570 lines
25 KiB
ReStructuredText
570 lines
25 KiB
ReStructuredText
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:mod:`time` --- Time access and conversions
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===========================================
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.. module:: time
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:synopsis: Time access and conversions.
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This module provides various time-related functions. For related
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functionality, see also the :mod:`datetime` and :mod:`calendar` modules.
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Although this module is always available,
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not all functions are available on all platforms. Most of the functions
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defined in this module call platform C library functions with the same name. It
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may sometimes be helpful to consult the platform documentation, because the
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semantics of these functions varies among platforms.
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An explanation of some terminology and conventions is in order.
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.. index:: single: epoch
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* The :dfn:`epoch` is the point where the time starts. On January 1st of that
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year, at 0 hours, the "time since the epoch" is zero. For Unix, the epoch is
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1970. To find out what the epoch is, look at ``gmtime(0)``.
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.. index:: single: Year 2038
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* The functions in this module do not handle dates and times before the epoch or
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far in the future. The cut-off point in the future is determined by the C
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library; for Unix, it is typically in 2038.
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.. index::
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single: Year 2000
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single: Y2K
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.. _time-y2kissues:
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* **Year 2000 (Y2K) issues**: Python depends on the platform's C library, which
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generally doesn't have year 2000 issues, since all dates and times are
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represented internally as seconds since the epoch. Functions accepting a
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:class:`struct_time` (see below) generally require a 4-digit year. For backward
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compatibility, 2-digit years are supported if the module variable
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``accept2dyear`` is a non-zero integer; this variable is initialized to ``1``
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unless the environment variable :envvar:`PYTHONY2K` is set to a non-empty
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string, in which case it is initialized to ``0``. Thus, you can set
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:envvar:`PYTHONY2K` to a non-empty string in the environment to require 4-digit
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years for all year input. When 2-digit years are accepted, they are converted
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according to the POSIX or X/Open standard: values 69-99 are mapped to 1969-1999,
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and values 0--68 are mapped to 2000--2068. Values 100--1899 are always illegal.
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Note that this is new as of Python 1.5.2(a2); earlier versions, up to Python
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1.5.1 and 1.5.2a1, would add 1900 to year values below 1900.
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.. index::
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single: UTC
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single: Coordinated Universal Time
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single: Greenwich Mean Time
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* UTC is Coordinated Universal Time (formerly known as Greenwich Mean Time, or
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GMT). The acronym UTC is not a mistake but a compromise between English and
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French.
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.. index:: single: Daylight Saving Time
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* DST is Daylight Saving Time, an adjustment of the timezone by (usually) one
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hour during part of the year. DST rules are magic (determined by local law) and
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can change from year to year. The C library has a table containing the local
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rules (often it is read from a system file for flexibility) and is the only
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source of True Wisdom in this respect.
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* The precision of the various real-time functions may be less than suggested by
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the units in which their value or argument is expressed. E.g. on most Unix
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systems, the clock "ticks" only 50 or 100 times a second.
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* On the other hand, the precision of :func:`.time` and :func:`sleep` is better
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than their Unix equivalents: times are expressed as floating point numbers,
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:func:`.time` returns the most accurate time available (using Unix
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:c:func:`gettimeofday` where available), and :func:`sleep` will accept a time
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with a nonzero fraction (Unix :c:func:`select` is used to implement this, where
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available).
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* The time value as returned by :func:`gmtime`, :func:`localtime`, and
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:func:`strptime`, and accepted by :func:`asctime`, :func:`mktime` and
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:func:`strftime`, may be considered as a sequence of 9 integers. The return
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values of :func:`gmtime`, :func:`localtime`, and :func:`strptime` also offer
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attribute names for individual fields.
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See :class:`struct_time` for a description of these objects.
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.. versionchanged:: 2.2
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The time value sequence was changed from a tuple to a :class:`struct_time`, with
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the addition of attribute names for the fields.
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* Use the following functions to convert between time representations:
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+-------------------------+-------------------------+-------------------------+
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| From | To | Use |
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+=========================+=========================+=========================+
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| seconds since the epoch | :class:`struct_time` in | :func:`gmtime` |
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| | UTC | |
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+-------------------------+-------------------------+-------------------------+
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| seconds since the epoch | :class:`struct_time` in | :func:`localtime` |
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| | local time | |
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+-------------------------+-------------------------+-------------------------+
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| :class:`struct_time` in | seconds since the epoch | :func:`calendar.timegm` |
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| UTC | | |
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+-------------------------+-------------------------+-------------------------+
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| :class:`struct_time` in | seconds since the epoch | :func:`mktime` |
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| local time | | |
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+-------------------------+-------------------------+-------------------------+
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The module defines the following functions and data items:
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.. data:: accept2dyear
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Boolean value indicating whether two-digit year values will be accepted. This
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is true by default, but will be set to false if the environment variable
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:envvar:`PYTHONY2K` has been set to a non-empty string. It may also be modified
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at run time.
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.. data:: altzone
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The offset of the local DST timezone, in seconds west of UTC, if one is defined.
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This is negative if the local DST timezone is east of UTC (as in Western Europe,
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including the UK). Only use this if ``daylight`` is nonzero.
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.. function:: asctime([t])
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Convert a tuple or :class:`struct_time` representing a time as returned by
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:func:`gmtime` or :func:`localtime` to a 24-character string of the following
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form: ``'Sun Jun 20 23:21:05 1993'``. If *t* is not provided, the current time
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as returned by :func:`localtime` is used. Locale information is not used by
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:func:`asctime`.
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.. note::
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Unlike the C function of the same name, there is no trailing newline.
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.. versionchanged:: 2.1
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Allowed *t* to be omitted.
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.. function:: clock()
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.. index::
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single: CPU time
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single: processor time
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single: benchmarking
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On Unix, return the current processor time as a floating point number expressed
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in seconds. The precision, and in fact the very definition of the meaning of
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"processor time", depends on that of the C function of the same name, but in any
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case, this is the function to use for benchmarking Python or timing algorithms.
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On Windows, this function returns wall-clock seconds elapsed since the first
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call to this function, as a floating point number, based on the Win32 function
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:c:func:`QueryPerformanceCounter`. The resolution is typically better than one
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microsecond.
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.. function:: ctime([secs])
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Convert a time expressed in seconds since the epoch to a string representing
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local time. If *secs* is not provided or :const:`None`, the current time as
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returned by :func:`.time` is used. ``ctime(secs)`` is equivalent to
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``asctime(localtime(secs))``. Locale information is not used by :func:`ctime`.
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.. versionchanged:: 2.1
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Allowed *secs* to be omitted.
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.. versionchanged:: 2.4
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If *secs* is :const:`None`, the current time is used.
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.. data:: daylight
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Nonzero if a DST timezone is defined.
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.. function:: gmtime([secs])
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Convert a time expressed in seconds since the epoch to a :class:`struct_time` in
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UTC in which the dst flag is always zero. If *secs* is not provided or
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:const:`None`, the current time as returned by :func:`.time` is used. Fractions
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of a second are ignored. See above for a description of the
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:class:`struct_time` object. See :func:`calendar.timegm` for the inverse of this
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function.
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.. versionchanged:: 2.1
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Allowed *secs* to be omitted.
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.. versionchanged:: 2.4
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If *secs* is :const:`None`, the current time is used.
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.. function:: localtime([secs])
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Like :func:`gmtime` but converts to local time. If *secs* is not provided or
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:const:`None`, the current time as returned by :func:`.time` is used. The dst
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flag is set to ``1`` when DST applies to the given time.
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.. versionchanged:: 2.1
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Allowed *secs* to be omitted.
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.. versionchanged:: 2.4
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If *secs* is :const:`None`, the current time is used.
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.. function:: mktime(t)
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This is the inverse function of :func:`localtime`. Its argument is the
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:class:`struct_time` or full 9-tuple (since the dst flag is needed; use ``-1``
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as the dst flag if it is unknown) which expresses the time in *local* time, not
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UTC. It returns a floating point number, for compatibility with :func:`.time`.
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If the input value cannot be represented as a valid time, either
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:exc:`OverflowError` or :exc:`ValueError` will be raised (which depends on
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whether the invalid value is caught by Python or the underlying C libraries).
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The earliest date for which it can generate a time is platform-dependent.
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.. function:: sleep(secs)
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Suspend execution for the given number of seconds. The argument may be a
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floating point number to indicate a more precise sleep time. The actual
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suspension time may be less than that requested because any caught signal will
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terminate the :func:`sleep` following execution of that signal's catching
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routine. Also, the suspension time may be longer than requested by an arbitrary
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amount because of the scheduling of other activity in the system.
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.. function:: strftime(format[, t])
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Convert a tuple or :class:`struct_time` representing a time as returned by
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:func:`gmtime` or :func:`localtime` to a string as specified by the *format*
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argument. If *t* is not provided, the current time as returned by
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:func:`localtime` is used. *format* must be a string. :exc:`ValueError` is
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raised if any field in *t* is outside of the allowed range. :func:`strftime`
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returns a locale depedent byte string; the result may be converted to unicode
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by doing ``strftime(<myformat>).decode(locale.getlocale()[1])``.
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.. versionchanged:: 2.1
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Allowed *t* to be omitted.
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.. versionchanged:: 2.4
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:exc:`ValueError` raised if a field in *t* is out of range.
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.. versionchanged:: 2.5
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0 is now a legal argument for any position in the time tuple; if it is normally
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illegal the value is forced to a correct one.
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The following directives can be embedded in the *format* string. They are shown
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without the optional field width and precision specification, and are replaced
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by the indicated characters in the :func:`strftime` result:
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+-----------+--------------------------------+-------+
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| Directive | Meaning | Notes |
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+===========+================================+=======+
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| ``%a`` | Locale's abbreviated weekday | |
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| | name. | |
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+-----------+--------------------------------+-------+
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| ``%A`` | Locale's full weekday name. | |
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+-----------+--------------------------------+-------+
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| ``%b`` | Locale's abbreviated month | |
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| | name. | |
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+-----------+--------------------------------+-------+
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| ``%B`` | Locale's full month name. | |
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+-----------+--------------------------------+-------+
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| ``%c`` | Locale's appropriate date and | |
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| | time representation. | |
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+-----------+--------------------------------+-------+
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| ``%d`` | Day of the month as a decimal | |
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| | number [01,31]. | |
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+-----------+--------------------------------+-------+
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| ``%H`` | Hour (24-hour clock) as a | |
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| | decimal number [00,23]. | |
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+-----------+--------------------------------+-------+
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| ``%I`` | Hour (12-hour clock) as a | |
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| | decimal number [01,12]. | |
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+-----------+--------------------------------+-------+
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| ``%j`` | Day of the year as a decimal | |
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| | number [001,366]. | |
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+-----------+--------------------------------+-------+
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| ``%m`` | Month as a decimal number | |
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| | [01,12]. | |
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+-----------+--------------------------------+-------+
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| ``%M`` | Minute as a decimal number | |
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| | [00,59]. | |
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+-----------+--------------------------------+-------+
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| ``%p`` | Locale's equivalent of either | \(1) |
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| | AM or PM. | |
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+-----------+--------------------------------+-------+
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| ``%S`` | Second as a decimal number | \(2) |
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| | [00,61]. | |
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+-----------+--------------------------------+-------+
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| ``%U`` | Week number of the year | \(3) |
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| | (Sunday as the first day of | |
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| | the week) as a decimal number | |
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| | [00,53]. All days in a new | |
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| | year preceding the first | |
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| | Sunday are considered to be in | |
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| | week 0. | |
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+-----------+--------------------------------+-------+
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| ``%w`` | Weekday as a decimal number | |
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| | [0(Sunday),6]. | |
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+-----------+--------------------------------+-------+
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| ``%W`` | Week number of the year | \(3) |
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| | (Monday as the first day of | |
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| | the week) as a decimal number | |
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| | [00,53]. All days in a new | |
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| | year preceding the first | |
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| | Monday are considered to be in | |
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| | week 0. | |
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+-----------+--------------------------------+-------+
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| ``%x`` | Locale's appropriate date | |
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| | representation. | |
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+-----------+--------------------------------+-------+
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| ``%X`` | Locale's appropriate time | |
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| | representation. | |
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+-----------+--------------------------------+-------+
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| ``%y`` | Year without century as a | |
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| | decimal number [00,99]. | |
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+-----------+--------------------------------+-------+
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| ``%Y`` | Year with century as a decimal | |
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| | number. | |
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+-----------+--------------------------------+-------+
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| ``%Z`` | Time zone name (no characters | |
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| | if no time zone exists). | |
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+-----------+--------------------------------+-------+
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| ``%%`` | A literal ``'%'`` character. | |
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+-----------+--------------------------------+-------+
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Notes:
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(1)
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When used with the :func:`strptime` function, the ``%p`` directive only affects
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the output hour field if the ``%I`` directive is used to parse the hour.
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(2)
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The range really is ``0`` to ``61``; this accounts for leap seconds and the
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(very rare) double leap seconds.
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(3)
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When used with the :func:`strptime` function, ``%U`` and ``%W`` are only used in
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calculations when the day of the week and the year are specified.
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Here is an example, a format for dates compatible with that specified in the
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:rfc:`2822` Internet email standard. [#]_ ::
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>>> from time import gmtime, strftime
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>>> strftime("%a, %d %b %Y %H:%M:%S +0000", gmtime())
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'Thu, 28 Jun 2001 14:17:15 +0000'
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Additional directives may be supported on certain platforms, but only the
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ones listed here have a meaning standardized by ANSI C. To see the full set
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of format codes supported on your platform, consult the :manpage:`strftime(3)`
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documentation.
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On some platforms, an optional field width and precision specification can
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immediately follow the initial ``'%'`` of a directive in the following order;
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this is also not portable. The field width is normally 2 except for ``%j`` where
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it is 3.
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.. function:: strptime(string[, format])
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Parse a string representing a time according to a format. The return value is
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a :class:`struct_time` as returned by :func:`gmtime` or :func:`localtime`.
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The *format* parameter uses the same directives as those used by
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:func:`strftime`; it defaults to ``"%a %b %d %H:%M:%S %Y"`` which matches the
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formatting returned by :func:`ctime`. If *string* cannot be parsed according to
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*format*, or if it has excess data after parsing, :exc:`ValueError` is raised.
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The default values used to fill in any missing data when more accurate values
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cannot be inferred are ``(1900, 1, 1, 0, 0, 0, 0, 1, -1)``.
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For example:
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>>> import time
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>>> time.strptime("30 Nov 00", "%d %b %y") # doctest: +NORMALIZE_WHITESPACE
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time.struct_time(tm_year=2000, tm_mon=11, tm_mday=30, tm_hour=0, tm_min=0,
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tm_sec=0, tm_wday=3, tm_yday=335, tm_isdst=-1)
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Support for the ``%Z`` directive is based on the values contained in ``tzname``
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and whether ``daylight`` is true. Because of this, it is platform-specific
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except for recognizing UTC and GMT which are always known (and are considered to
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be non-daylight savings timezones).
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Only the directives specified in the documentation are supported. Because
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``strftime()`` is implemented per platform it can sometimes offer more
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directives than those listed. But ``strptime()`` is independent of any platform
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and thus does not necessarily support all directives available that are not
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documented as supported.
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.. class:: struct_time
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The type of the time value sequence returned by :func:`gmtime`,
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:func:`localtime`, and :func:`strptime`. It is an object with a :term:`named
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tuple` interface: values can be accessed by index and by attribute name. The
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following values are present:
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+-------+-------------------+---------------------------------+
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| Index | Attribute | Values |
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+=======+===================+=================================+
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| 0 | :attr:`tm_year` | (for example, 1993) |
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+-------+-------------------+---------------------------------+
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| 1 | :attr:`tm_mon` | range [1, 12] |
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+-------+-------------------+---------------------------------+
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| 2 | :attr:`tm_mday` | range [1, 31] |
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+-------+-------------------+---------------------------------+
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| 3 | :attr:`tm_hour` | range [0, 23] |
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+-------+-------------------+---------------------------------+
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| 4 | :attr:`tm_min` | range [0, 59] |
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+-------+-------------------+---------------------------------+
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| 5 | :attr:`tm_sec` | range [0, 61]; see **(2)** in |
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| | | :func:`strftime` description |
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+-------+-------------------+---------------------------------+
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| 6 | :attr:`tm_wday` | range [0, 6], Monday is 0 |
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+-------+-------------------+---------------------------------+
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| 7 | :attr:`tm_yday` | range [1, 366] |
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+-------+-------------------+---------------------------------+
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| 8 | :attr:`tm_isdst` | 0, 1 or -1; see below |
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+-------+-------------------+---------------------------------+
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.. versionadded:: 2.2
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Note that unlike the C structure, the month value is a range of [1, 12], not
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[0, 11]. A year value will be handled as described under :ref:`Year 2000
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(Y2K) issues <time-y2kissues>` above. A ``-1`` argument as the daylight
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savings flag, passed to :func:`mktime` will usually result in the correct
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daylight savings state to be filled in.
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When a tuple with an incorrect length is passed to a function expecting a
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:class:`struct_time`, or having elements of the wrong type, a
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:exc:`TypeError` is raised.
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.. function:: time()
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Return the time in seconds since the epoch as a floating point number.
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Note that even though the time is always returned as a floating point
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number, not all systems provide time with a better precision than 1 second.
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While this function normally returns non-decreasing values, it can return a
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lower value than a previous call if the system clock has been set back between
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the two calls.
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.. data:: timezone
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The offset of the local (non-DST) timezone, in seconds west of UTC (negative in
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most of Western Europe, positive in the US, zero in the UK).
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.. data:: tzname
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A tuple of two strings: the first is the name of the local non-DST timezone, the
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second is the name of the local DST timezone. If no DST timezone is defined,
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the second string should not be used.
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.. function:: tzset()
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Resets the time conversion rules used by the library routines. The environment
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variable :envvar:`TZ` specifies how this is done.
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.. versionadded:: 2.3
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Availability: Unix.
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.. note::
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Although in many cases, changing the :envvar:`TZ` environment variable may
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affect the output of functions like :func:`localtime` without calling
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:func:`tzset`, this behavior should not be relied on.
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The :envvar:`TZ` environment variable should contain no whitespace.
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The standard format of the :envvar:`TZ` environment variable is (whitespace
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added for clarity)::
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std offset [dst [offset [,start[/time], end[/time]]]]
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Where the components are:
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``std`` and ``dst``
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Three or more alphanumerics giving the timezone abbreviations. These will be
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propagated into time.tzname
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``offset``
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The offset has the form: ``± hh[:mm[:ss]]``. This indicates the value
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added the local time to arrive at UTC. If preceded by a '-', the timezone
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is east of the Prime Meridian; otherwise, it is west. If no offset follows
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dst, summer time is assumed to be one hour ahead of standard time.
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``start[/time], end[/time]``
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Indicates when to change to and back from DST. The format of the
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start and end dates are one of the following:
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:samp:`J{n}`
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The Julian day *n* (1 <= *n* <= 365). Leap days are not counted, so in
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all years February 28 is day 59 and March 1 is day 60.
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:samp:`{n}`
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The zero-based Julian day (0 <= *n* <= 365). Leap days are counted, and
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it is possible to refer to February 29.
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:samp:`M{m}.{n}.{d}`
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The *d*'th day (0 <= *d* <= 6) or week *n* of month *m* of the year (1
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<= *n* <= 5, 1 <= *m* <= 12, where week 5 means "the last *d* day in
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month *m*" which may occur in either the fourth or the fifth
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week). Week 1 is the first week in which the *d*'th day occurs. Day
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zero is Sunday.
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``time`` has the same format as ``offset`` except that no leading sign
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('-' or '+') is allowed. The default, if time is not given, is 02:00:00.
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::
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>>> os.environ['TZ'] = 'EST+05EDT,M4.1.0,M10.5.0'
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>>> time.tzset()
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>>> time.strftime('%X %x %Z')
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'02:07:36 05/08/03 EDT'
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>>> os.environ['TZ'] = 'AEST-10AEDT-11,M10.5.0,M3.5.0'
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>>> time.tzset()
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>>> time.strftime('%X %x %Z')
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'16:08:12 05/08/03 AEST'
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On many Unix systems (including \*BSD, Linux, Solaris, and Darwin), it is more
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convenient to use the system's zoneinfo (:manpage:`tzfile(5)`) database to
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specify the timezone rules. To do this, set the :envvar:`TZ` environment
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variable to the path of the required timezone datafile, relative to the root of
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the systems 'zoneinfo' timezone database, usually located at
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:file:`/usr/share/zoneinfo`. For example, ``'US/Eastern'``,
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``'Australia/Melbourne'``, ``'Egypt'`` or ``'Europe/Amsterdam'``. ::
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>>> os.environ['TZ'] = 'US/Eastern'
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>>> time.tzset()
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>>> time.tzname
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('EST', 'EDT')
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>>> os.environ['TZ'] = 'Egypt'
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>>> time.tzset()
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>>> time.tzname
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('EET', 'EEST')
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.. seealso::
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Module :mod:`datetime`
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More object-oriented interface to dates and times.
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Module :mod:`locale`
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Internationalization services. The locale setting affects the interpretation
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of many format specifiers in :func:`strftime` and :func:`strptime`.
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Module :mod:`calendar`
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General calendar-related functions. :func:`~calendar.timegm` is the
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inverse of :func:`gmtime` from this module.
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.. rubric:: Footnotes
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.. [#] The use of ``%Z`` is now deprecated, but the ``%z`` escape that expands to the
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preferred hour/minute offset is not supported by all ANSI C libraries. Also, a
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strict reading of the original 1982 :rfc:`822` standard calls for a two-digit
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year (%y rather than %Y), but practice moved to 4-digit years long before the
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year 2000. After that, :rfc:`822` became obsolete and the 4-digit year has
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been first recommended by :rfc:`1123` and then mandated by :rfc:`2822`.
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