Doc/lib/libtime.tex

   1 \section{\module{time} ---
   2          Time access and conversions.}
   3 \declaremodule{builtin}{time}
   4
   5 \modulesynopsis{Time access and conversions.}
   6
   7
   8 This module provides various time-related functions.
   9 It is always available.
  10
  11 An explanation of some terminology and conventions is in order.
  12
  13 \begin{itemize}
  14
  15 \item
  16 The \dfn{epoch}\index{epoch} is the point where the time starts.  On
  17 January 1st of that year, at 0 hours, the ``time since the epoch'' is
  18 zero.  For \UNIX{}, the epoch is 1970.  To find out what the epoch is,
  19 look at \code{gmtime(0)}.
  20
  21 \item
  22 The functions in this module do not handle dates and times before the
  23 epoch or far in the future.  The cut-off point in the future is
  24 determined by the C library; for \UNIX{}, it is typically in
  25 2038\index{Year 2038}.
  26
  27 \item
  28 \strong{Year 2000 (Y2K) issues}:\index{Year 2000}\index{Y2K}  Python
  29 depends on the platform's C library, which generally doesn't have year
  30 2000 issues, since all dates and times are represented internally as
  31 seconds since the epoch.  Functions accepting a time tuple (see below)
  32 generally require a 4-digit year.  For backward compatibility, 2-digit
  33 years are supported if the module variable \code{accept2dyear} is a
  34 non-zero integer; this variable is initialized to \code{1} unless the
  35 environment variable \envvar{PYTHONY2K} is set to a non-empty string,
  36 in which case it is initialized to \code{0}.  Thus, you can set
  37 \envvar{PYTHONY2K} to a non-empty string in the environment to require 4-digit
  38 years for all year input.  When 2-digit years are accepted, they are
  39 converted according to the \POSIX{} or X/Open standard: values 69-99
  40 are mapped to 1969-1999, and values 0--68 are mapped to 2000--2068.
  41 Values 100--1899 are always illegal.  Note that this is new as of
  42 Python 1.5.2(a2); earlier versions, up to Python 1.5.1 and 1.5.2a1,
  43 would add 1900 to year values below 1900.
  44
  45 \item
  46 UTC\index{UTC} is Coordinated Universal Time\index{Coordinated
  47 Universal Time} (formerly known as Greenwich Mean
  48 Time,\index{Greenwich Mean Time} or GMT).  The acronym UTC is not a
  49 mistake but a compromise between English and French.
  50
  51 \item
  52 DST is Daylight Saving Time,\index{Daylight Saving Time} an adjustment
  53 of the timezone by (usually) one hour during part of the year.  DST
  54 rules are magic (determined by local law) and can change from year to
  55 year.  The C library has a table containing the local rules (often it
  56 is read from a system file for flexibility) and is the only source of
  57 True Wisdom in this respect.
  58
  59 \item
  60 The precision of the various real-time functions may be less than
  61 suggested by the units in which their value or argument is expressed.
  62 E.g.\ on most \UNIX{} systems, the clock ``ticks'' only 50 or 100 times a
  63 second, and on the Mac, times are only accurate to whole seconds.
  64
  65 \item
  66 On the other hand, the precision of \function{time()} and
  67 \function{sleep()} is better than their \UNIX{} equivalents: times are
  68 expressed as floating point numbers, \function{time()} returns the
  69 most accurate time available (using \UNIX{} \cfunction{gettimeofday()}
  70 where available), and \function{sleep()} will accept a time with a
  71 nonzero fraction (\UNIX{} \cfunction{select()} is used to implement
  72 this, where available).
  73
  74 \item
  75 The time tuple as returned by \function{gmtime()},
  76 \function{localtime()}, and \function{strptime()}, and accepted by
  77 \function{asctime()}, \function{mktime()} and \function{strftime()},
  78 is a tuple of 9 integers: year (e.g.\ 1993), month (1--12), day
  79 (1--31), hour (0--23), minute (0--59), second (0--59), weekday (0--6,
  80 monday is 0), Julian day (1--366) and daylight savings flag (-1, 0 or
  81 1).  Note that unlike the C structure, the month value is a range
  82 of 1-12, not 0-11.  A year value will be handled as descibed under
  83 ``Year 2000 (Y2K) issues'' above.  A \code{-1} argument as daylight
  84 savings flag, passed to \function{mktime()} will usually result in the
  85 correct daylight savings state to be filled in.
  86
  87 \end{itemize}
  88
  89 The module defines the following functions and data items:
  90
  91
  92 \begin{datadesc}{accept2dyear}
  93 Boolean value indicating whether two-digit year values will be
  94 accepted.  This is true by default, but will be set to false if the
  95 environment variable \envvar{PYTHONY2K} has been set to a non-empty
  96 string.  It may also be modified at run time.
  97 \end{datadesc}
  98
  99 \begin{datadesc}{altzone}
 100 The offset of the local DST timezone, in seconds west of the 0th
 101 meridian, if one is defined.  Negative if the local DST timezone is
 102 east of the 0th meridian (as in Western Europe, including the UK).
 103 Only use this if \code{daylight} is nonzero.
 104 \end{datadesc}
 105
 106 \begin{funcdesc}{asctime}{tuple}
 107 Convert a tuple representing a time as returned by \function{gmtime()}
 108 or \function{localtime()} to a 24-character string of the following form:
 109 \code{'Sun Jun 20 23:21:05 1993'}.  Note: unlike the C function of
 110 the same name, there is no trailing newline.
 111 \end{funcdesc}
 112
 113 \begin{funcdesc}{clock}{}
 114 Return the current CPU time as a floating point number expressed in
 115 seconds.  The precision, and in fact the very definiton of the meaning
 116 of ``CPU time''\index{CPU time}, depends on that of the C function
 117 of the same name, but in any case, this is the function to use for
 118 benchmarking\index{benchmarking} Python or timing algorithms.
 119 \end{funcdesc}
 120
 121 \begin{funcdesc}{ctime}{secs}
 122 Convert a time expressed in seconds since the epoch to a string
 123 representing local time.  \code{ctime(\var{secs})} is equivalent to
 124 \code{asctime(localtime(\var{secs}))}.
 125 \end{funcdesc}
 126
 127 \begin{datadesc}{daylight}
 128 Nonzero if a DST timezone is defined.
 129 \end{datadesc}
 130
 131 \begin{funcdesc}{gmtime}{secs}
 132 Convert a time expressed in seconds since the epoch to a time tuple
 133 in UTC in which the dst flag is always zero.  Fractions of a second are
 134 ignored.  See above for a description of the tuple lay-out.
 135 \end{funcdesc}
 136
 137 \begin{funcdesc}{localtime}{secs}
 138 Like \function{gmtime()} but converts to local time.  The dst flag is
 139 set to \code{1} when DST applies to the given time.
 140 \end{funcdesc}
 141
 142 \begin{funcdesc}{mktime}{tuple}
 143 This is the inverse function of \function{localtime()}.  Its argument
 144 is the full 9-tuple (since the dst flag is needed --- pass \code{-1}
 145 as the dst flag if it is unknown) which expresses the time in
 146 \emph{local} time, not UTC.  It returns a floating point number, for
 147 compatibility with \function{time()}.  If the input value cannot be
 148 represented as a valid time, \exception{OverflowError} is raised.
 149 \end{funcdesc}
 150
 151 \begin{funcdesc}{sleep}{secs}
 152 Suspend execution for the given number of seconds.  The argument may
 153 be a floating point number to indicate a more precise sleep time.
 154 The actual suspension time may be less than that requested because any
 155 caught signal will terminate the \function{sleep()} following
 156 execution of that signal's catching routine.  Also, the suspension
 157 time may be longer than requested by an arbitrary amount because of
 158 the scheduling of other activity in the system.
 159 \end{funcdesc}
 160
 161 \begin{funcdesc}{strftime}{format, tuple}
 162 Convert a tuple representing a time as returned by \function{gmtime()}
 163 or \function{localtime()} to a string as specified by the \var{format}
 164 argument.  \var{format} must be a string.
 165
 166 The following directives can be embedded in the \var{format} string.
 167 They are shown without the optional field width and precision
 168 specification, and are replaced by the indicated characters in the
 169 \function{strftime()} result:
 170
 171 \begin{tableii}{c|p{24em}}{code}{Directive}{Meaning}
 172   \lineii{\%a}{Locale's abbreviated weekday name.}
 173   \lineii{\%A}{Locale's full weekday name.}
 174   \lineii{\%b}{Locale's abbreviated month name.}
 175   \lineii{\%B}{Locale's full month name.}
 176   \lineii{\%c}{Locale's appropriate date and time representation.}
 177   \lineii{\%d}{Day of the month as a decimal number [01,31].}
 178   \lineii{\%H}{Hour (24-hour clock) as a decimal number [00,23].}
 179   \lineii{\%I}{Hour (12-hour clock) as a decimal number [01,12].}
 180   \lineii{\%j}{Day of the year as a decimal number [001,366].}
 181   \lineii{\%m}{Month as a decimal number [01,12].}
 182   \lineii{\%M}{Minute as a decimal number [00,59].}
 183   \lineii{\%p}{Locale's equivalent of either AM or PM.}
 184   \lineii{\%S}{Second as a decimal number [00,61].}
 185   \lineii{\%U}{Week number of the year (Sunday as the first day of the
 186                week) as a decimal number [00,53].  All days in a new year
 187                preceding the first Sunday are considered to be in week 0.}
 188   \lineii{\%w}{Weekday as a decimal number [0(Sunday),6].}
 189   \lineii{\%W}{Week number of the year (Monday as the first day of the
 190                week) as a decimal number [00,53].  All days in a new year
 191                preceding the first Sunday are considered to be in week 0.}
 192   \lineii{\%x}{Locale's appropriate date representation.}
 193   \lineii{\%X}{Locale's appropriate time representation.}
 194   \lineii{\%y}{Year without century as a decimal number [00,99].}
 195   \lineii{\%Y}{Year with century as a decimal number.}
 196   \lineii{\%Z}{Time zone name (or by no characters if no time zone exists).}
 197   \lineii{\%\%}{\%}
 198 \end{tableii}
 199
 200 Additional directives may be supported on certain platforms, but
 201 only the ones listed here have a meaning standardized by ANSI C.
 202
 203 On some platforms, an optional field width and precision
 204 specification can immediately follow the initial \character{\%} of a
 205 directive in the following order; this is also not portable.
 206 The field width is normally 2 except for \code{\%j} where it is 3.
 207 \end{funcdesc}
 208
 209 \begin{funcdesc}{strptime}{string\optional{, format}}
 210 Parse a string representing a time according to a format.  The return
 211 value is a tuple as returned by \function{gmtime()} or
 212 \function{localtime()}.  The \var{format} parameter uses the same
 213 directives as those used by \function{strftime()}; it defaults to
 214 \code{"\%a \%b \%d \%H:\%M:\%S \%Y"} which matches the formatting
 215 returned by \function{ctime()}.  The same platform caveats apply; see
 216 the local \UNIX{} documentation for restrictions or additional
 217 supported directives.  If \var{string} cannot be parsed according to
 218 \var{format}, \exception{ValueError} is raised.  This function may not
 219 be defined on all platforms.
 220 \end{funcdesc}
 221
 222 \begin{funcdesc}{time}{}
 223 Return the time as a floating point number expressed in seconds since
 224 the epoch, in UTC.  Note that even though the time is always returned
 225 as a floating point number, not all systems provide time with a better
 226 precision than 1 second.
 227 \end{funcdesc}
 228
 229 \begin{datadesc}{timezone}
 230 The offset of the local (non-DST) timezone, in seconds west of the 0th
 231 meridian (i.e. negative in most of Western Europe, positive in the US,
 232 zero in the UK).
 233 \end{datadesc}
 234
 235 \begin{datadesc}{tzname}
 236 A tuple of two strings: the first is the name of the local non-DST
 237 timezone, the second is the name of the local DST timezone.  If no DST
 238 timezone is defined, the second string should not be used.
 239 \end{datadesc}
 240