mirror of https://github.com/python/cpython
313 lines
14 KiB
TeX
313 lines
14 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.
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It is always available, but not all functions are available
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on all 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 time tuple (see below)
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generally require a 4-digit year. For backward compatibility, 2-digit
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years are supported if the module variable \code{accept2dyear} is a
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non-zero integer; this variable is initialized to \code{1} unless the
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environment variable \envvar{PYTHONY2K} is set to a non-empty string,
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in which case it is 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 tuple 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 tuple of 9 integers:
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\begin{tableiii}{r|l|l}{textrm}{Index}{Field}{Values}
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\lineiii{0}{year}{(for example, 1993)}
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\lineiii{1}{month}{range [1,12]}
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\lineiii{2}{day}{range [1,31]}
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\lineiii{3}{hour}{range [0,23]}
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\lineiii{4}{minute}{range [0,59]}
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\lineiii{5}{second}{range [0,61]; see \strong{(1)} in \function{strftime()} description}
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\lineiii{6}{weekday}{range [0,6], Monday is 0}
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\lineiii{7}{Julian day}{range [1,366]}
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\lineiii{8}{daylight savings flag}{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
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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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\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{tuple}}
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Convert a tuple representing a time as returned 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{tuple} is not provided, the
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current time as returned by \function{localtime()} is used. Note: unlike
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the C function of the same name, there is no trailing newline.
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\versionchanged[Allowed \var{tuple} 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 the nearest approximation to
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wall-clock time since the first call to this function, based on the
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Win32 function \cfunction{QueryPerformanceCounter()}. The resolution
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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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\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 time tuple
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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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tuple lay-out.
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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}{tuple}
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This is the inverse function of \function{localtime()}. Its argument
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is the full 9-tuple (since the dst flag is needed; use \code{-1} as
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the dst flag if it is unknown) which 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, \exception{OverflowError} is raised.
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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{, tuple}}
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Convert a tuple representing a time as returned by \function{gmtime()}
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or \function{localtime()} to a string as specified by the \var{format}
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argument. If \var{tuple} is not provided, the current time as returned by
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\function{localtime()} is used. \var{format} must be a string.
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\versionchanged[Allowed \var{tuple} 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 Sunday 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 (or by 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 tuple 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()}. The same platform caveats apply; see
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the local \UNIX{} documentation for restrictions or additional
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supported directives. If \var{string} cannot be parsed according to
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\var{format}, \exception{ValueError} is raised. Values which are not
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provided as part of the input string are filled in with default
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values; the specific values are platform-dependent as the XPG standard
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does not provide sufficient information to constrain the result.
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\strong{Note:} This function relies entirely on the underlying
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platform's C library for the date parsing, and some of these libraries
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are buggy. There's nothing to be done about this short of a new,
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portable implementation of \cfunction{strptime()}.
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Availability: Most modern \UNIX{} systems.
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\end{funcdesc}
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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{seealso}
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\seemodule{locale}{Internationalization services. The locale
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settings can affect the return values for some of
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the functions in the \module{time} module.}
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\end{seealso}
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