[Tutor] Through a glass, darkly: the datetime module

[eryksun]

On Sat, Oct 6, 2012 at 10:37 PM, Richard D. Moores <rdmoores at gmail.com> wrote:
> On Sat, Oct 6, 2012 at 5:22 PM, eryksun <eryksun at gmail.com> wrote:
>
>>     >>> from datetime import date
>>     >>> date(2014, 2, 18).strftime("%A")
>>     'Tuesday'
>>
>> http://docs.python.org/library/datetime#strftime-and-strptime-behavior
>
> I remain bewildered. Where did these strangely named things come from,
> strftime and strptime? I see that

These are named for the C standard library functions for [f]ormatting
and [p]arsing broken-down [time] structures to and from [str]ings. For
examples, see the GNU libc docs:

http://www.gnu.org/software/libc/manual/html_node/Calendar-Time.html

I tend to agree that in Python the names seem... oddly wonkish, and
the reliance on terse formatting codes seems... oddly low-level. You
just have to get used to it, like working with regular expressions and
string formatting. Or use (or write your own) higher-level APIs such
as calendar.day_name, which uses datetime.date.strftime:


    class _localized_day:

        # January 1, 2001, was a Monday.
        _days = [datetime.date(2001, 1, i+1).strftime
                 for i in range(7)]

        def __init__(self, format):
            self.format = format

        def __getitem__(self, i):
            funcs = self._days[i]
            if isinstance(i, slice):
                return [f(self.format) for f in funcs]
            else:
                return funcs(self.format)

        def __len__(self):
            return 7

    day_name = _localized_day('%A')


Notice day_name is initialized with the "%A" format code, and
_localized_day has a class attribute "_days" that's a list of 7
strftime bound methods. It uses these in __getitem__ to format a
single day or a slice of several days:

    >>> calendar.day_name[:3]
    ['Monday', 'Tuesday', 'Wednesday']


> "date, datetime, and time objects all support a strftime(format)
> method, to create a string representing the time under the control of
> an explicit format string. Broadly speaking, d.strftime(fmt) acts like
> the time module’s time.strftime(fmt, d.timetuple()) although not all
> objects support a timetuple() method."
>
> Total gibberish. I feel like I've hit a brick wall. Where can I go to
> learn to understand it? I need some very basic, specific information.

They're comparing the datetime methods to the similarly-named methods
in the time module, such as time.strftime:

http://docs.python.org/library/time#time.strftime

which formats an instance of time.struct_time:

http://docs.python.org/library/time#time.struct_time

datetime.date.timetuple() and datetime.datetime.timetuple() both
return an instance of time.struct_time:

http://docs.python.org/library/datetime#datetime.date.timetuple
http://docs.python.org/library/datetime#datetime.datetime.timetuple

Compare struct_time to a libc broken-down time structure:

http://www.gnu.org/software/libc/manual/html_node/Broken_002ddown-Time.html