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Python / Date/Time

Python / Date/Time

  • datetime — Basic date and time types — Python 3.7.3 documentation #ril

    • The datetime module supplies classes for manipulating dates and times in both simple and complex ways. While date and time arithmetic is supported, the focus of the implementation is on EFFICIENT ATTRIBUTE EXTRACTION FOR OUTPUT FORMATTING AND MANIPULATION. For related functionality, see also the time and calendar modules.

      為什麼說 time/calendar 較多時間的計算? 因為時間差 (timedelta) 跟比較大小都在 datetime module 裡。

    • There are two kinds of date and time objects: “NAIVE” and “AWARE”.

      An aware object has sufficient knowledge of applicable algorithmic and political time adjustments, such as time zone and daylight saving time information, to LOCATE ITSELF RELATIVE TO OTHER AWARE OBJECTS. An aware object is used to represent a SPECIFIC MOMENT in time that is NOT OPEN TO INTERPRETATION.

      A naive object does not contain enough information to UNAMBIGUOUSLY locate itself relative to other date/time objects. Whether a naive object represents Coordinated Universal Time (UTC), local time, or time in some other timezone is PURELY UP TO THE PROGRAM, just like it is up to the program whether a particular number represents metres, miles, or mass.

    • Naive objects are EASY TO UNDERSTAND AND TO WORK WITH, at the cost of ignoring some aspects of reality.

      從 "easy to work with" 及 "applications requiring aware object" 看來，似乎是比較推 naive 的，由於 open to interpretation 的特性，統一將 naive 視為 UTC 即可。

    • For applications requiring aware objects, datetime and time objects have an OPTIONAL time zone information attribute, tzinfo, that can be set to an instance of a subclass of the abstract tzinfo class. These tzinfo objects capture information about the OFFSET FROM UTC TIME, the time zone name, and whether Daylight Saving Time is in effect.

      Note that only one concrete tzinfo class, the timezone class, is supplied by the datetime module. The timezone class can represent simple timezones with FIXED OFFSET from UTC, such as UTC ITSELF or North American EST and EDT timezones.

      這裡 "fixed offset" 要強調的是 timezone 未將 DST 等政治因素考量進去；唯一不受政治影響的是 UTC -- timezone.utc。

      注意 Python 2 完全沒為 tzinfo 提供實作 8.1. datetime — Basic date and time types — Python 2.7.16 documentation，datetime.timezone 是 Python 3 才有的：

      Note that no concrete tzinfo classes are supplied by the datetime module.

    • Supporting timezones at deeper levels of detail is UP TO THE APPLICATION. The rules for time adjustment across the world are more POLITICAL than rational, CHANGE FREQUENTLY, and there is NO STANDARD suitable for every application aside from UTC.

    Available Types

    • class datetime.date

      An idealized ?? naive date, assuming the current Gregorian calendar always was, and always will be, in effect. Attributes: year, month, and day.

    • class datetime.time

      An idealized time, independent of any particular day, assuming that every day has exactly 246060 seconds (there is no notion of “leap seconds” here). Attributes: hour, minute, second, microsecond, and tzinfo.

    • class datetime.datetime

      A combination of a date and a time. Attributes: year, month, day, hour, minute, second, microsecond, and tzinfo.

    • class datetime.timedelta

      A duration expressing the difference between two date, time, or datetime instances to microsecond resolution.

    • class datetime.tzinfo

      An abstract base class for time zone information objects. These are used by the datetime and time classes to provide a customizable notion of time adjustment (for example, to account for time zone and/or daylight saving time).

    • class datetime.timezone

      A class that implements the tzinfo abstract base class as a fixed offset from the UTC. New in version 3.2.

    • Objects of these types are IMMUTABLE.

      python - Do datetime objects need to be deep-copied? - Stack Overflow 提到，因此所有對 datetime instance 的操作都會傳回新的 instance，不用怕動到原來的 instance。

    • Objects of the date type are ALWAYS NAIVE.

      An object of type time or datetime may be naive or aware. A datetime object d is aware if d.tzinfo is not None and d.tzinfo.utcoffset(d) does not return None. If d.tzinfo is None, or if d.tzinfo is not None but d.tzinfo.utcoffset(d) returns None, d is naive. A time object t is aware if t.tzinfo is not None and t.tzinfo.utcoffset(None) does not return None. Otherwise, t is naive.

    • The distinction between naive and aware doesn’t apply to timedelta objects.

      因為 timedelta 本身就是相對的概念，相對於 naive 或 aware 都沒問題。

    • Subclass relationships:

      object
    timedelta
    tzinfo
        timezone
    time
    date
        datetime

  • 8.1. datetime — Basic date and time types — Python 2.7.14 documentation 專注在取用 date/time 的 attribute extraction - 取年、月、日、時等 #ril

    • Objects of these types are immutable，這也代表著所有的 datetime.* types 都是 hashable，可以搭配 hashable collection 使用。

  • 15.3. time — Time access and conversions — Python 2.7.14 documentation 大部份 function 背後都是同名的 C function，可能因 platform 不同而異 #ril

  • 8.2. calendar — General calendar-related functions — Python 2.7.14 documentation 會輪出類似 Unix-like cal 的日曆 #ril

解析/格式化 ??

>>> d = datetime.strptime('2017-02-02 18:00', '%Y-%m-%d %H:%M')
>>> d.isoformat()
'2017-02-02T18:00:00'

參考資料：

Time Zone ??

  • Unicode 有所謂的 Unicode Sandwich -- 進入程式都轉成 Unicode，輸出都用 byte string。在 time zone 的部份，實務上該如何取捨 naive 與 aware？

    程式跟儲存裡都用 naive，輸出時再轉 aware。

參考資料：

  • 8.1. datetime — Basic date and time types — Python 2.7.14 documentation

    • date/time object 區分為 naive (天真的) 與 aware 兩種。所謂 aware 是知道 timezone、daylight saving time (DST) 是否作用中 (in effect)，可以明確地跟其他 aware object 比較 (unambiguously)，相對於 naive 就沒有這些資訊 (可以自己決定該如何解讀)。
    • datetime 跟 time 都有 tzinfo 屬性 (不是 None 時就是 aware，型態繼承自 datetime.tzinfo abstract class)，記錄著與 UTC 的偏移 (offset)、time zone 名稱、DST (Daylight Saving Time) 是否作用中等。
    • 有趣的是 datetime module 並未提供任何 datetime.tzinfo 的實作；或許跟 "Naive objects are easy to understand and to work with, at the cost of ignoring some aspects of reality" 與 "The rules for time adjustment across the world are more political than rational, and there is no standard suitable for every application." 這兩段話有關，time zone 並非必要。

  • datetime.astimezone(tz) - 8.1. datetime — Basic date and time types — Python 2.7.14 documentation 將 aware datetime 轉換成另一個 timezone，若想直接代換掉 timezone 則用 replace(tzinfo=tz)，其中 tz 若傳入 None，則會把 aware datetime 變成 naive object。

  • tzinfo Objects - 8.1. datetime — Basic date and time types — Python 2.7.14 documentation #ril

    • datetime.tzinfo 的實作一定要提供 utcoffset(self, dt)、dst(self, dt) 與 tzname(self, dt)。
    • utcoffset() 會傳回與 UTC 間的偏移 (timedelta)，並把 DST 考量進去。
    • 從 LocalTimezone 的實作看來，utcoffset()、dst() 及 tzname() 的回傳值都跟 _isdst(dt) 有關，也就是某個 date/time 是否 DST 有在作用中，也難怪所有的 method 都要傳 dt 進去；tzname() 的 "return different names depending on the specific value of dt passed, especially if the tzinfo class is accounting for daylight time" 印證了這個說法。

  • Timezones and Python | Julien Danjou (2015-06-16) #ril

  • pytz - World Timezone Definitions for Python — pytz 2014.10 documentation #ril

Time Delta ??

  • Available Types - 8.1. datetime — Basic date and time types — Python 2.7.15 documentation 提到 datetime.timedelta 是 "A DURATION expressing the difference between two date, time, or datetime instances to MICROSECOND resolution."，也提到 The distinction between naive and aware doesn’t apply to timedelta objects. 沒有 time zone naive/aware 的概念。
  • timedelta Objects - 8.1. datetime — Basic date and time types — Python 2.7.14 documentation
    • class datetime.timedelta([days[, seconds[, microseconds[, milliseconds[, minutes[, hours[, weeks]]]]]]]) 所有 argument 預設都是 0，可以接受 integer/float，也可以是 positive/negative，彈性很大；但 microsecond 已經最小精度，再給小數好像也沒意思?
    • 雖然 constructor 提供了 milliseconds、minutes、hours、weeks 等，但內部只存 days、seconds 跟 microseconds (也就是 timedelta instance 的 3 個 attribute)；也就是內部會做 normalization 以確保 "unique representation"，所以 milliseconds => x 1000 macroseconds、minutes => x 60 seconds、hours => x 3600 seconds、weeks => x 7 days。
    • 除了 supported operations 提到的 timedelta objects 間可以做一些數學運算，例如 t1 + t2、t1 - t2、t * i、t // i、+t、-t 還有 abs(t)，另外 timedelta 間也可以比較，比的是 duration 的長短。
    • timedelta 本身是 hashable，也支援 pickling。在 boolean context 裡，只要不是 timedelta(0) 就會被視為 True。
    • 中間提到 timedelta 可以跟 date/datetime 做加減運算 (跟 time 不行??)，不過細節就在 date/datetime objects 的章節說明了。
  • 8.1. datetime — Basic date and time types — Python 2.7.14 documentation 出現不少 timedelta(0) 的用法，甚至有 ZERO = timedelta(0)。

Date Time - date, datetime, time ??

如何解析 ISO 8601 字串?

如果要解析 ISO 8601 (或它的子集 RFC 3339)，都不建議用 datetime.strptime()，處理各種變化型會很苦。建議用 python-dateutil：

>>> from dateutil import parser
>>> parser.parse('2017-12-29T13:54:01.2732344+00:00')
datetime.datetime(2017, 12, 29, 13, 54, 1, 273234, tzinfo=tzutc())
>>> parser.parse('2017-12-29T13:54:01.2732344')
datetime.datetime(2017, 12, 29, 13, 54, 1, 273234)

參考資料：

如何取得當地時間年、月、日、時、分等不同欄位的值??

from datetime import datetime

now = datetime.now()
year, month, day, hour, minute, second =
    now.year, now.month, now.day,
    now.hour, now.minute, now.second

參考資料：

  • 8.1. datetime — Basic date and time types — Python 2.7.14 documentation https://docs.python.org/2/library/datetime.html datetime.now() 可以取得 datetime.datetime 的 instance，然後透過 .year、.month、.day、.hour、.minute、.second 就能取得不同欄位的值。提到 datetime.today() 等同於 datetime.fromtimestamp(time.time())，也就是等同於 datetime.now()。不過 date.today() 的可讀性似乎比 datetime.today()?

如何取得當下 UTC (aware) 的 datetime??

參考資料：

TypeError: datetime.datetime(...) is not JSON serializable

from datetime import datetime, date
import json
import pytz

def test_default_serializer():
    def default(obj):
        if isinstance(obj, (date, datetime)):
            return obj.isoformat()
        else:
            return str(obj)
            # raise TypeError('%r is not JSON serializable' % obj)

    data = [
        date(2018, 1, 2),
        datetime(2018, 1, 2, 13, 50, tzinfo=pytz.utc),
    ]
    s = json.dumps(data, default=default)
    assert s == '["2018-01-02", "2018-01-02T13:50:00+00:00"]'

參考資料：