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