185 lines
7.4 KiB
TeX
185 lines
7.4 KiB
TeX
\section{Built-in Module \sectcode{time}}
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\label{module-time}
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\bimodindex{time}
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This module provides various time-related functions.
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It is always available.
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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 ``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
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epoch is 1970. To find out what the epoch is, look at \code{gmtime(0)}.
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\item
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UTC is Coordinated Universal Time (formerly known as Greenwich Mean
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Time). The acronym UTC is not a mistake but a compromise between
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English and French.
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\item
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DST is Daylight Saving Time, an adjustment of the timezone by
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(usually) one hour during part of the year. DST rules are magic
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(determined by local law) and can change from year to year. The C
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library has a table containing the local rules (often it is read from
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a system file for flexibility) and is the only source of True Wisdom
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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 \code{time()} and \code{sleep()}
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is better than their \UNIX{} equivalents: times are expressed as floating
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point numbers, \code{time()} returns the most accurate time available
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(using \UNIX{} \code{gettimeofday()} where available), and \code{sleep()}
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will accept a time with a nonzero fraction (\UNIX{} \code{select()} is
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used to implement this, where available).
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\item
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The time tuple as returned by \code{gmtime()} and \code{localtime()},
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or as accpted by \code{mktime()} is a tuple of 9
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integers: year (e.g.\ 1993), month (1--12), day (1--31), hour
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(0--23), minute (0--59), second (0--59), weekday (0--6, monday is 0),
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Julian day (1--366) and daylight savings flag (-1, 0 or 1).
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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 of $<$ 100 will typically be silently converted to
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1900 $+$ year value. A -1 argument as daylight savings flag, passed to
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\code{mktime()} will usually result in the correct daylight savings
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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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\renewcommand{\indexsubitem}{(in module time)}
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\begin{datadesc}{altzone}
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The offset of the local DST timezone, in seconds west of the 0th
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meridian, if one is defined. Negative if the local DST timezone is
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east of the 0th meridian (as in Western Europe, including the UK).
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Only use this if \code{daylight} is nonzero.
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\end{datadesc}
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\begin{funcdesc}{asctime}{tuple}
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Convert a tuple representing a time as returned by \code{gmtime()} or
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\code{localtime()} to a 24-character string of the following form:
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\code{'Sun Jun 20 23:21:05 1993'}. Note: unlike the C function of
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the same name, there is no trailing newline.
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\end{funcdesc}
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\begin{funcdesc}{clock}{}
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Return the current CPU time as a floating point number expressed in
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seconds. The precision, and in fact the very definiton of the meaning
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of ``CPU time'', depends on that of the C function of the same name,
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but in any case, this is the function to use for benchmarking Python
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or timing algorithms.
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\end{funcdesc}
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\begin{funcdesc}{ctime}{secs}
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Convert a time expressed in seconds since the epoch to a string
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representing local time. \code{ctime(t)} is equivalent to
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\code{asctime(localtime(t))}.
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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}{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. Fractions of a second are
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ignored.
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\end{funcdesc}
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\begin{funcdesc}{localtime}{secs}
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Like \code{gmtime} but converts to local time. The dst flag is set
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to 1 when DST applies to the given time.
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\end{funcdesc}
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\begin{funcdesc}{mktime}{tuple}
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This is the inverse function of \code{localtime}. Its argument is the
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full 9-tuple (since the dst flag is needed --- pass -1 as the dst flag if
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it is unknown) which expresses the time
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in \emph{local} time, not UTC. It returns a floating
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point number, for compatibility with \code{time.time()}. If the input
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value can't be represented as a valid time, 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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\end{funcdesc}
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\begin{funcdesc}{strftime}{format, tuple}
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Convert a tuple representing a time as returned by \code{gmtime()} or
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\code{localtime()} to a string as specified by the format argument.
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The following directives, shown without the optional field width and
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precision specification, are replaced by the indicated characters:
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\begin{tableii}{|c|p{24em}|}{code}{Directive}{Meaning}
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\lineii{\%a}{Locale's abbreviated weekday name.}
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\lineii{\%A}{Locale's full weekday name.}
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\lineii{\%b}{Locale's abbreviated month name.}
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\lineii{\%B}{Locale's full month name.}
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\lineii{\%c}{Locale's appropriate date and time representation.}
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\lineii{\%d}{Day of the month as a decimal number [01,31].}
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\lineii{\%H}{Hour (24-hour clock) as a decimal number [00,23].}
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\lineii{\%I}{Hour (12-hour clock) as a decimal number [01,12].}
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\lineii{\%j}{Day of the year as a decimal number [001,366].}
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\lineii{\%m}{Month as a decimal number [01,12].}
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\lineii{\%M}{Minute as a decimal number [00,59].}
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\lineii{\%p}{Locale's equivalent of either AM or PM.}
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\lineii{\%S}{Second as a decimal number [00,61].}
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\lineii{\%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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\lineii{\%w}{Weekday as a decimal number [0(Sunday),6].}
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\lineii{\%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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\lineii{\%x}{Locale's appropriate date representation.}
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\lineii{\%X}{Locale's appropriate time representation.}
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\lineii{\%y}{Year without century as a decimal number [00,99].}
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\lineii{\%Y}{Year with century as a decimal number.}
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\lineii{\%Z}{Time zone name (or by no characters if no time zone exists).}
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\lineii{\%\%}{\%}
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\end{tableii}
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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 \% 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 \%j where it is 3.
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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.
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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 the 0th
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meridian (i.e. negative in most of Western Europe, positive in the US,
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zero in the 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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