the
:mod:`datetime` module. The :class:`timezone` class can represent simple
timezones with fixed offset from UTC such as UTC itself or North American EST and
EDT timezones. Supporting timezones at whatever level of detail is
required is up to the application. The rules for time adjustment across the
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".
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 is up to the program whether a
particular number represents metres, 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 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 time zone name, and whether Daylight Saving Time is in effect. Note
that only one concrete :class:`tzinfo` class, the :class:`timezone` class, is
supplied by the :mod:`datetime` module. The :class:`timezone` class can
represent simple timezones with fixed offset from UTC, such as UTC itself or
North American EST and EDT timezones. Supporting timezones at deeper levels of
detail is up to the application. The rules for time adjustment across the
world are more political than rational, change frequently, and there is no
standard suitable for every application aside from UTC.
......@@ -114,10 +120,13 @@ 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.
......@@ -1846,3 +1855,7 @@ Notes:
When the ``%z`` directive is provided to the :meth:`strptime` method, an
aware :class:`.datetime` object will be produced. The ``tzinfo`` of the
result will be set to a :class:`timezone` instance.
.. rubric:: Footnotes
.. [#] If, that is, we ignore the effects of Relativity
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