/Doc/library/time.rst
ReStructuredText | 558 lines | 424 code | 134 blank | 0 comment | 0 complexity | f6fc870e78c81b541e8eb80693ac9813 MD5 | raw file
1 2:mod:`time` --- Time access and conversions 3=========================================== 4 5.. module:: time 6 :synopsis: Time access and conversions. 7 8 9This module provides various time-related functions. For related 10functionality, see also the :mod:`datetime` and :mod:`calendar` modules. 11 12Although this module is always available, 13not all functions are available on all platforms. Most of the functions 14defined in this module call platform C library functions with the same name. It 15may sometimes be helpful to consult the platform documentation, because the 16semantics of these functions varies among platforms. 17 18An explanation of some terminology and conventions is in order. 19 20 .. index:: single: epoch 21 22* The :dfn:`epoch` is the point where the time starts. On January 1st of that 23 year, at 0 hours, the "time since the epoch" is zero. For Unix, the epoch is 24 1970. To find out what the epoch is, look at ``gmtime(0)``. 25 26 .. index:: single: Year 2038 27 28* The functions in this module do not handle dates and times before the epoch or 29 far in the future. The cut-off point in the future is determined by the C 30 library; for Unix, it is typically in 2038. 31 32 .. index:: 33 single: Year 2000 34 single: Y2K 35 36* **Year 2000 (Y2K) issues**: Python depends on the platform's C library, which 37 generally doesn't have year 2000 issues, since all dates and times are 38 represented internally as seconds since the epoch. Functions accepting a 39 :class:`struct_time` (see below) generally require a 4-digit year. For backward 40 compatibility, 2-digit years are supported if the module variable 41 ``accept2dyear`` is a non-zero integer; this variable is initialized to ``1`` 42 unless the environment variable :envvar:`PYTHONY2K` is set to a non-empty 43 string, in which case it is initialized to ``0``. Thus, you can set 44 :envvar:`PYTHONY2K` to a non-empty string in the environment to require 4-digit 45 years for all year input. When 2-digit years are accepted, they are converted 46 according to the POSIX or X/Open standard: values 69-99 are mapped to 1969-1999, 47 and values 0--68 are mapped to 2000--2068. Values 100--1899 are always illegal. 48 Note that this is new as of Python 1.5.2(a2); earlier versions, up to Python 49 1.5.1 and 1.5.2a1, would add 1900 to year values below 1900. 50 51 .. index:: 52 single: UTC 53 single: Coordinated Universal Time 54 single: Greenwich Mean Time 55 56* UTC is Coordinated Universal Time (formerly known as Greenwich Mean Time, or 57 GMT). The acronym UTC is not a mistake but a compromise between English and 58 French. 59 60 .. index:: single: Daylight Saving Time 61 62* DST is Daylight Saving Time, an adjustment of the timezone by (usually) one 63 hour during part of the year. DST rules are magic (determined by local law) and 64 can change from year to year. The C library has a table containing the local 65 rules (often it is read from a system file for flexibility) and is the only 66 source of True Wisdom in this respect. 67 68* The precision of the various real-time functions may be less than suggested by 69 the units in which their value or argument is expressed. E.g. on most Unix 70 systems, the clock "ticks" only 50 or 100 times a second. 71 72* On the other hand, the precision of :func:`time` and :func:`sleep` is better 73 than their Unix equivalents: times are expressed as floating point numbers, 74 :func:`time` returns the most accurate time available (using Unix 75 :cfunc:`gettimeofday` where available), and :func:`sleep` will accept a time 76 with a nonzero fraction (Unix :cfunc:`select` is used to implement this, where 77 available). 78 79* The time value as returned by :func:`gmtime`, :func:`localtime`, and 80 :func:`strptime`, and accepted by :func:`asctime`, :func:`mktime` and 81 :func:`strftime`, may be considered as a sequence of 9 integers. The return 82 values of :func:`gmtime`, :func:`localtime`, and :func:`strptime` also offer 83 attribute names for individual fields. 84 85 +-------+-------------------+---------------------------------+ 86 | Index | Attribute | Values | 87 +=======+===================+=================================+ 88 | 0 | :attr:`tm_year` | (for example, 1993) | 89 +-------+-------------------+---------------------------------+ 90 | 1 | :attr:`tm_mon` | range [1,12] | 91 +-------+-------------------+---------------------------------+ 92 | 2 | :attr:`tm_mday` | range [1,31] | 93 +-------+-------------------+---------------------------------+ 94 | 3 | :attr:`tm_hour` | range [0,23] | 95 +-------+-------------------+---------------------------------+ 96 | 4 | :attr:`tm_min` | range [0,59] | 97 +-------+-------------------+---------------------------------+ 98 | 5 | :attr:`tm_sec` | range [0,61]; see **(1)** in | 99 | | | :func:`strftime` description | 100 +-------+-------------------+---------------------------------+ 101 | 6 | :attr:`tm_wday` | range [0,6], Monday is 0 | 102 +-------+-------------------+---------------------------------+ 103 | 7 | :attr:`tm_yday` | range [1,366] | 104 +-------+-------------------+---------------------------------+ 105 | 8 | :attr:`tm_isdst` | 0, 1 or -1; see below | 106 +-------+-------------------+---------------------------------+ 107 108 Note that unlike the C structure, the month value is a range of 1-12, not 0-11. 109 A year value will be handled as described under "Year 2000 (Y2K) issues" above. 110 A ``-1`` argument as the daylight savings flag, passed to :func:`mktime` will 111 usually result in the correct daylight savings state to be filled in. 112 113 When a tuple with an incorrect length is passed to a function expecting a 114 :class:`struct_time`, or having elements of the wrong type, a :exc:`TypeError` 115 is raised. 116 117 .. versionchanged:: 2.2 118 The time value sequence was changed from a tuple to a :class:`struct_time`, with 119 the addition of attribute names for the fields. 120 121* Use the following functions to convert between time representations: 122 123 +-------------------------+-------------------------+-------------------------+ 124 | From | To | Use | 125 +=========================+=========================+=========================+ 126 | seconds since the epoch | :class:`struct_time` in | :func:`gmtime` | 127 | | UTC | | 128 +-------------------------+-------------------------+-------------------------+ 129 | seconds since the epoch | :class:`struct_time` in | :func:`localtime` | 130 | | local time | | 131 +-------------------------+-------------------------+-------------------------+ 132 | :class:`struct_time` in | seconds since the epoch | :func:`calendar.timegm` | 133 | UTC | | | 134 +-------------------------+-------------------------+-------------------------+ 135 | :class:`struct_time` in | seconds since the epoch | :func:`mktime` | 136 | local time | | | 137 +-------------------------+-------------------------+-------------------------+ 138 139 140The module defines the following functions and data items: 141 142.. data:: accept2dyear 143 144 Boolean value indicating whether two-digit year values will be accepted. This 145 is true by default, but will be set to false if the environment variable 146 :envvar:`PYTHONY2K` has been set to a non-empty string. It may also be modified 147 at run time. 148 149 150.. data:: altzone 151 152 The offset of the local DST timezone, in seconds west of UTC, if one is defined. 153 This is negative if the local DST timezone is east of UTC (as in Western Europe, 154 including the UK). Only use this if ``daylight`` is nonzero. 155 156 157.. function:: asctime([t]) 158 159 Convert a tuple or :class:`struct_time` representing a time as returned by 160 :func:`gmtime` or :func:`localtime` to a 24-character string of the following 161 form: ``'Sun Jun 20 23:21:05 1993'``. If *t* is not provided, the current time 162 as returned by :func:`localtime` is used. Locale information is not used by 163 :func:`asctime`. 164 165 .. note:: 166 167 Unlike the C function of the same name, there is no trailing newline. 168 169 .. versionchanged:: 2.1 170 Allowed *t* to be omitted. 171 172 173.. function:: clock() 174 175 .. index:: 176 single: CPU time 177 single: processor time 178 single: benchmarking 179 180 On Unix, return the current processor time as a floating point number expressed 181 in seconds. The precision, and in fact the very definition of the meaning of 182 "processor time", depends on that of the C function of the same name, but in any 183 case, this is the function to use for benchmarking Python or timing algorithms. 184 185 On Windows, this function returns wall-clock seconds elapsed since the first 186 call to this function, as a floating point number, based on the Win32 function 187 :cfunc:`QueryPerformanceCounter`. The resolution is typically better than one 188 microsecond. 189 190 191.. function:: ctime([secs]) 192 193 Convert a time expressed in seconds since the epoch to a string representing 194 local time. If *secs* is not provided or :const:`None`, the current time as 195 returned by :func:`time` is used. ``ctime(secs)`` is equivalent to 196 ``asctime(localtime(secs))``. Locale information is not used by :func:`ctime`. 197 198 .. versionchanged:: 2.1 199 Allowed *secs* to be omitted. 200 201 .. versionchanged:: 2.4 202 If *secs* is :const:`None`, the current time is used. 203 204 205.. data:: daylight 206 207 Nonzero if a DST timezone is defined. 208 209 210.. function:: gmtime([secs]) 211 212 Convert a time expressed in seconds since the epoch to a :class:`struct_time` in 213 UTC in which the dst flag is always zero. If *secs* is not provided or 214 :const:`None`, the current time as returned by :func:`time` is used. Fractions 215 of a second are ignored. See above for a description of the 216 :class:`struct_time` object. See :func:`calendar.timegm` for the inverse of this 217 function. 218 219 .. versionchanged:: 2.1 220 Allowed *secs* to be omitted. 221 222 .. versionchanged:: 2.4 223 If *secs* is :const:`None`, the current time is used. 224 225 226.. function:: localtime([secs]) 227 228 Like :func:`gmtime` but converts to local time. If *secs* is not provided or 229 :const:`None`, the current time as returned by :func:`time` is used. The dst 230 flag is set to ``1`` when DST applies to the given time. 231 232 .. versionchanged:: 2.1 233 Allowed *secs* to be omitted. 234 235 .. versionchanged:: 2.4 236 If *secs* is :const:`None`, the current time is used. 237 238 239.. function:: mktime(t) 240 241 This is the inverse function of :func:`localtime`. Its argument is the 242 :class:`struct_time` or full 9-tuple (since the dst flag is needed; use ``-1`` 243 as the dst flag if it is unknown) which expresses the time in *local* time, not 244 UTC. It returns a floating point number, for compatibility with :func:`time`. 245 If the input value cannot be represented as a valid time, either 246 :exc:`OverflowError` or :exc:`ValueError` will be raised (which depends on 247 whether the invalid value is caught by Python or the underlying C libraries). 248 The earliest date for which it can generate a time is platform-dependent. 249 250 251.. function:: sleep(secs) 252 253 Suspend execution for the given number of seconds. The argument may be a 254 floating point number to indicate a more precise sleep time. The actual 255 suspension time may be less than that requested because any caught signal will 256 terminate the :func:`sleep` following execution of that signal's catching 257 routine. Also, the suspension time may be longer than requested by an arbitrary 258 amount because of the scheduling of other activity in the system. 259 260 261.. function:: strftime(format[, t]) 262 263 Convert a tuple or :class:`struct_time` representing a time as returned by 264 :func:`gmtime` or :func:`localtime` to a string as specified by the *format* 265 argument. If *t* is not provided, the current time as returned by 266 :func:`localtime` is used. *format* must be a string. :exc:`ValueError` is 267 raised if any field in *t* is outside of the allowed range. 268 269 .. versionchanged:: 2.1 270 Allowed *t* to be omitted. 271 272 .. versionchanged:: 2.4 273 :exc:`ValueError` raised if a field in *t* is out of range. 274 275 .. versionchanged:: 2.5 276 0 is now a legal argument for any position in the time tuple; if it is normally 277 illegal the value is forced to a correct one.. 278 279 The following directives can be embedded in the *format* string. They are shown 280 without the optional field width and precision specification, and are replaced 281 by the indicated characters in the :func:`strftime` result: 282 283 +-----------+--------------------------------+-------+ 284 | Directive | Meaning | Notes | 285 +===========+================================+=======+ 286 | ``%a`` | Locale's abbreviated weekday | | 287 | | name. | | 288 +-----------+--------------------------------+-------+ 289 | ``%A`` | Locale's full weekday name. | | 290 +-----------+--------------------------------+-------+ 291 | ``%b`` | Locale's abbreviated month | | 292 | | name. | | 293 +-----------+--------------------------------+-------+ 294 | ``%B`` | Locale's full month name. | | 295 +-----------+--------------------------------+-------+ 296 | ``%c`` | Locale's appropriate date and | | 297 | | time representation. | | 298 +-----------+--------------------------------+-------+ 299 | ``%d`` | Day of the month as a decimal | | 300 | | number [01,31]. | | 301 +-----------+--------------------------------+-------+ 302 | ``%H`` | Hour (24-hour clock) as a | | 303 | | decimal number [00,23]. | | 304 +-----------+--------------------------------+-------+ 305 | ``%I`` | Hour (12-hour clock) as a | | 306 | | decimal number [01,12]. | | 307 +-----------+--------------------------------+-------+ 308 | ``%j`` | Day of the year as a decimal | | 309 | | number [001,366]. | | 310 +-----------+--------------------------------+-------+ 311 | ``%m`` | Month as a decimal number | | 312 | | [01,12]. | | 313 +-----------+--------------------------------+-------+ 314 | ``%M`` | Minute as a decimal number | | 315 | | [00,59]. | | 316 +-----------+--------------------------------+-------+ 317 | ``%p`` | Locale's equivalent of either | \(1) | 318 | | AM or PM. | | 319 +-----------+--------------------------------+-------+ 320 | ``%S`` | Second as a decimal number | \(2) | 321 | | [00,61]. | | 322 +-----------+--------------------------------+-------+ 323 | ``%U`` | Week number of the year | \(3) | 324 | | (Sunday as the first day of | | 325 | | the week) as a decimal number | | 326 | | [00,53]. All days in a new | | 327 | | year preceding the first | | 328 | | Sunday are considered to be in | | 329 | | week 0. | | 330 +-----------+--------------------------------+-------+ 331 | ``%w`` | Weekday as a decimal number | | 332 | | [0(Sunday),6]. | | 333 +-----------+--------------------------------+-------+ 334 | ``%W`` | Week number of the year | \(3) | 335 | | (Monday as the first day of | | 336 | | the week) as a decimal number | | 337 | | [00,53]. All days in a new | | 338 | | year preceding the first | | 339 | | Monday are considered to be in | | 340 | | week 0. | | 341 +-----------+--------------------------------+-------+ 342 | ``%x`` | Locale's appropriate date | | 343 | | representation. | | 344 +-----------+--------------------------------+-------+ 345 | ``%X`` | Locale's appropriate time | | 346 | | representation. | | 347 +-----------+--------------------------------+-------+ 348 | ``%y`` | Year without century as a | | 349 | | decimal number [00,99]. | | 350 +-----------+--------------------------------+-------+ 351 | ``%Y`` | Year with century as a decimal | | 352 | | number. | | 353 +-----------+--------------------------------+-------+ 354 | ``%Z`` | Time zone name (no characters | | 355 | | if no time zone exists). | | 356 +-----------+--------------------------------+-------+ 357 | ``%%`` | A literal ``'%'`` character. | | 358 +-----------+--------------------------------+-------+ 359 360 Notes: 361 362 (1) 363 When used with the :func:`strptime` function, the ``%p`` directive only affects 364 the output hour field if the ``%I`` directive is used to parse the hour. 365 366 (2) 367 The range really is ``0`` to ``61``; this accounts for leap seconds and the 368 (very rare) double leap seconds. 369 370 (3) 371 When used with the :func:`strptime` function, ``%U`` and ``%W`` are only used in 372 calculations when the day of the week and the year are specified. 373 374 Here is an example, a format for dates compatible with that specified in the 375 :rfc:`2822` Internet email standard. [#]_ :: 376 377 >>> from time import gmtime, strftime 378 >>> strftime("%a, %d %b %Y %H:%M:%S +0000", gmtime()) 379 'Thu, 28 Jun 2001 14:17:15 +0000' 380 381 Additional directives may be supported on certain platforms, but only the ones 382 listed here have a meaning standardized by ANSI C. 383 384 On some platforms, an optional field width and precision specification can 385 immediately follow the initial ``'%'`` of a directive in the following order; 386 this is also not portable. The field width is normally 2 except for ``%j`` where 387 it is 3. 388 389 390.. function:: strptime(string[, format]) 391 392 Parse a string representing a time according to a format. The return value is 393 a :class:`struct_time` as returned by :func:`gmtime` or :func:`localtime`. 394 395 The *format* parameter uses the same directives as those used by 396 :func:`strftime`; it defaults to ``"%a %b %d %H:%M:%S %Y"`` which matches the 397 formatting returned by :func:`ctime`. If *string* cannot be parsed according to 398 *format*, or if it has excess data after parsing, :exc:`ValueError` is raised. 399 The default values used to fill in any missing data when more accurate values 400 cannot be inferred are ``(1900, 1, 1, 0, 0, 0, 0, 1, -1)``. 401 402 For example: 403 404 >>> import time 405 >>> time.strptime("30 Nov 00", "%d %b %y") # doctest: +NORMALIZE_WHITESPACE 406 time.struct_time(tm_year=2000, tm_mon=11, tm_mday=30, tm_hour=0, tm_min=0, 407 tm_sec=0, tm_wday=3, tm_yday=335, tm_isdst=-1) 408 409 Support for the ``%Z`` directive is based on the values contained in ``tzname`` 410 and whether ``daylight`` is true. Because of this, it is platform-specific 411 except for recognizing UTC and GMT which are always known (and are considered to 412 be non-daylight savings timezones). 413 414 Only the directives specified in the documentation are supported. Because 415 ``strftime()`` is implemented per platform it can sometimes offer more 416 directives than those listed. But ``strptime()`` is independent of any platform 417 and thus does not necessarily support all directives available that are not 418 documented as supported. 419 420 421.. data:: struct_time 422 423 The type of the time value sequence returned by :func:`gmtime`, 424 :func:`localtime`, and :func:`strptime`. 425 426 .. versionadded:: 2.2 427 428 429.. function:: time() 430 431 Return the time as a floating point number expressed in seconds since the epoch, 432 in UTC. Note that even though the time is always returned as a floating point 433 number, not all systems provide time with a better precision than 1 second. 434 While this function normally returns non-decreasing values, it can return a 435 lower value than a previous call if the system clock has been set back between 436 the two calls. 437 438 439.. data:: timezone 440 441 The offset of the local (non-DST) timezone, in seconds west of UTC (negative in 442 most of Western Europe, positive in the US, zero in the UK). 443 444 445.. data:: tzname 446 447 A tuple of two strings: the first is the name of the local non-DST timezone, the 448 second is the name of the local DST timezone. If no DST timezone is defined, 449 the second string should not be used. 450 451 452.. function:: tzset() 453 454 Resets the time conversion rules used by the library routines. The environment 455 variable :envvar:`TZ` specifies how this is done. 456 457 .. versionadded:: 2.3 458 459 Availability: Unix. 460 461 .. note:: 462 463 Although in many cases, changing the :envvar:`TZ` environment variable may 464 affect the output of functions like :func:`localtime` without calling 465 :func:`tzset`, this behavior should not be relied on. 466 467 The :envvar:`TZ` environment variable should contain no whitespace. 468 469 The standard format of the :envvar:`TZ` environment variable is (whitespace 470 added for clarity):: 471 472 std offset [dst [offset [,start[/time], end[/time]]]] 473 474 Where the components are: 475 476 ``std`` and ``dst`` 477 Three or more alphanumerics giving the timezone abbreviations. These will be 478 propagated into time.tzname 479 480 ``offset`` 481 The offset has the form: ``Âą hh[:mm[:ss]]``. This indicates the value 482 added the local time to arrive at UTC. If preceded by a '-', the timezone 483 is east of the Prime Meridian; otherwise, it is west. If no offset follows 484 dst, summer time is assumed to be one hour ahead of standard time. 485 486 ``start[/time], end[/time]`` 487 Indicates when to change to and back from DST. The format of the 488 start and end dates are one of the following: 489 490 :samp:`J{n}` 491 The Julian day *n* (1 <= *n* <= 365). Leap days are not counted, so in 492 all years February 28 is day 59 and March 1 is day 60. 493 494 :samp:`{n}` 495 The zero-based Julian day (0 <= *n* <= 365). Leap days are counted, and 496 it is possible to refer to February 29. 497 498 :samp:`M{m}.{n}.{d}` 499 The *d*'th day (0 <= *d* <= 6) or week *n* of month *m* of the year (1 500 <= *n* <= 5, 1 <= *m* <= 12, where week 5 means "the last *d* day in 501 month *m*" which may occur in either the fourth or the fifth 502 week). Week 1 is the first week in which the *d*'th day occurs. Day 503 zero is Sunday. 504 505 ``time`` has the same format as ``offset`` except that no leading sign 506 ('-' or '+') is allowed. The default, if time is not given, is 02:00:00. 507 508 :: 509 510 >>> os.environ['TZ'] = 'EST+05EDT,M4.1.0,M10.5.0' 511 >>> time.tzset() 512 >>> time.strftime('%X %x %Z') 513 '02:07:36 05/08/03 EDT' 514 >>> os.environ['TZ'] = 'AEST-10AEDT-11,M10.5.0,M3.5.0' 515 >>> time.tzset() 516 >>> time.strftime('%X %x %Z') 517 '16:08:12 05/08/03 AEST' 518 519 On many Unix systems (including \*BSD, Linux, Solaris, and Darwin), it is more 520 convenient to use the system's zoneinfo (:manpage:`tzfile(5)`) database to 521 specify the timezone rules. To do this, set the :envvar:`TZ` environment 522 variable to the path of the required timezone datafile, relative to the root of 523 the systems 'zoneinfo' timezone database, usually located at 524 :file:`/usr/share/zoneinfo`. For example, ``'US/Eastern'``, 525 ``'Australia/Melbourne'``, ``'Egypt'`` or ``'Europe/Amsterdam'``. :: 526 527 >>> os.environ['TZ'] = 'US/Eastern' 528 >>> time.tzset() 529 >>> time.tzname 530 ('EST', 'EDT') 531 >>> os.environ['TZ'] = 'Egypt' 532 >>> time.tzset() 533 >>> time.tzname 534 ('EET', 'EEST') 535 536 537.. seealso:: 538 539 Module :mod:`datetime` 540 More object-oriented interface to dates and times. 541 542 Module :mod:`locale` 543 Internationalization services. The locale settings can affect the return values 544 for some of the functions in the :mod:`time` module. 545 546 Module :mod:`calendar` 547 General calendar-related functions. :func:`timegm` is the inverse of 548 :func:`gmtime` from this module. 549 550.. rubric:: Footnotes 551 552.. [#] The use of ``%Z`` is now deprecated, but the ``%z`` escape that expands to the 553 preferred hour/minute offset is not supported by all ANSI C libraries. Also, a 554 strict reading of the original 1982 :rfc:`822` standard calls for a two-digit 555 year (%y rather than %Y), but practice moved to 4-digit years long before the 556 year 2000. The 4-digit year has been mandated by :rfc:`2822`, which obsoletes 557 :rfc:`822`. 558