python标准库介绍——12 time 模块详解

==time 模块==

``time`` 模块提供了一些处理日期和一天内时间的函数. 它是建立在 C 运行时库的简单封装. 

给定的日期和时间可以被表示为浮点型(从参考时间, 通常是 1970.1.1 到现在经过的秒数.
即 Unix 格式), 或者一个表示时间的 struct (类元组). 

=== 获得当前时间===

[Example 1-79 #eg-1-79] 展示了如何使用 ``time`` 模块获取当前时间.

====Example 1-79. 使用 time 模块获取当前时间====[eg-1-79]

```
File: time-example-1.py

import time

now = time.time()

print now, "seconds since", time.gmtime(0)[:6]
print
print "or in other words:"
print "- local time:", time.localtime(now)
print "- utc:", time.gmtime(now)

*B*937758359.77 seconds since (1970, 1, 1, 0, 0, 0)

or in other words:
- local time: (1999, 9, 19, 18, 25, 59, 6, 262, 1)
- utc: (1999, 9, 19, 16, 25, 59, 6, 262, 0)*b*
```

``localtime`` 和 ``gmtime`` 返回的类元组包括年, 月, 日, 时, 分, 秒, 星期, 一年的第几天, 日光标志.
其中年是一个四位数(在有千年虫问题的平台上另有规定, 但还是四位数), 星期从星期一(数字 0 代表)开始,
1月1日是一年的第一天. 

=== 将时间值转换为字符串===

你可以使用标准的格式化字符串把时间对象转换为字符串, 不过 ``time`` 模块已经提供了许多标准转换函数,
如 [Example 1-80 #eg-1-80] 所示. 

====Example 1-80. 使用 time 模块格式化时间输出====[eg-1-80]

```
File: time-example-2.py

import time

now = time.localtime(time.time())

print time.asctime(now)
print time.strftime("%y/%m/%d %H:%M", now)
print time.strftime("%a %b %d", now)
print time.strftime("%c", now)
print time.strftime("%I %p", now)
print time.strftime("%Y-%m-%d %H:%M:%S %Z", now)

# do it by hand...
year, month, day, hour, minute, second, weekday, yearday, daylight = now
print "%04d-%02d-%02d" % (year, month, day)
print "%02d:%02d:%02d" % (hour, minute, second)
print ("MON", "TUE", "WED", "THU", "FRI", "SAT", "SUN")[weekday], yearday

*B*Sun Oct 10 21:39:24 1999
99/10/10 21:39
Sun Oct 10
Sun Oct 10 21:39:24 1999
09 PM
1999-10-10 21:39:24 CEST
1999-10-10
21:39:24
SUN 283*b*
```

===将字符串转换为时间对象===

在一些平台上, ``time`` 模块包含了 ``strptime`` 函数, 它的作用与 ``strftime`` 相反.
给定一个字符串和模式, 它返回相应的时间对象, 如 [Example 1-81 #eg-1-81] 所示. 

====Example 1-81. 使用 time.strptime 函数解析时间====[eg-1-81]

```
File: time-example-6.py

import time

# make sure we have a strptime function!
# 确认有函数 strptime
try:
    strptime = time.strptime
except AttributeError:
    from strptime import strptime

print strptime("31 Nov 00", "%d %b %y")
print strptime("1 Jan 70 1:30pm", "%d %b %y %I:%M%p")
```

只有在系统的 C 库提供了相应的函数的时候, ``time.strptime`` 函数才可以使用.
对于没有提供标准实现的平台, [Example 1-82 #eg-1-82] 提供了一个不完全的实现. 

====Example 1-82. strptime 实现====[eg-1-82]

```
File: strptime.py

import re
import string

MONTHS = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug",
          "Sep", "Oct", "Nov", "Dec"]

SPEC = {
    # map formatting code to a regular expression fragment
    "%a": "(?P<weekday>[a-z]+)",
    "%A": "(?P<weekday>[a-z]+)",
    "%b": "(?P<month>[a-z]+)",
    "%B": "(?P<month>[a-z]+)",
    "%C": "(?P<century>\d\d?)",
    "%d": "(?P<day>\d\d?)",
    "%D": "(?P<month>\d\d?)/(?P<day>\d\d?)/(?P<year>\d\d)",
    "%e": "(?P<day>\d\d?)",
    "%h": "(?P<month>[a-z]+)",
    "%H": "(?P<hour>\d\d?)",
    "%I": "(?P<hour12>\d\d?)",
    "%j": "(?P<yearday>\d\d?\d?)",
    "%m": "(?P<month>\d\d?)",
    "%M": "(?P<minute>\d\d?)",
    "%p": "(?P<ampm12>am|pm)",
    "%R": "(?P<hour>\d\d?):(?P<minute>\d\d?)",
    "%S": "(?P<second>\d\d?)",
    "%T": "(?P<hour>\d\d?):(?P<minute>\d\d?):(?P<second>\d\d?)",
    "%U": "(?P<week>\d\d)",
    "%w": "(?P<weekday>\d)",
    "%W": "(?P<weekday>\d\d)",
    "%y": "(?P<year>\d\d)",
    "%Y": "(?P<year>\d\d\d\d)",
    "%%": "%"
}

class TimeParser:
    def _ _init_ _(self, format):
        # convert strptime format string to regular expression
        format = string.join(re.split("(?:\s|%t|%n)+", format))
        pattern = []
        try:
            for spec in re.findall("%\w|%%|.", format):
                if spec[0] == "%":
                    spec = SPEC[spec]
                pattern.append(spec)
        except KeyError:
            raise ValueError, "unknown specificer: %s" % spec
        self.pattern = re.compile("(?i)" + string.join(pattern, ""))
    def match(self, daytime):
        # match time string
        match = self.pattern.match(daytime)
        if not match:
            raise ValueError, "format mismatch"
        get = match.groupdict().get
        tm = [0] * 9
        # extract date elements
        y = get("year")
        if y:
            y = int(y)
            if y < 68:
                y = 2000 + y
            elif y < 100:
                y = 1900 + y
            tm[0] = y
        m = get("month")
        if m:
            if m in MONTHS:
                m = MONTHS.index(m) + 1
            tm[1] = int(m)
        d = get("day")
        if d: tm[2] = int(d)
        # extract time elements
        h = get("hour")
        if h:
            tm[3] = int(h)
        else:
            h = get("hour12")
            if h:
                h = int(h)
                if string.lower(get("ampm12", "")) == "pm":
                    h = h + 12
                tm[3] = h
        m = get("minute")
        if m: tm[4] = int(m)
        s = get("second")
        if s: tm[5] = int(s)
        # ignore weekday/yearday for now
        return tuple(tm)

def strptime(string, format="%a %b %d %H:%M:%S %Y"):
    return TimeParser(format).match(string)

if _ _name_ _ == "_ _main_ _":
    # try it out
    import time
    print strptime("2000-12-20 01:02:03", "%Y-%m-%d %H:%M:%S")
    print strptime(time.ctime(time.time()))

*B*(2000, 12, 20, 1, 2, 3, 0, 0, 0)
(2000, 11, 15, 12, 30, 45, 0, 0, 0)*b*
```

=== 转换时间值===

将时间元组转换回时间值非常简单, 至少我们谈论的当地时间 (local time) 如此.
只要把时间元组传递给 ``mktime`` 函数, 如 [Example 1-83 #eg-1-83] 所示. 

====Example 1-83. 使用 time 模块将本地时间元组转换为时间值(整数)====[eg-1-83]

```
File: time-example-3.py

import time

t0 = time.time()
tm = time.localtime(t0)

print tm

print t0
print time.mktime(tm)

*B*(1999, 9, 9, 0, 11, 8, 3, 252, 1)
936828668.16
936828668.0*b*
```

但是, 1.5.2 版本的标准库没有提供能将 UTC 时间
(Universal Time, Coordinated: 特林威治标准时间)转换为时间值的函数
( Python 和对应底层 C 库都没有提供). [Example 1-84 #eg-1-84] 提供了该函数的一个
Python 实现, 称为 ``timegm`` . 

====Example 1-84. 将 UTC 时间元组转换为时间值(整数)====[eg-1-84]

```
File: time-example-4.py

import time

def _d(y, m, d, days=(0,31,59,90,120,151,181,212,243,273,304,334,365)):
    # map a date to the number of days from a reference point
    return (((y - 1901)*1461)/4 + days[m-1] + d +
        ((m > 2 and not y % 4 and (y % 100 or not y % 400)) and 1))

def timegm(tm, epoch=_d(1970,1,1)):
    year, month, day, h, m, s = tm[:6]
    assert year >= 1970
    assert 1 <= month <= 12
    return (_d(year, month, day) - epoch)*86400 + h*3600 + m*60 + s

t0 = time.time()
tm = time.gmtime(t0)

print tm

print t0
print timegm(tm)

*B*(1999, 9, 8, 22, 12, 12, 2, 251, 0)
936828732.48
936828732*b*
```

从 1.6 版本开始, ``calendar`` 模块提供了一个类似的函数 ``calendar.timegm`` . 

=== Timing 相关===

``time`` 模块可以计算 Python 程序的执行时间, 如 [Example 1-85 #eg-1-85] 所示.
你可以测量 "wall time" (real world time), 或是"进程时间" (消耗的 CPU 时间). 

====Example 1-85. 使用 time 模块评价算法====[eg-1-85]

```
File: time-example-5.py

import time

def procedure():
    time.sleep(2.5)

# measure process time
t0 = time.clock()
procedure()
print time.clock() - t0, "seconds process time"

# measure wall time
t0 = time.time()
procedure()
print time.time() - t0, "seconds wall time"

*B*0.0 seconds process time
2.50903499126 seconds wall time*b*
```

并不是所有的系统都能测量真实的进程时间. 一些系统中(包括 Windows ),
``clock`` 函数通常测量从程序启动到测量时的 wall time. 

进程时间的精度受限制. 在一些系统中, 它超过 30 分钟后进程会被清理.
(原文: On many systems, it wraps around after just over 30 minutes.) 

另参见 ``timing`` 模块( Windows 下的朋友不用忙活了,没有地~), 它可以测量两个事件之间的 wall time.