[Python-Dev] PEP 431 Time zone support improvements - Update

Lennart Regebro regebro at gmail.com
Fri Dec 28 21:23:46 CET 2012

Happy Holidays! Here is the update of PEP 431 with the changes that emerged
after the earlier discussion.

A raw download is here:

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


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.


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information in a Python release, that information would be outdated just a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longer recieving updates and therefore has an outdated database.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 creating a new submodule called `timezone``, 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 ambigious 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 ``timezone`` 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 timezone.

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

The current time zone API can not differentiating 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=timezone('Europe/Stockholm'))
    # But we can not specify which:
    >>> dt.astimezone(timezone('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

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

The new ``datetime.timezone``-module

The public API of the new ``timezone``-module contains one new class, one new
function and one new exception.

* New class: ``DstTzInfo``

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

* New function :``timezone(name=None, db_path=None)``

  This function takes a name string that must be a string specifying a
  valid zoneinfo timezone, ie "US/Eastern", "Europe/Warsaw" or "Etc/GMT+11".
  If not given, the local timezone will be looked up. If an invalid zone name
  are given, or the local timezone can not be retrieved, the function raises

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

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

  3. Check 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 Exception: ``UnknownTimeZoneError``

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

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

Changes in the ``datetime``-module

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

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

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

* ``tzinfo.tzname(self, dt, 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, ie that the time specified is after
the change from DST.

``True`` will specify that the given datetime should be interpreted as happening
during daylight savings time, ie 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.

There are also three new exceptions:

* ``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
are ambigious
  while setting ``is_dst`` to None::

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

* ``NonExistentTimeError``

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

    >>> datetime(2012, 3, 25, 2, 0,
tzinfo=timezone('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

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.

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.

* ``timezone()``  will return the local time zone if called without parameters.

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

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


Should the windows installer include the data package?

It has been suggested that the Windows installer should include the data
package. This would mean that an explicit installation no longer would be
needed on Windows. On the other hand, that would mean that many using Windows
would not be aware that the database quickly becomes outdated and would not
keep it updated.


* 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


This document has been placed in the public domain.
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