python-peps/pep-0431.txt

PEP: 431
Title: Time zone support improvements
Version: $Revision$
Last-Modified: $Date$
Author: Lennart Regebro <regebro@gmail.com>
BDFL-Delegate: Barry Warsaw <barry@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 11-Dec-2012
Post-History: 11-Dec-2012, 28-Dec-2012, 28-Jan-2013


Abstract
========

This PEP proposes the implementation of concrete time zone support in the
Python standard library, and also improvements to the time zone API to deal
with ambiguous time specifications during DST changes.


Proposal
========

Concrete time zone support
--------------------------

The time zone support in Python has no concrete implementation in the
standard library outside of a tzinfo baseclass that supports fixed offsets.
To properly support time zones you need to include a database over all time
zones, both current and historical, including daylight saving changes.
But such information changes frequently, so even if we include the last
information in a Python release, that information would be outdated just a
few months later.

Time zone support has therefore only been available through two third-party
modules, ``pytz`` and ``dateutil``, both who include and wrap the "zoneinfo"
database. This database, also called "tz" or "The Olsen database", is the
de-facto standard time zone database over time zones, and it is included in
most Unix and Unix-like operating systems, including OS X.

This gives us the opportunity to include the code that supports the zoneinfo
data in the standard library, but by default use the operating system's copy
of the data, which typically will be kept updated by the updating mechanism
of the operating system or distribution.

For those who have an operating system that does not include the zoneinfo
database, for example Windows, the Python source distribution will include a
copy of the zoneinfo database, and a distribution containing the latest
zoneinfo database will also be available at the Python Package Index, so it
can be easily installed with the Python packaging tools such as
``easy_install`` or ``pip``. This could also be done on Unices that are no
longer receiving updates and therefore have an outdated database.

With such a mechanism Python would have full time zone support in the
standard library on any platform, and a simple package installation would
provide an updated time zone database on those platforms where the zoneinfo
database isn't included, such as Windows, or on platforms where OS updates
are no longer provided.

The time zone support will be implemented by making the ``datetime`` module
into a package, and adding time zone support to ``datetime`` based on Stuart
Bishop's ``pytz`` module.


Getting the local time zone
---------------------------

On Unix there is no standard way of finding the name of the time zone that is
being used. All the information that is available is the time zone
abbreviations, such as ``EST`` and ``PDT``, but many of those abbreviations
are ambiguous and therefore you can't rely on them to figure out which time
zone you are located in.

There is however a standard for finding the compiled time zone information
since it's located in ``/etc/localtime``. Therefore it is possible to create
a local time zone object with the correct time zone information even though
you don't know the name of the time zone. A function in ``datetime`` should
be provided to return the local time zone.

The support for this will be made by integrating Lennart Regebro's
``tzlocal`` module into the new ``datetime`` module.

For Windows it will look up the local Windows time zone name, and use a
mapping between Windows time zone names and zoneinfo time zone names provided
by the Unicode consortium to convert that to a zoneinfo time zone.

The mapping should be updated before each major or bugfix release, scripts
for doing so will be provided in the ``Tools/`` directory.


Ambiguous times
---------------

When changing over from daylight savings time (DST) the clock is turned back
one hour. This means that the times during that hour happens twice, once
with DST and then once without DST. Similarly, when changing to daylight
savings time, one hour goes missing.

The current time zone API can not differentiate between the two ambiguous
times during a change from DST. For example, in Stockholm the time of
2012-11-28 02:00:00 happens twice, both at UTC 2012-11-28 00:00:00 and also
at 2012-11-28 01:00:00.

The current time zone API can not disambiguate this and therefore it's
unclear which time should be returned::

    # This could be either 00:00 or 01:00 UTC:
    >>> dt = datetime(2012, 10, 28, 2, 0, tzinfo=zoneinfo('Europe/Stockholm'))
    # But we can not specify which:
    >>> dt.astimezone(zoneinfo('UTC'))
    datetime.datetime(2012, 10, 28, 1, 0, tzinfo=<UTC>)

``pytz`` solved this problem by adding ``is_dst`` parameters to several
methods of the tzinfo objects to make it possible to disambiguate times when
this is desired.

This PEP proposes to add these ``is_dst`` parameters to the relevant methods
of the ``datetime`` API, and therefore add this functionality directly to
``datetime``. This is likely the hardest part of this PEP as this involves
updating the C version of the ``datetime`` library with this functionality,
as this involved writing new code, and not just reorganizing existing
external libraries.


Implementation API
==================

The zoneinfo database
---------------------

The latest version of the zoneinfo database should exist in the
``Lib/tzdata`` directory of the Python source control system. This copy of
the database should be updated before every Python feature and bug-fix
release, but not for releases of Python versions that are in
security-fix-only-mode.

Scripts to update the database will be provided in ``Tools/``, and the
release instructions will be updated to include this update.

New configure options ``--enable-internal-timezone-database`` and
``--disable-internal-timezone-database`` will be implemented to enable and
disable the installation of this database when installing from source. A
source install will default to installing them.

Binary installers for systems that have a system-provided zoneinfo database
may skip installing the included database since it would never be used for
these platforms. For other platforms, for example Windows, binary installers
must install the included database.


Changes in the ``datetime``-module
----------------------------------

The public API of the new time zone support contains one new class, one new
function, one new exception and four new collections. In addition to this, several
methods on the datetime object gets a new ``is_dst`` parameter.

New class ``dsttimezone``
^^^^^^^^^^^^^^^^^^^^^^^^^

This class provides a concrete implementation of the ``tzinfo`` base
class that implements DST support.


New function ``zoneinfo(name=None, db_path=None)``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This function takes a name string that must be a string specifying a
valid zoneinfo time zone, i.e. "US/Eastern", "Europe/Warsaw" or "Etc/GMT".
If not given, the local time zone will be looked up. If an invalid zone name
is given, or the local time zone can not be retrieved, the function raises
`UnknownTimeZoneError`.

The function also takes an optional path to the location of the zoneinfo
database which should be used. If not specified, the function will look for
databases in the following order:

1. Check if the `tzdata-update` module is installed, and then use that
   database.

2. Use the database in ``/usr/share/zoneinfo``, if it exists.

3. Use the Python-provided database in ``Lib/tzdata``.

If no database is found an ``UnknownTimeZoneError`` or subclass thereof will
be raised with a message explaining that no zoneinfo database can be found,
but that you can install one with the ``tzdata-update`` package.


New parameter ``is_dst``
^^^^^^^^^^^^^^^^^^^^^^^^

A new ``is_dst`` parameter is added to several methods to handle time
ambiguity during DST changeovers.

* ``tzinfo.utcoffset(dt, is_dst=False)``

* ``tzinfo.dst(dt, is_dst=False)``

* ``tzinfo.tzname(dt, is_dst=False)``

* ``datetime.astimezone(tz, is_dst=False)``

The ``is_dst`` parameter can be ``False`` (default), ``True``, or ``None``.

``False`` will specify that the given datetime should be interpreted as not
happening during daylight savings time, i.e. that the time specified is after
the change from DST. This is default to preserve existing behavior.

``True`` will specify that the given datetime should be interpreted as happening
during daylight savings time, i.e. that the time specified is before the change
from DST.

``None`` will raise an ``AmbiguousTimeError`` exception if the time specified
was during a DST change over. It will also raise a ``NonExistentTimeError``
if a time is specified during the "missing time" in a change to DST.

New exceptions
^^^^^^^^^^^^^^

* ``UnknownTimeZoneError``

  This exception is a subclass of KeyError and raised when giving a time
  zone specification that can't be found::

     >>> datetime.zoneinfo('Europe/New_York')
     Traceback (most recent call last):
     ...
     UnknownTimeZoneError: There is no time zone called 'Europe/New_York'

* ``InvalidTimeError``

  This exception serves as a base for ``AmbiguousTimeError`` and
  ``NonExistentTimeError``, to enable you to trap these two separately. It
  will subclass from ValueError, so that you can catch these errors together
  with inputs like the 29th of February 2011.

* ``AmbiguousTimeError``

  This exception is raised when giving a datetime specification that is ambiguous
  while setting ``is_dst`` to None::

    >>> datetime(2012, 11, 28, 2, 0, tzinfo=zoneinfo('Europe/Stockholm'), is_dst=None)
    >>>
    Traceback (most recent call last):
    ...
    AmbiguousTimeError: 2012-10-28 02:00:00 is ambiguous in time zone Europe/Stockholm


* ``NonExistentTimeError``

  This exception is raised when giving a datetime specification for a time that due to
  daylight saving does not exist, while setting ``is_dst`` to None::

    >>> datetime(2012, 3, 25, 2, 0, tzinfo=zoneinfo('Europe/Stockholm'), is_dst=None)
    >>>
    Traceback (most recent call last):
    ...
    NonExistentTimeError: 2012-03-25 02:00:00 does not exist in time zone Europe/Stockholm


New collections
^^^^^^^^^^^^^^^

* ``all_timezones`` is the exhaustive list of the time zone names that can
  be used, listed alphabethically.

* ``common_timezones`` is a list of useful, current time zones, listed
  alphabethically.


The ``tzdata-update``-package
-----------------------------

The zoneinfo database will be packaged for easy installation with
``easy_install``/``pip``/``buildout``. This package will not install any
Python code, and will not contain any Python code except that which is needed
for installation.

It will be kept updated with the same tools as the internal database, but
released whenever the ``zoneinfo``-database is updated, and use the same
version schema.


Differences from the ``pytz`` API
=================================

* ``pytz`` has the functions ``localize()`` and ``normalize()`` to work
  around that ``tzinfo`` doesn't have is_dst. When ``is_dst`` is
  implemented directly in ``datetime.tzinfo`` they are no longer needed.

* The ``timezone()`` function is called ``zoneinfo()`` to avoid clashing with
  the ``timezone`` class introduced in Python 3.2.

* ``zoneinfo()`` will return the local time zone if called without arguments.

* The class ``pytz.StaticTzInfo`` is there to provide the ``is_dst`` support for static
  time zones. When ``is_dst`` support is included in ``datetime.tzinfo`` it is no longer needed.

* ``InvalidTimeError`` subclasses from ``ValueError``.


Resources
=========

* http://pytz.sourceforge.net/

* http://pypi.python.org/pypi/tzlocal

* http://pypi.python.org/pypi/python-dateutil

* http://unicode.org/cldr/data/common/supplemental/windowsZones.xml

Copyright
=========

This document has been placed in the public domain.