dateutil 解析器在从各种来源正确猜测日期和时间方面做得很好。

我们正在处理每个文件仅使用一种日期/时间格式的文件，但文件之间的格式会有所不同。分析显示 dateutil.parser.parse 使用了很多时间。由于每个文件只需要确定一次，因此实现每次都不会猜测格式的东西可以加快速度。

我实际上并不事先知道格式，我仍然需要推断格式。比如:

from MysteryPackage import date_string_to_format_string
import datetime

# e.g. mystring = '1 Jan 2016'
myformat = None

...

# somewhere in a loop reading from a file or connection:
if myformat is None:
    myformat = date_string_to_format_string(mystring)

# do the usual checks to see if that worked, then:
mydatetime = datetime.strptime(mystring, myformat)

有这样的功能吗？

最佳答案

这是一个棘手的问题。我的方法使用正则表达式和 (?(DEFINE)...) 语法，只有较新的 regex 才支持该语法。 模块。

    (?(DEFINE)
        (?P<year_def>[12]\d{3})
        (?P<year_short_def>\d{2})
        (?P<month_def>January|February|March|April|May|June|
        July|August|September|October|November|December)
        (?P<month_short_def>Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)
        (?P<day_def>(?:0[1-9]|[1-9]|[12][0-9]|3[01]))
        (?P<weekday_def>(?:Mon|Tue|Wednes|Thurs|Fri|Satur|Sun)day)
        (?P<weekday_short_def>Mon|Tue|Wed|Thu|Fri|Sat|Sun)
        (?P<hms_def>\d{2}:\d{2}:\d{2})
        (?P<hm_def>\d{2}:\d{2})
            (?P<ms_def>\d{5,6})
            (?P<delim_def>([-/., ]+|(?<=\d|^)T))
        )
        # actually match them
        (?P<hms>^(?&hms_def)$)|(?P<year>^(?&year_def)$)|(?P<month>^(?&month_def)$)|(?P<month_short>^(?&month_short_def)$)|(?P<day>^(?&day_def)$)|
        (?P<weekday>^(?&weekday_def)$)|(?P<weekday_short>^(?&weekday_short_def)$)|(?P<hm>^(?&hm_def)$)|(?P<delim>^(?&delim_def)$)|(?P<ms>^(?&ms_def)$)
        """, re.VERBOSE)

在这之后，我们需要考虑可能的分隔符:

# delim
delim = re.compile(r'([-/., ]+|(?<=\d)T)')

格式映射:

# formats
formats = {'ms': '%f', 'year': '%Y', 'month': '%B', 'month_dec': '%m', 'day': '%d', 'weekday': '%A', 'hms': '%H:%M:%S', 'weekday_short': '%a', 'month_short': '%b', 'hm': '%H:%M', 'delim': ''}

函数GuessFormat()在分隔符的帮助下分割部分，尝试匹配它们并为strftime()输出对应的代码:

def GuessFormat(datestring):

    # define the bricks
    bricks = re.compile(r"""
            (?(DEFINE)
                (?P<year_def>[12]\d{3})
                (?P<year_short_def>\d{2})
                (?P<month_def>January|February|March|April|May|June|
                July|August|September|October|November|December)
                (?P<month_short_def>Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)
                (?P<day_def>(?:0[1-9]|[1-9]|[12][0-9]|3[01]))
                (?P<weekday_def>(?:Mon|Tue|Wednes|Thurs|Fri|Satur|Sun)day)
                (?P<weekday_short_def>Mon|Tue|Wed|Thu|Fri|Sat|Sun)
                (?P<hms_def>T?\d{2}:\d{2}:\d{2})
                (?P<hm_def>T?\d{2}:\d{2})
                (?P<ms_def>\d{5,6})
                (?P<delim_def>([-/., ]+|(?<=\d|^)T))
            )
            # actually match them
            (?P<hms>^(?&hms_def)$)|(?P<year>^(?&year_def)$)|(?P<month>^(?&month_def)$)|(?P<month_short>^(?&month_short_def)$)|(?P<day>^(?&day_def)$)|
            (?P<weekday>^(?&weekday_def)$)|(?P<weekday_short>^(?&weekday_short_def)$)|(?P<hm>^(?&hm_def)$)|(?P<delim>^(?&delim_def)$)|(?P<ms>^(?&ms_def)$)
            """, re.VERBOSE)

    # delim
    delim = re.compile(r'([-/., ]+|(?<=\d)T)')

    # formats
    formats = {'ms': '%f', 'year': '%Y', 'month': '%B', 'month_dec': '%m', 'day': '%d', 'weekday': '%A', 'hms': '%H:%M:%S', 'weekday_short': '%a', 'month_short': '%b', 'hm': '%H:%M', 'delim': ''}

    parts = delim.split(datestring)
    out = []
    for index, part in enumerate(parts):
        try:
            brick = dict(filter(lambda x: x[1] is not None, bricks.match(part).groupdict().items()))
            key = next(iter(brick))

            # ambiguities
            if key == 'day' and index == 2:
                key = 'month_dec'

            item = part if key == 'delim' else formats[key]
            out.append(item)
        except AttributeError:
            out.append(part)

    return "".join(out)

最后一个测试:

import regex as re

datestrings = [datetime.now().isoformat(), '2006-11-02', 'Thursday, 10 August 2006 08:42:51', 'August 9, 1995', 'Aug 9, 1995', 'Thu, 01 Jan 1970 00:00:00', '21/11/06 16:30', 
'06 Jun 2017 20:33:10']

# test
for dt in datestrings:
    print("Date: {}, Format: {}".format(dt, GuessFormat(dt)))

这会产生:

Date: 2017-06-07T22:02:05.001811, Format: %Y-%m-%dT%H:%M:%S.%f
Date: 2006-11-02, Format: %Y-%m-%d
Date: Thursday, 10 August 2006 08:42:51, Format: %A, %m %B %Y %H:%M:%S
Date: August 9, 1995, Format: %B %m, %Y
Date: Aug 9, 1995, Format: %b %m, %Y
Date: Thu, 01 Jan 1970 00:00:00, Format: %a, %m %b %Y %H:%M:%S
Date: 21/11/06 16:30, Format: %d/%m/%d %H:%M
Date: 06 Jun 2017 20:33:10, Format: %d %b %Y %H:%M:%S

关于python - 如何从日期字符串中确定适当的 strftime 格式？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/44321601/