"zoneinfo" — IANA time zone support
***********************************

New in version 3.9.

======================================================================

The "zoneinfo" module provides a concrete time zone implementation to
support the IANA time zone database as originally specified in **PEP
615**. By default, "zoneinfo" uses the system’s time zone data if
available; if no system time zone data is available, the library will
fall back to using the first-party tzdata package available on PyPI.

See also:

  Module: "datetime"
     Provides the "time" and "datetime" types with which the
     "ZoneInfo" class is designed to be used.

  Package tzdata
     First-party package maintained by the CPython core developers to
     supply time zone data via PyPI.


Using "ZoneInfo"
================

"ZoneInfo" is a concrete implementation of the "datetime.tzinfo"
abstract base class, and is intended to be attached to "tzinfo",
either via the constructor, the "datetime.replace" method or
"datetime.astimezone":

   >>> from zoneinfo import ZoneInfo
   >>> from datetime import datetime, timedelta

   >>> dt = datetime(2020, 10, 31, 12, tzinfo=ZoneInfo("America/Los_Angeles"))
   >>> print(dt)
   2020-10-31 12:00:00-07:00

   >>> dt.tzname()
   'PDT'

Datetimes constructed in this way are compatible with datetime
arithmetic and handle daylight saving time transitions with no further
intervention:

   >>> dt_add = dt + timedelta(days=1)

   >>> print(dt_add)
   2020-11-01 12:00:00-08:00

   >>> dt_add.tzname()
   'PST'

These time zones also support the "fold" attribute introduced in **PEP
495**.  During offset transitions which induce ambiguous times (such
as a daylight saving time to standard time transition), the offset
from *before* the transition is used when "fold=0", and the offset
*after* the transition is used when "fold=1", for example:

   >>> dt = datetime(2020, 11, 1, 1, tzinfo=ZoneInfo("America/Los_Angeles"))
   >>> print(dt)
   2020-11-01 01:00:00-07:00

   >>> print(dt.replace(fold=1))
   2020-11-01 01:00:00-08:00

When converting from another time zone, the fold will be set to the
correct value:

   >>> from datetime import timezone
   >>> LOS_ANGELES = ZoneInfo("America/Los_Angeles")
   >>> dt_utc = datetime(2020, 11, 1, 8, tzinfo=timezone.utc)

   >>> # Before the PDT -> PST transition
   >>> print(dt_utc.astimezone(LOS_ANGELES))
   2020-11-01 01:00:00-07:00

   >>> # After the PDT -> PST transition
   >>> print((dt_utc + timedelta(hours=1)).astimezone(LOS_ANGELES))
   2020-11-01 01:00:00-08:00


Data sources
============

The "zoneinfo" module does not directly provide time zone data, and
instead pulls time zone information from the system time zone database
or the first-party PyPI package tzdata, if available. Some systems,
including notably Windows systems, do not have an IANA database
available, and so for projects targeting cross-platform compatibility
that require time zone data, it is recommended to declare a dependency
on tzdata. If neither system data nor tzdata are available, all calls
to "ZoneInfo" will raise "ZoneInfoNotFoundError".


Configuring the data sources
----------------------------

When "ZoneInfo(key)" is called, the constructor first searches the
directories specified in "TZPATH" for a file matching "key", and on
failure looks for a match in the tzdata package. This behavior can be
configured in three ways:

1. The default "TZPATH" when not otherwise specified can be configured
   at compile time.

2. "TZPATH" can be configured using an environment variable.

3. At runtime, the search path can be manipulated using the
   "reset_tzpath()" function.


Compile-time configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~

The default "TZPATH" includes several common deployment locations for
the time zone database (except on Windows, where there are no “well-
known” locations for time zone data). On POSIX systems, downstream
distributors and those building Python from source who know where
their system time zone data is deployed may change the default time
zone path by specifying the compile-time option "TZPATH" (or, more
likely, the "configure" flag "--with-tzpath"), which should be a
string delimited by "os.pathsep".

On all platforms, the configured value is available as the "TZPATH"
key in "sysconfig.get_config_var()".


Environment configuration
~~~~~~~~~~~~~~~~~~~~~~~~~

When initializing "TZPATH" (either at import time or whenever
"reset_tzpath()" is called with no arguments), the "zoneinfo" module
will use the environment variable "PYTHONTZPATH", if it exists, to set
the search path.

PYTHONTZPATH

   This is an "os.pathsep"-separated string containing the time zone
   search path to use. It must consist of only absolute rather than
   relative paths. Relative components specified in "PYTHONTZPATH"
   will not be used, but otherwise the behavior when a relative path
   is specified is implementation-defined; CPython will raise
   "InvalidTZPathWarning", but other implementations are free to
   silently ignore the erroneous component or raise an exception.

To set the system to ignore the system data and use the tzdata package
instead, set "PYTHONTZPATH=""".


Runtime configuration
~~~~~~~~~~~~~~~~~~~~~

The TZ search path can also be configured at runtime using the
"reset_tzpath()" function. This is generally not an advisable
operation, though it is reasonable to use it in test functions that
require the use of a specific time zone path (or require disabling
access to the system time zones).


The "ZoneInfo" class
====================

class zoneinfo.ZoneInfo(key)

   A concrete "datetime.tzinfo" subclass that represents an IANA time
   zone specified by the string "key". Calls to the primary
   constructor will always return objects that compare identically;
   put another way, barring cache invalidation via
   "ZoneInfo.clear_cache()", for all values of "key", the following
   assertion will always be true:

      a = ZoneInfo(key)
      b = ZoneInfo(key)
      assert a is b

   "key" must be in the form of a relative, normalized POSIX path,
   with no up-level references. The constructor will raise
   "ValueError" if a non-conforming key is passed.

   If no file matching "key" is found, the constructor will raise
   "ZoneInfoNotFoundError".

The "ZoneInfo" class has two alternate constructors:

classmethod ZoneInfo.from_file(fobj, /, key=None)

   Constructs a "ZoneInfo" object from a file-like object returning
   bytes (e.g. a file opened in binary mode or an "io.BytesIO"
   object). Unlike the primary constructor, this always constructs a
   new object.

   The "key" parameter sets the name of the zone for the purposes of
   "__str__()" and "__repr__()".

   Objects created via this constructor cannot be pickled (see
   pickling).

classmethod ZoneInfo.no_cache(key)

   An alternate constructor that bypasses the constructor’s cache. It
   is identical to the primary constructor, but returns a new object
   on each call. This is most likely to be useful for testing or
   demonstration purposes, but it can also be used to create a system
   with a different cache invalidation strategy.

   Objects created via this constructor will also bypass the cache of
   a deserializing process when unpickled.

   Caution:

     Using this constructor may change the semantics of your datetimes
     in surprising ways, only use it if you know that you need to.

The following class methods are also available:

classmethod ZoneInfo.clear_cache(*, only_keys=None)

   A method for invalidating the cache on the "ZoneInfo" class. If no
   arguments are passed, all caches are invalidated and the next call
   to the primary constructor for each key will return a new instance.

   If an iterable of key names is passed to the "only_keys" parameter,
   only the specified keys will be removed from the cache. Keys passed
   to "only_keys" but not found in the cache are ignored.

   Warning:

     Invoking this function may change the semantics of datetimes
     using "ZoneInfo" in surprising ways; this modifies process-wide
     global state and thus may have wide-ranging effects. Only use it
     if you know that you need to.

The class has one attribute:

ZoneInfo.key

   This is a read-only *attribute* that returns the value of "key"
   passed to the constructor, which should be a lookup key in the IANA
   time zone database (e.g. "America/New_York", "Europe/Paris" or
   "Asia/Tokyo").

   For zones constructed from file without specifying a "key"
   parameter, this will be set to "None".

   Note:

     Although it is a somewhat common practice to expose these to end
     users, these values are designed to be primary keys for
     representing the relevant zones and not necessarily user-facing
     elements.  Projects like CLDR (the Unicode Common Locale Data
     Repository) can be used to get more user-friendly strings from
     these keys.


String representations
----------------------

The string representation returned when calling "str" on a "ZoneInfo"
object defaults to using the "ZoneInfo.key" attribute (see the note on
usage in the attribute documentation):

   >>> zone = ZoneInfo("Pacific/Kwajalein")
   >>> str(zone)
   'Pacific/Kwajalein'

   >>> dt = datetime(2020, 4, 1, 3, 15, tzinfo=zone)
   >>> f"{dt.isoformat()} [{dt.tzinfo}]"
   '2020-04-01T03:15:00+12:00 [Pacific/Kwajalein]'

For objects constructed from a file without specifying a "key"
parameter, "str" falls back to calling "repr()". "ZoneInfo"’s "repr"
is implementation-defined and not necessarily stable between versions,
but it is guaranteed not to be a valid "ZoneInfo" key.


Pickle serialization
--------------------

Rather than serializing all transition data, "ZoneInfo" objects are
serialized by key, and "ZoneInfo" objects constructed from files (even
those with a value for "key" specified) cannot be pickled.

The behavior of a "ZoneInfo" file depends on how it was constructed:

1. "ZoneInfo(key)": When constructed with the primary constructor, a
   "ZoneInfo" object is serialized by key, and when deserialized, the
   deserializing process uses the primary and thus it is expected that
   these are expected to be the same object as other references to the
   same time zone.  For example, if "europe_berlin_pkl" is a string
   containing a pickle constructed from "ZoneInfo("Europe/Berlin")",
   one would expect the following behavior:

      >>> a = ZoneInfo("Europe/Berlin")
      >>> b = pickle.loads(europe_berlin_pkl)
      >>> a is b
      True

2. "ZoneInfo.no_cache(key)": When constructed from the cache-bypassing
   constructor, the "ZoneInfo" object is also serialized by key, but
   when deserialized, the deserializing process uses the cache
   bypassing constructor. If "europe_berlin_pkl_nc" is a string
   containing a pickle constructed from
   "ZoneInfo.no_cache("Europe/Berlin")", one would expect the
   following behavior:

      >>> a = ZoneInfo("Europe/Berlin")
      >>> b = pickle.loads(europe_berlin_pkl_nc)
      >>> a is b
      False

3. "ZoneInfo.from_file(fobj, /, key=None)": When constructed from a
   file, the "ZoneInfo" object raises an exception on pickling. If an
   end user wants to pickle a "ZoneInfo" constructed from a file, it
   is recommended that they use a wrapper type or a custom
   serialization function: either serializing by key or storing the
   contents of the file object and serializing that.

This method of serialization requires that the time zone data for the
required key be available on both the serializing and deserializing
side, similar to the way that references to classes and functions are
expected to exist in both the serializing and deserializing
environments. It also means that no guarantees are made about the
consistency of results when unpickling a "ZoneInfo" pickled in an
environment with a different version of the time zone data.


Functions
=========

zoneinfo.available_timezones()

   Get a set containing all the valid keys for IANA time zones
   available anywhere on the time zone path. This is recalculated on
   every call to the function.

   This function only includes canonical zone names and does not
   include “special” zones such as those under the "posix/" and
   "right/" directories, or the "posixrules" zone.

   Caution:

     This function may open a large number of files, as the best way
     to determine if a file on the time zone path is a valid time zone
     is to read the “magic string” at the beginning.

   Note:

     These values are not designed to be exposed to end-users; for
     user facing elements, applications should use something like CLDR
     (the Unicode Common Locale Data Repository) to get more user-
     friendly strings. See also the cautionary note on "ZoneInfo.key".

zoneinfo.reset_tzpath(to=None)

   Sets or resets the time zone search path ("TZPATH") for the module.
   When called with no arguments, "TZPATH" is set to the default
   value.

   Calling "reset_tzpath" will not invalidate the "ZoneInfo" cache,
   and so calls to the primary "ZoneInfo" constructor will only use
   the new "TZPATH" in the case of a cache miss.

   The "to" parameter must be a *sequence* of strings or "os.PathLike"
   and not a string, all of which must be absolute paths. "ValueError"
   will be raised if something other than an absolute path is passed.


Globals
=======

zoneinfo.TZPATH

   A read-only sequence representing the time zone search path – when
   constructing a "ZoneInfo" from a key, the key is joined to each
   entry in the "TZPATH", and the first file found is used.

   "TZPATH" may contain only absolute paths, never relative paths,
   regardless of how it is configured.

   The object that "zoneinfo.TZPATH" points to may change in response
   to a call to "reset_tzpath()", so it is recommended to use
   "zoneinfo.TZPATH" rather than importing "TZPATH" from "zoneinfo" or
   assigning a long-lived variable to "zoneinfo.TZPATH".

   For more information on configuring the time zone search path, see
   Configuring the data sources.


Exceptions and warnings
=======================

exception zoneinfo.ZoneInfoNotFoundError

   Raised when construction of a "ZoneInfo" object fails because the
   specified key could not be found on the system. This is a subclass
   of "KeyError".

exception zoneinfo.InvalidTZPathWarning

   Raised when "PYTHONTZPATH" contains an invalid component that will
   be filtered out, such as a relative path.
