8.1. "datetime" — Basic date and time types
*******************************************

2.3 新版功能.

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.

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 [1].

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’s 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.

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 no concrete "tzinfo" classes are supplied by the
"datetime" module.  Supporting timezones at whatever level of detail
is required is up to the application.  The rules for time adjustment
across the world are more political than rational, and there is no
standard suitable for every application.

The "datetime" module exports the following constants:

datetime.MINYEAR

   The smallest year number allowed in a "date" or "datetime" object.
   "MINYEAR" is "1".

datetime.MAXYEAR

   The largest year number allowed in a "date" or "datetime" object.
   "MAXYEAR" is "9999".

参见:

  Module "calendar"
     General calendar related functions.

  Module "time"
     Time access and conversions.


8.1.1. 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 24*60*60 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).

Objects of these types are immutable.

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.

Subclass relationships:

   object
       timedelta
       tzinfo
       time
       date
           datetime


8.1.2. "timedelta" Objects
==========================

A "timedelta" object represents a duration, the difference between two
dates or times.

class datetime.timedelta([days[, seconds[, microseconds[, milliseconds[, minutes[, hours[, weeks]]]]]]])

   All arguments are optional and default to "0".  Arguments may be
   ints, longs, or floats, and may be positive or negative.

   Only *days*, *seconds* and *microseconds* are stored internally.
   Arguments are converted to those units:

   * A millisecond is converted to 1000 microseconds.

   * A minute is converted to 60 seconds.

   * An hour is converted to 3600 seconds.

   * A week is converted to 7 days.

   and days, seconds and microseconds are then normalized so that the
   representation is unique, with

   * "0 <= microseconds < 1000000"

   * "0 <= seconds < 3600*24" (the number of seconds in one day)

   * "-999999999 <= days <= 999999999"

   If any argument is a float and there are fractional microseconds,
   the fractional microseconds left over from all arguments are
   combined and their sum is rounded to the nearest microsecond.  If
   no argument is a float, the conversion and normalization processes
   are exact (no information is lost).

   If the normalized value of days lies outside the indicated range,
   "OverflowError" is raised.

   Note that normalization of negative values may be surprising at
   first. For example,

   >>> from datetime import timedelta
   >>> d = timedelta(microseconds=-1)
   >>> (d.days, d.seconds, d.microseconds)
   (-1, 86399, 999999)

Class attributes are:

timedelta.min

   The most negative "timedelta" object, "timedelta(-999999999)".

timedelta.max

   The most positive "timedelta" object, "timedelta(days=999999999,
   hours=23, minutes=59, seconds=59, microseconds=999999)".

timedelta.resolution

   The smallest possible difference between non-equal "timedelta"
   objects, "timedelta(microseconds=1)".

Note that, because of normalization, "timedelta.max" >
"-timedelta.min". "-timedelta.max" is not representable as a
"timedelta" object.

Instance attributes (read-only):

+--------------------+----------------------------------------------+
| 属性               | Value                                        |
+====================+==============================================+
| "days"             | Between -999999999 and 999999999 inclusive   |
+--------------------+----------------------------------------------+
| "seconds"          | Between 0 and 86399 inclusive                |
+--------------------+----------------------------------------------+
| "microseconds"     | Between 0 and 999999 inclusive               |
+--------------------+----------------------------------------------+

Supported operations:

+----------------------------------+-------------------------------------------------+
| Operation                        | 结果                                            |
+==================================+=================================================+
| "t1 = t2 + t3"                   | Sum of *t2* and *t3*. Afterwards *t1*-*t2* ==   |
|                                  | *t3* and *t1*-*t3* == *t2* are true. (1)        |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 - t3"                   | Difference of *t2* and *t3*. Afterwards *t1* == |
|                                  | *t2* - *t3* and *t2* == *t1* + *t3* are true.   |
|                                  | (1)                                             |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 * i or t1 = i * t2"     | Delta multiplied by an integer or long.         |
|                                  | Afterwards *t1* // i == *t2* is true, provided  |
|                                  | "i != 0".                                       |
+----------------------------------+-------------------------------------------------+
|                                  | In general, *t1* * i == *t1* * (i-1) + *t1* is  |
|                                  | true. (1)                                       |
+----------------------------------+-------------------------------------------------+
| "t1 = t2 // i"                   | The floor is computed and the remainder (if     |
|                                  | any) is thrown away. (3)                        |
+----------------------------------+-------------------------------------------------+
| "+t1"                            | Returns a "timedelta" object with the same      |
|                                  | value. (2)                                      |
+----------------------------------+-------------------------------------------------+
| "-t1"                            | equivalent to "timedelta"(-*t1.days*,           |
|                                  | -*t1.seconds*, -*t1.microseconds*), and to      |
|                                  | *t1** -1. (1)(4)                                |
+----------------------------------+-------------------------------------------------+
| "abs(t)"                         | equivalent to +*t* when "t.days >= 0", and to   |
|                                  | -*t* when "t.days < 0". (2)                     |
+----------------------------------+-------------------------------------------------+
| "str(t)"                         | Returns a string in the form "[D day[s],        |
|                                  | ][H]H:MM:SS[.UUUUUU]", where D is negative for  |
|                                  | negative "t". (5)                               |
+----------------------------------+-------------------------------------------------+
| "repr(t)"                        | Returns a string in the form                    |
|                                  | "datetime.timedelta(D[, S[, U]])", where D is   |
|                                  | negative for negative "t". (5)                  |
+----------------------------------+-------------------------------------------------+

注释:

1. This is exact, but may overflow.

2. This is exact, and cannot overflow.

3. Division by 0 raises "ZeroDivisionError".

4. -*timedelta.max* is not representable as a "timedelta" object.

5. String representations of "timedelta" objects are normalized
   similarly to their internal representation.  This leads to somewhat
   unusual results for negative timedeltas.  For example:

   >>> timedelta(hours=-5)
   datetime.timedelta(-1, 68400)
   >>> print(_)
   -1 day, 19:00:00

In addition to the operations listed above "timedelta" objects support
certain additions and subtractions with "date" and "datetime" objects
(see below).

Comparisons of "timedelta" objects are supported with the "timedelta"
object representing the smaller duration considered to be the smaller
timedelta. In order to stop mixed-type comparisons from falling back
to the default comparison by object address, when a "timedelta" object
is compared to an object of a different type, "TypeError" is raised
unless the comparison is "==" or "!=".  The latter cases return
"False" or "True", respectively.

"timedelta" objects are *hashable* (usable as dictionary keys),
support efficient pickling, and in Boolean contexts, a "timedelta"
object is considered to be true if and only if it isn’t equal to
"timedelta(0)".

Instance methods:

timedelta.total_seconds()

   Return the total number of seconds contained in the duration.
   Equivalent to "(td.microseconds + (td.seconds + td.days * 24 *
   3600) * 10**6) / 10**6" computed with true division enabled.

   Note that for very large time intervals (greater than 270 years on
   most platforms) this method will lose microsecond accuracy.

   2.7 新版功能.

Example usage:

>>> from datetime import timedelta
>>> year = timedelta(days=365)
>>> another_year = timedelta(weeks=40, days=84, hours=23,
...                          minutes=50, seconds=600)  # adds up to 365 days
>>> year.total_seconds()
31536000.0
>>> year == another_year
True
>>> ten_years = 10 * year
>>> ten_years, ten_years.days // 365
(datetime.timedelta(3650), 10)
>>> nine_years = ten_years - year
>>> nine_years, nine_years.days // 365
(datetime.timedelta(3285), 9)
>>> three_years = nine_years // 3;
>>> three_years, three_years.days // 365
(datetime.timedelta(1095), 3)
>>> abs(three_years - ten_years) == 2 * three_years + year
True


8.1.3. "date" Objects
=====================

A "date" object represents a date (year, month and day) in an
idealized calendar, the current Gregorian calendar indefinitely
extended in both directions.  January 1 of year 1 is called day number
1, January 2 of year 1 is called day number 2, and so on.  This
matches the definition of the “proleptic Gregorian” calendar in
Dershowitz and Reingold’s book Calendrical Calculations, where it’s
the base calendar for all computations.  See the book for algorithms
for converting between proleptic Gregorian ordinals and many other
calendar systems.

class datetime.date(year, month, day)

   All arguments are required.  Arguments may be ints or longs, in the
   following ranges:

   * "MINYEAR <= year <= MAXYEAR"

   * "1 <= month <= 12"

   * "1 <= day <= number of days in the given month and year"

   If an argument outside those ranges is given, "ValueError" is
   raised.

Other constructors, all class methods:

classmethod date.today()

   Return the current local date.  This is equivalent to
   "date.fromtimestamp(time.time())".

classmethod date.fromtimestamp(timestamp)

   Return the local date corresponding to the POSIX timestamp, such as
   is returned by "time.time()".  This may raise "ValueError", if the
   timestamp is out of the range of values supported by the platform C
   "localtime()" function. It’s common for this to be restricted to
   years from 1970 through 2038.  Note that on non-POSIX systems that
   include leap seconds in their notion of a timestamp, leap seconds
   are ignored by "fromtimestamp()".

classmethod date.fromordinal(ordinal)

   Return the date corresponding to the proleptic Gregorian ordinal,
   where January 1 of year 1 has ordinal 1.  "ValueError" is raised
   unless "1 <= ordinal <= date.max.toordinal()". For any date *d*,
   "date.fromordinal(d.toordinal()) == d".

Class attributes:

date.min

   The earliest representable date, "date(MINYEAR, 1, 1)".

date.max

   The latest representable date, "date(MAXYEAR, 12, 31)".

date.resolution

   The smallest possible difference between non-equal date objects,
   "timedelta(days=1)".

Instance attributes (read-only):

date.year

   Between "MINYEAR" and "MAXYEAR" inclusive.

date.month

   Between 1 and 12 inclusive.

date.day

   Between 1 and the number of days in the given month of the given
   year.

Supported operations:

+---------------------------------+------------------------------------------------+
| Operation                       | 结果                                           |
+=================================+================================================+
| "date2 = date1 + timedelta"     | *date2* is "timedelta.days" days removed from  |
|                                 | *date1*.  (1)                                  |
+---------------------------------+------------------------------------------------+
| "date2 = date1 - timedelta"     | Computes *date2* such that "date2 + timedelta  |
|                                 | == date1". (2)                                 |
+---------------------------------+------------------------------------------------+
| "timedelta = date1 - date2"     | (3)                                            |
+---------------------------------+------------------------------------------------+
| "date1 < date2"                 | *date1* is considered less than *date2* when   |
|                                 | *date1* precedes *date2* in time. (4)          |
+---------------------------------+------------------------------------------------+

注释:

1. *date2* is moved forward in time if "timedelta.days > 0", or
   backward if "timedelta.days < 0".  Afterward "date2 - date1 ==
   timedelta.days". "timedelta.seconds" and "timedelta.microseconds"
   are ignored. "OverflowError" is raised if "date2.year" would be
   smaller than "MINYEAR" or larger than "MAXYEAR".

2. This isn’t quite equivalent to date1 + (-timedelta), because
   -timedelta in isolation can overflow in cases where date1 -
   timedelta does not. "timedelta.seconds" and
   "timedelta.microseconds" are ignored.

3. This is exact, and cannot overflow.  timedelta.seconds and
   timedelta.microseconds are 0, and date2 + timedelta == date1 after.

4. In other words, "date1 < date2" if and only if
   "date1.toordinal() < date2.toordinal()". In order to stop
   comparison from falling back to the default scheme of comparing
   object addresses, date comparison normally raises "TypeError" if
   the other comparand isn’t also a "date" object. However,
   "NotImplemented" is returned instead if the other comparand has a
   "timetuple()" attribute.  This hook gives other kinds of date
   objects a chance at implementing mixed- type comparison. If not,
   when a "date" object is compared to an object of a different type,
   "TypeError" is raised unless the comparison is "==" or "!=".  The
   latter cases return "False" or "True", respectively.

Dates can be used as dictionary keys. In Boolean contexts, all "date"
objects are considered to be true.

Instance methods:

date.replace(year, month, day)

   Return a date with the same value, except for those parameters
   given new values by whichever keyword arguments are specified.  For
   example, if "d == date(2002, 12, 31)", then "d.replace(day=26) ==
   date(2002, 12, 26)".

date.timetuple()

   Return a "time.struct_time" such as returned by "time.localtime()".
   The hours, minutes and seconds are 0, and the DST flag is -1.
   "d.timetuple()" is equivalent to "time.struct_time((d.year,
   d.month, d.day, 0, 0, 0, d.weekday(), yday, -1))", where "yday =
   d.toordinal() - date(d.year, 1, 1).toordinal() + 1" is the day
   number within the current year starting with "1" for January 1st.

date.toordinal()

   Return the proleptic Gregorian ordinal of the date, where January 1
   of year 1 has ordinal 1.  For any "date" object *d*,
   "date.fromordinal(d.toordinal()) == d".

date.weekday()

   Return the day of the week as an integer, where Monday is 0 and
   Sunday is 6. For example, "date(2002, 12, 4).weekday() == 2", a
   Wednesday. See also "isoweekday()".

date.isoweekday()

   Return the day of the week as an integer, where Monday is 1 and
   Sunday is 7. For example, "date(2002, 12, 4).isoweekday() == 3", a
   Wednesday. See also "weekday()", "isocalendar()".

date.isocalendar()

   Return a 3-tuple, (ISO year, ISO week number, ISO weekday).

   The ISO calendar is a widely used variant of the Gregorian
   calendar. See
   https://www.staff.science.uu.nl/~gent0113/calendar/isocalendar.htm
   for a good explanation.

   The ISO year consists of 52 or 53 full weeks, and where a week
   starts on a Monday and ends on a Sunday.  The first week of an ISO
   year is the first (Gregorian) calendar week of a year containing a
   Thursday. This is called week number 1, and the ISO year of that
   Thursday is the same as its Gregorian year.

   For example, 2004 begins on a Thursday, so the first week of ISO
   year 2004 begins on Monday, 29 Dec 2003 and ends on Sunday, 4 Jan
   2004, so that "date(2003, 12, 29).isocalendar() == (2004, 1, 1)"
   and "date(2004, 1, 4).isocalendar() == (2004, 1, 7)".

date.isoformat()

   Return a string representing the date in ISO 8601 format, ‘YYYY-MM-
   DD’.  For example, "date(2002, 12, 4).isoformat() == '2002-12-04'".

date.__str__()

   For a date *d*, "str(d)" is equivalent to "d.isoformat()".

date.ctime()

   Return a string representing the date, for example "date(2002, 12,
   4).ctime() == 'Wed Dec 4 00:00:00 2002'". "d.ctime()" is equivalent
   to "time.ctime(time.mktime(d.timetuple()))" on platforms where the
   native C "ctime()" function (which "time.ctime()" invokes, but
   which "date.ctime()" does not invoke) conforms to the C standard.

date.strftime(format)

   Return a string representing the date, controlled by an explicit
   format string. Format codes referring to hours, minutes or seconds
   will see 0 values. For a complete list of formatting directives,
   see section strftime() and strptime() Behavior.

date.__format__(format)

   Same as "date.strftime()". This makes it possible to specify a
   format string for a "date" object when using "str.format()". See
   section strftime() and strptime() Behavior.

Example of counting days to an event:

   >>> import time
   >>> from datetime import date
   >>> today = date.today()
   >>> today
   datetime.date(2007, 12, 5)
   >>> today == date.fromtimestamp(time.time())
   True
   >>> my_birthday = date(today.year, 6, 24)
   >>> if my_birthday < today:
   ...     my_birthday = my_birthday.replace(year=today.year + 1)
   >>> my_birthday
   datetime.date(2008, 6, 24)
   >>> time_to_birthday = abs(my_birthday - today)
   >>> time_to_birthday.days
   202

Example of working with "date":

   >>> from datetime import date
   >>> d = date.fromordinal(730920) # 730920th day after 1. 1. 0001
   >>> d
   datetime.date(2002, 3, 11)
   >>> t = d.timetuple()
   >>> for i in t:     
   ...     print i
   2002                # year
   3                   # month
   11                  # day
   0
   0
   0
   0                   # weekday (0 = Monday)
   70                  # 70th day in the year
   -1
   >>> ic = d.isocalendar()
   >>> for i in ic:    
   ...     print i
   2002                # ISO year
   11                  # ISO week number
   1                   # ISO day number ( 1 = Monday )
   >>> d.isoformat()
   '2002-03-11'
   >>> d.strftime("%d/%m/%y")
   '11/03/02'
   >>> d.strftime("%A %d. %B %Y")
   'Monday 11. March 2002'
   >>> 'The {1} is {0:%d}, the {2} is {0:%B}.'.format(d, "day", "month")
   'The day is 11, the month is March.'


8.1.4. "datetime" Objects
=========================

A "datetime" object is a single object containing all the information
from a "date" object and a "time" object.  Like a "date" object,
"datetime" assumes the current Gregorian calendar extended in both
directions; like a time object, "datetime" assumes there are exactly
3600*24 seconds in every day.

Constructor:

class datetime.datetime(year, month, day[, hour[, minute[, second[, microsecond[, tzinfo]]]]])

   The year, month and day arguments are required.  *tzinfo* may be
   "None", or an instance of a "tzinfo" subclass.  The remaining
   arguments may be ints or longs, in the following ranges:

   * "MINYEAR <= year <= MAXYEAR"

   * "1 <= month <= 12"

   * "1 <= day <= number of days in the given month and year"

   * "0 <= hour < 24"

   * "0 <= minute < 60"

   * "0 <= second < 60"

   * "0 <= microsecond < 1000000"

   If an argument outside those ranges is given, "ValueError" is
   raised.

Other constructors, all class methods:

classmethod datetime.today()

   Return the current local datetime, with "tzinfo" "None". This is
   equivalent to "datetime.fromtimestamp(time.time())". See also
   "now()", "fromtimestamp()".

classmethod datetime.now([tz])

   Return the current local date and time.  If optional argument *tz*
   is "None" or not specified, this is like "today()", but, if
   possible, supplies more precision than can be gotten from going
   through a "time.time()" timestamp (for example, this may be
   possible on platforms supplying the C "gettimeofday()" function).

   If *tz* is not "None", it must be an instance of a "tzinfo"
   subclass, and the current date and time are converted to *tz*’s
   time zone.  In this case the result is equivalent to
   "tz.fromutc(datetime.utcnow().replace(tzinfo=tz))". See also
   "today()", "utcnow()".

classmethod datetime.utcnow()

   Return the current UTC date and time, with "tzinfo" "None". This is
   like "now()", but returns the current UTC date and time, as a naive
   "datetime" object. See also "now()".

classmethod datetime.fromtimestamp(timestamp[, tz])

   Return the local date and time corresponding to the POSIX
   timestamp, such as is returned by "time.time()". If optional
   argument *tz* is "None" or not specified, the timestamp is
   converted to the platform’s local date and time, and the returned
   "datetime" object is naive.

   If *tz* is not "None", it must be an instance of a "tzinfo"
   subclass, and the timestamp is converted to *tz*’s time zone.  In
   this case the result is equivalent to "tz.fromutc(datetime.utcfrom
   timestamp(timestamp).replace(tzinfo=tz))".

   "fromtimestamp()" may raise "ValueError", if the timestamp is out
   of the range of values supported by the platform C "localtime()" or
   "gmtime()" functions.  It’s common for this to be restricted to
   years in 1970 through 2038. Note that on non-POSIX systems that
   include leap seconds in their notion of a timestamp, leap seconds
   are ignored by "fromtimestamp()", and then it’s possible to have
   two timestamps differing by a second that yield identical
   "datetime" objects. See also "utcfromtimestamp()".

classmethod datetime.utcfromtimestamp(timestamp)

   Return the UTC "datetime" corresponding to the POSIX timestamp,
   with "tzinfo" "None". This may raise "ValueError", if the timestamp
   is out of the range of values supported by the platform C
   "gmtime()" function. It’s common for this to be restricted to years
   in 1970 through 2038. See also "fromtimestamp()".

classmethod datetime.fromordinal(ordinal)

   Return the "datetime" corresponding to the proleptic Gregorian
   ordinal, where January 1 of year 1 has ordinal 1. "ValueError" is
   raised unless "1 <= ordinal <= datetime.max.toordinal()".  The
   hour, minute, second and microsecond of the result are all 0, and
   "tzinfo" is "None".

classmethod datetime.combine(date, time)

   Return a new "datetime" object whose date components are equal to
   the given "date" object’s, and whose time components and "tzinfo"
   attributes are equal to the given "time" object’s. For any
   "datetime" object *d*, "d == datetime.combine(d.date(),
   d.timetz())".  If date is a "datetime" object, its time components
   and "tzinfo" attributes are ignored.

classmethod datetime.strptime(date_string, format)

   Return a "datetime" corresponding to *date_string*, parsed
   according to *format*.  This is equivalent to
   "datetime(*(time.strptime(date_string, format)[0:6]))".
   "ValueError" is raised if the date_string and format can’t be
   parsed by "time.strptime()" or if it returns a value which isn’t a
   time tuple. For a complete list of formatting directives, see
   section strftime() and strptime() Behavior.

   2.5 新版功能.

Class attributes:

datetime.min

   The earliest representable "datetime", "datetime(MINYEAR, 1, 1,
   tzinfo=None)".

datetime.max

   The latest representable "datetime", "datetime(MAXYEAR, 12, 31, 23,
   59, 59, 999999, tzinfo=None)".

datetime.resolution

   The smallest possible difference between non-equal "datetime"
   objects, "timedelta(microseconds=1)".

Instance attributes (read-only):

datetime.year

   Between "MINYEAR" and "MAXYEAR" inclusive.

datetime.month

   Between 1 and 12 inclusive.

datetime.day

   Between 1 and the number of days in the given month of the given
   year.

datetime.hour

   In "range(24)".

datetime.minute

   In "range(60)".

datetime.second

   In "range(60)".

datetime.microsecond

   In "range(1000000)".

datetime.tzinfo

   The object passed as the *tzinfo* argument to the "datetime"
   constructor, or "None" if none was passed.

Supported operations:

+-----------------------------------------+----------------------------------+
| Operation                               | 结果                             |
+=========================================+==================================+
| "datetime2 = datetime1 + timedelta"     | (1)                              |
+-----------------------------------------+----------------------------------+
| "datetime2 = datetime1 - timedelta"     | (2)                              |
+-----------------------------------------+----------------------------------+
| "timedelta = datetime1 - datetime2"     | (3)                              |
+-----------------------------------------+----------------------------------+
| "datetime1 < datetime2"                 | Compares "datetime" to           |
|                                         | "datetime". (4)                  |
+-----------------------------------------+----------------------------------+

1. datetime2 is a duration of timedelta removed from datetime1,
   moving forward in time if "timedelta.days" > 0, or backward if
   "timedelta.days" < 0.  The result has the same "tzinfo" attribute
   as the input datetime, and datetime2 - datetime1 == timedelta
   after. "OverflowError" is raised if datetime2.year would be smaller
   than "MINYEAR" or larger than "MAXYEAR". Note that no time zone
   adjustments are done even if the input is an aware object.

2. Computes the datetime2 such that datetime2 + timedelta ==
   datetime1. As for addition, the result has the same "tzinfo"
   attribute as the input datetime, and no time zone adjustments are
   done even if the input is aware. This isn’t quite equivalent to
   datetime1 + (-timedelta), because -timedelta in isolation can
   overflow in cases where datetime1 - timedelta does not.

3. Subtraction of a "datetime" from a "datetime" is defined only if
   both operands are naive, or if both are aware.  If one is aware and
   the other is naive, "TypeError" is raised.

   If both are naive, or both are aware and have the same "tzinfo"
   attribute, the "tzinfo" attributes are ignored, and the result is a
   "timedelta" object *t* such that "datetime2 + t == datetime1".  No
   time zone adjustments are done in this case.

   If both are aware and have different "tzinfo" attributes, "a-b"
   acts as if *a* and *b* were first converted to naive UTC datetimes
   first.  The result is "(a.replace(tzinfo=None) - a.utcoffset()) -
   (b.replace(tzinfo=None) - b.utcoffset())" except that the
   implementation never overflows.

4. *datetime1* is considered less than *datetime2* when *datetime1*
   precedes *datetime2* in time.

   If one comparand is naive and the other is aware, "TypeError" is
   raised. If both comparands are aware, and have the same "tzinfo"
   attribute, the common "tzinfo" attribute is ignored and the base
   datetimes are compared.  If both comparands are aware and have
   different "tzinfo" attributes, the comparands are first adjusted by
   subtracting their UTC offsets (obtained from "self.utcoffset()").

   注解: In order to stop comparison from falling back to the
     default scheme of comparing object addresses, datetime comparison
     normally raises "TypeError" if the other comparand isn’t also a
     "datetime" object.  However, "NotImplemented" is returned instead
     if the other comparand has a "timetuple()" attribute.  This hook
     gives other kinds of date objects a chance at implementing mixed-
     type comparison.  If not, when a "datetime" object is compared to
     an object of a different type, "TypeError" is raised unless the
     comparison is "==" or "!=".  The latter cases return "False" or
     "True", respectively.

"datetime" objects can be used as dictionary keys. In Boolean
contexts, all "datetime" objects are considered to be true.

Instance methods:

datetime.date()

   Return "date" object with same year, month and day.

datetime.time()

   Return "time" object with same hour, minute, second and
   microsecond. "tzinfo" is "None".  See also method "timetz()".

datetime.timetz()

   Return "time" object with same hour, minute, second, microsecond,
   and tzinfo attributes.  See also method "time()".

datetime.replace([year[, month[, day[, hour[, minute[, second[, microsecond[, tzinfo]]]]]]]])

   Return a datetime with the same attributes, except for those
   attributes given new values by whichever keyword arguments are
   specified.  Note that "tzinfo=None" can be specified to create a
   naive datetime from an aware datetime with no conversion of date
   and time data.

datetime.astimezone(tz)

   Return a "datetime" object with new "tzinfo" attribute *tz*,
   adjusting the date and time data so the result is the same UTC time
   as *self*, but in *tz*’s local time.

   *tz* must be an instance of a "tzinfo" subclass, and its
   "utcoffset()" and "dst()" methods must not return "None".  *self*
   must be aware ("self.tzinfo" must not be "None", and
   "self.utcoffset()" must not return "None").

   If "self.tzinfo" is *tz*, "self.astimezone(tz)" is equal to *self*:
   no adjustment of date or time data is performed. Else the result is
   local time in time zone *tz*, representing the same UTC time as
   *self*:  after "astz = dt.astimezone(tz)", "astz -
   astz.utcoffset()" will usually have the same date and time data as
   "dt - dt.utcoffset()". The discussion of class "tzinfo" explains
   the cases at Daylight Saving Time transition boundaries where this
   cannot be achieved (an issue only if *tz* models both standard and
   daylight time).

   If you merely want to attach a time zone object *tz* to a datetime
   *dt* without adjustment of date and time data, use
   "dt.replace(tzinfo=tz)".  If you merely want to remove the time
   zone object from an aware datetime *dt* without conversion of date
   and time data, use "dt.replace(tzinfo=None)".

   Note that the default "tzinfo.fromutc()" method can be overridden
   in a "tzinfo" subclass to affect the result returned by
   "astimezone()". Ignoring error cases, "astimezone()" acts like:

      def astimezone(self, tz):
          if self.tzinfo is tz:
              return self
          # Convert self to UTC, and attach the new time zone object.
          utc = (self - self.utcoffset()).replace(tzinfo=tz)
          # Convert from UTC to tz's local time.
          return tz.fromutc(utc)

datetime.utcoffset()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.utcoffset(self)", and raises an exception if the
   latter doesn’t return "None", or a "timedelta" object representing
   a whole number of minutes with magnitude less than one day.

datetime.dst()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.dst(self)", and raises an exception if the latter
   doesn’t return "None", or a "timedelta" object representing a whole
   number of minutes with magnitude less than one day.

datetime.tzname()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.tzname(self)", raises an exception if the latter
   doesn’t return "None" or a string object,

datetime.timetuple()

   Return a "time.struct_time" such as returned by "time.localtime()".
   "d.timetuple()" is equivalent to "time.struct_time((d.year,
   d.month, d.day, d.hour, d.minute, d.second, d.weekday(), yday,
   dst))", where "yday = d.toordinal() - date(d.year, 1,
   1).toordinal() + 1" is the day number within the current year
   starting with "1" for January 1st. The "tm_isdst" flag of the
   result is set according to the "dst()" method: "tzinfo" is "None"
   or "dst()" returns "None", "tm_isdst" is set to "-1"; else if
   "dst()" returns a non-zero value, "tm_isdst" is set to "1"; else
   "tm_isdst" is set to "0".

datetime.utctimetuple()

   If "datetime" instance *d* is naive, this is the same as
   "d.timetuple()" except that "tm_isdst" is forced to 0 regardless of
   what "d.dst()" returns.  DST is never in effect for a UTC time.

   If *d* is aware, *d* is normalized to UTC time, by subtracting
   "d.utcoffset()", and a "time.struct_time" for the normalized time
   is returned.  "tm_isdst" is forced to 0. Note that the result’s
   "tm_year" member may be "MINYEAR"-1 or "MAXYEAR"+1, if *d*.year was
   "MINYEAR" or "MAXYEAR" and UTC adjustment spills over a year
   boundary.

datetime.toordinal()

   Return the proleptic Gregorian ordinal of the date.  The same as
   "self.date().toordinal()".

datetime.weekday()

   Return the day of the week as an integer, where Monday is 0 and
   Sunday is 6. The same as "self.date().weekday()". See also
   "isoweekday()".

datetime.isoweekday()

   Return the day of the week as an integer, where Monday is 1 and
   Sunday is 7. The same as "self.date().isoweekday()". See also
   "weekday()", "isocalendar()".

datetime.isocalendar()

   Return a 3-tuple, (ISO year, ISO week number, ISO weekday).  The
   same as "self.date().isocalendar()".

datetime.isoformat([sep])

   Return a string representing the date and time in ISO 8601 format,
   YYYY-MM-DDTHH:MM:SS.mmmmmm or, if "microsecond" is 0, YYYY-MM-
   DDTHH:MM:SS

   If "utcoffset()" does not return "None", a 6-character string is
   appended, giving the UTC offset in (signed) hours and minutes:
   YYYY-MM-DDTHH:MM:SS.mmmmmm+HH:MM or, if "microsecond" is 0 YYYY-MM-
   DDTHH:MM:SS+HH:MM

   The optional argument *sep* (default "'T'") is a one-character
   separator, placed between the date and time portions of the result.
   For example,

   >>> from datetime import tzinfo, timedelta, datetime
   >>> class TZ(tzinfo):
   ...     def utcoffset(self, dt): return timedelta(minutes=-399)
   ...
   >>> datetime(2002, 12, 25, tzinfo=TZ()).isoformat(' ')
   '2002-12-25 00:00:00-06:39'

datetime.__str__()

   For a "datetime" instance *d*, "str(d)" is equivalent to
   "d.isoformat(' ')".

datetime.ctime()

   Return a string representing the date and time, for example
   "datetime(2002, 12, 4, 20, 30, 40).ctime() == 'Wed Dec  4 20:30:40
   2002'". "d.ctime()" is equivalent to
   "time.ctime(time.mktime(d.timetuple()))" on platforms where the
   native C "ctime()" function (which "time.ctime()" invokes, but
   which "datetime.ctime()" does not invoke) conforms to the C
   standard.

datetime.strftime(format)

   Return a string representing the date and time, controlled by an
   explicit format string.  For a complete list of formatting
   directives, see section strftime() and strptime() Behavior.

datetime.__format__(format)

   Same as "datetime.strftime()".  This makes it possible to specify a
   format string for a "datetime" object when using "str.format()".
   See section strftime() and strptime() Behavior.

Examples of working with datetime objects:

   >>> from datetime import datetime, date, time
   >>> # Using datetime.combine()
   >>> d = date(2005, 7, 14)
   >>> t = time(12, 30)
   >>> datetime.combine(d, t)
   datetime.datetime(2005, 7, 14, 12, 30)
   >>> # Using datetime.now() or datetime.utcnow()
   >>> datetime.now()   
   datetime.datetime(2007, 12, 6, 16, 29, 43, 79043)   # GMT +1
   >>> datetime.utcnow()   
   datetime.datetime(2007, 12, 6, 15, 29, 43, 79060)
   >>> # Using datetime.strptime()
   >>> dt = datetime.strptime("21/11/06 16:30", "%d/%m/%y %H:%M")
   >>> dt
   datetime.datetime(2006, 11, 21, 16, 30)
   >>> # Using datetime.timetuple() to get tuple of all attributes
   >>> tt = dt.timetuple()
   >>> for it in tt:   
   ...     print it
   ...
   2006    # year
   11      # month
   21      # day
   16      # hour
   30      # minute
   0       # second
   1       # weekday (0 = Monday)
   325     # number of days since 1st January
   -1      # dst - method tzinfo.dst() returned None
   >>> # Date in ISO format
   >>> ic = dt.isocalendar()
   >>> for it in ic:   
   ...     print it
   ...
   2006    # ISO year
   47      # ISO week
   2       # ISO weekday
   >>> # Formatting datetime
   >>> dt.strftime("%A, %d. %B %Y %I:%M%p")
   'Tuesday, 21. November 2006 04:30PM'
   >>> 'The {1} is {0:%d}, the {2} is {0:%B}, the {3} is {0:%I:%M%p}.'.format(dt, "day", "month", "time")
   'The day is 21, the month is November, the time is 04:30PM.'

Using datetime with tzinfo:

>>> from datetime import timedelta, datetime, tzinfo
>>> class GMT1(tzinfo):
...     def utcoffset(self, dt):
...         return timedelta(hours=1) + self.dst(dt)
...     def dst(self, dt):
...         # DST starts last Sunday in March
...         d = datetime(dt.year, 4, 1)   # ends last Sunday in October
...         self.dston = d - timedelta(days=d.weekday() + 1)
...         d = datetime(dt.year, 11, 1)
...         self.dstoff = d - timedelta(days=d.weekday() + 1)
...         if self.dston <=  dt.replace(tzinfo=None) < self.dstoff:
...             return timedelta(hours=1)
...         else:
...             return timedelta(0)
...     def tzname(self,dt):
...          return "GMT +1"
...
>>> class GMT2(tzinfo):
...     def utcoffset(self, dt):
...         return timedelta(hours=2) + self.dst(dt)
...     def dst(self, dt):
...         d = datetime(dt.year, 4, 1)
...         self.dston = d - timedelta(days=d.weekday() + 1)
...         d = datetime(dt.year, 11, 1)
...         self.dstoff = d - timedelta(days=d.weekday() + 1)
...         if self.dston <=  dt.replace(tzinfo=None) < self.dstoff:
...             return timedelta(hours=1)
...         else:
...             return timedelta(0)
...     def tzname(self,dt):
...         return "GMT +2"
...
>>> gmt1 = GMT1()
>>> # Daylight Saving Time
>>> dt1 = datetime(2006, 11, 21, 16, 30, tzinfo=gmt1)
>>> dt1.dst()
datetime.timedelta(0)
>>> dt1.utcoffset()
datetime.timedelta(0, 3600)
>>> dt2 = datetime(2006, 6, 14, 13, 0, tzinfo=gmt1)
>>> dt2.dst()
datetime.timedelta(0, 3600)
>>> dt2.utcoffset()
datetime.timedelta(0, 7200)
>>> # Convert datetime to another time zone
>>> dt3 = dt2.astimezone(GMT2())
>>> dt3     # doctest: +ELLIPSIS
datetime.datetime(2006, 6, 14, 14, 0, tzinfo=<GMT2 object at 0x...>)
>>> dt2     # doctest: +ELLIPSIS
datetime.datetime(2006, 6, 14, 13, 0, tzinfo=<GMT1 object at 0x...>)
>>> dt2.utctimetuple() == dt3.utctimetuple()
True


8.1.5. "time" Objects
=====================

A time object represents a (local) time of day, independent of any
particular day, and subject to adjustment via a "tzinfo" object.

class datetime.time([hour[, minute[, second[, microsecond[, tzinfo]]]]])

   All arguments are optional.  *tzinfo* may be "None", or an instance
   of a "tzinfo" subclass.  The remaining arguments may be ints or
   longs, in the following ranges:

   * "0 <= hour < 24"

   * "0 <= minute < 60"

   * "0 <= second < 60"

   * "0 <= microsecond < 1000000".

   If an argument outside those ranges is given, "ValueError" is
   raised.  All default to "0" except *tzinfo*, which defaults to
   "None".

Class attributes:

time.min

   The earliest representable "time", "time(0, 0, 0, 0)".

time.max

   The latest representable "time", "time(23, 59, 59, 999999)".

time.resolution

   The smallest possible difference between non-equal "time" objects,
   "timedelta(microseconds=1)", although note that arithmetic on
   "time" objects is not supported.

Instance attributes (read-only):

time.hour

   In "range(24)".

time.minute

   In "range(60)".

time.second

   In "range(60)".

time.microsecond

   In "range(1000000)".

time.tzinfo

   The object passed as the tzinfo argument to the "time" constructor,
   or "None" if none was passed.

Supported operations:

* comparison of "time" to "time", where *a* is considered less than
  *b* when *a* precedes *b* in time.  If one comparand is naive and
  the other is aware, "TypeError" is raised.  If both comparands are
  aware, and have the same "tzinfo" attribute, the common "tzinfo"
  attribute is ignored and the base times are compared.  If both
  comparands are aware and have different "tzinfo" attributes, the
  comparands are first adjusted by subtracting their UTC offsets
  (obtained from "self.utcoffset()"). In order to stop mixed-type
  comparisons from falling back to the default comparison by object
  address, when a "time" object is compared to an object of a
  different type, "TypeError" is raised unless the comparison is "=="
  or "!=".  The latter cases return "False" or "True", respectively.

* hash, use as dict key

* efficient pickling

* in Boolean contexts, a "time" object is considered to be true if
  and only if, after converting it to minutes and subtracting
  "utcoffset()" (or "0" if that’s "None"), the result is non-zero.

Instance methods:

time.replace([hour[, minute[, second[, microsecond[, tzinfo]]]]])

   Return a "time" with the same value, except for those attributes
   given new values by whichever keyword arguments are specified.
   Note that "tzinfo=None" can be specified to create a naive "time"
   from an aware "time", without conversion of the time data.

time.isoformat()

   Return a string representing the time in ISO 8601 format,
   HH:MM:SS.mmmmmm or, if self.microsecond is 0, HH:MM:SS If
   "utcoffset()" does not return "None", a 6-character string is
   appended, giving the UTC offset in (signed) hours and minutes:
   HH:MM:SS.mmmmmm+HH:MM or, if self.microsecond is 0, HH:MM:SS+HH:MM

time.__str__()

   For a time *t*, "str(t)" is equivalent to "t.isoformat()".

time.strftime(format)

   Return a string representing the time, controlled by an explicit
   format string. For a complete list of formatting directives, see
   section strftime() and strptime() Behavior.

time.__format__(format)

   Same as "time.strftime()". This makes it possible to specify a
   format string for a "time" object when using "str.format()". See
   section strftime() and strptime() Behavior.

time.utcoffset()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.utcoffset(None)", and raises an exception if the
   latter doesn’t return "None" or a "timedelta" object representing a
   whole number of minutes with magnitude less than one day.

time.dst()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.dst(None)", and raises an exception if the latter
   doesn’t return "None", or a "timedelta" object representing a whole
   number of minutes with magnitude less than one day.

time.tzname()

   If "tzinfo" is "None", returns "None", else returns
   "self.tzinfo.tzname(None)", or raises an exception if the latter
   doesn’t return "None" or a string object.

示例:

>>> from datetime import time, tzinfo, timedelta
>>> class GMT1(tzinfo):
...     def utcoffset(self, dt):
...         return timedelta(hours=1)
...     def dst(self, dt):
...         return timedelta(0)
...     def tzname(self,dt):
...         return "Europe/Prague"
...
>>> t = time(12, 10, 30, tzinfo=GMT1())
>>> t                               # doctest: +ELLIPSIS
datetime.time(12, 10, 30, tzinfo=<GMT1 object at 0x...>)
>>> gmt = GMT1()
>>> t.isoformat()
'12:10:30+01:00'
>>> t.dst()
datetime.timedelta(0)
>>> t.tzname()
'Europe/Prague'
>>> t.strftime("%H:%M:%S %Z")
'12:10:30 Europe/Prague'
>>> 'The {} is {:%H:%M}.'.format("time", t)
'The time is 12:10.'


8.1.6. "tzinfo" Objects
=======================

class datetime.tzinfo

   This is an abstract base class, meaning that this class should not
   be instantiated directly.  You need to derive a concrete subclass,
   and (at least) supply implementations of the standard "tzinfo"
   methods needed by the "datetime" methods you use.  The "datetime"
   module does not supply any concrete subclasses of "tzinfo".

   An instance of (a concrete subclass of) "tzinfo" can be passed to
   the constructors for "datetime" and "time" objects. The latter
   objects view their attributes as being in local time, and the
   "tzinfo" object supports methods revealing offset of local time
   from UTC, the name of the time zone, and DST offset, all relative
   to a date or time object passed to them.

   Special requirement for pickling:  A "tzinfo" subclass must have an
   "__init__()" method that can be called with no arguments, else it
   can be pickled but possibly not unpickled again.  This is a
   technical requirement that may be relaxed in the future.

   A concrete subclass of "tzinfo" may need to implement the following
   methods.  Exactly which methods are needed depends on the uses made
   of aware "datetime" objects.  If in doubt, simply implement all of
   them.

tzinfo.utcoffset(self, dt)

   Return offset of local time from UTC, in minutes east of UTC.  If
   local time is west of UTC, this should be negative.  Note that this
   is intended to be the total offset from UTC; for example, if a
   "tzinfo" object represents both time zone and DST adjustments,
   "utcoffset()" should return their sum.  If the UTC offset isn’t
   known, return "None".  Else the value returned must be a
   "timedelta" object specifying a whole number of minutes in the
   range -1439 to 1439 inclusive (1440 = 24*60; the magnitude of the
   offset must be less than one day).  Most implementations of
   "utcoffset()" will probably look like one of these two:

      return CONSTANT                 # fixed-offset class
      return CONSTANT + self.dst(dt)  # daylight-aware class

   If "utcoffset()" does not return "None", "dst()" should not return
   "None" either.

   The default implementation of "utcoffset()" raises
   "NotImplementedError".

tzinfo.dst(self, dt)

   Return the daylight saving time (DST) adjustment, in minutes east
   of UTC, or "None" if DST information isn’t known.  Return
   "timedelta(0)" if DST is not in effect. If DST is in effect, return
   the offset as a "timedelta" object (see "utcoffset()" for details).
   Note that DST offset, if applicable, has already been added to the
   UTC offset returned by "utcoffset()", so there’s no need to consult
   "dst()" unless you’re interested in obtaining DST info separately.
   For example, "datetime.timetuple()" calls its "tzinfo" attribute’s
   "dst()" method to determine how the "tm_isdst" flag should be set,
   and "tzinfo.fromutc()" calls "dst()" to account for DST changes
   when crossing time zones.

   An instance *tz* of a "tzinfo" subclass that models both standard
   and daylight times must be consistent in this sense:

   "tz.utcoffset(dt) - tz.dst(dt)"

   must return the same result for every "datetime" *dt* with
   "dt.tzinfo == tz"  For sane "tzinfo" subclasses, this expression
   yields the time zone’s “standard offset”, which should not depend
   on the date or the time, but only on geographic location.  The
   implementation of "datetime.astimezone()" relies on this, but
   cannot detect violations; it’s the programmer’s responsibility to
   ensure it.  If a "tzinfo" subclass cannot guarantee this, it may be
   able to override the default implementation of "tzinfo.fromutc()"
   to work correctly with "astimezone()" regardless.

   Most implementations of "dst()" will probably look like one of
   these two:

      def dst(self, dt):
          # a fixed-offset class:  doesn't account for DST
          return timedelta(0)

   or

      def dst(self, dt):
          # Code to set dston and dstoff to the time zone's DST
          # transition times based on the input dt.year, and expressed
          # in standard local time.  Then

          if dston <= dt.replace(tzinfo=None) < dstoff:
              return timedelta(hours=1)
          else:
              return timedelta(0)

   The default implementation of "dst()" raises "NotImplementedError".

tzinfo.tzname(self, dt)

   Return the time zone name corresponding to the "datetime" object
   *dt*, as a string. Nothing about string names is defined by the
   "datetime" module, and there’s no requirement that it mean anything
   in particular.  For example, “GMT”, “UTC”, “-500”, “-5:00”, “EDT”,
   “US/Eastern”, “America/New York” are all valid replies.  Return
   "None" if a string name isn’t known.  Note that this is a method
   rather than a fixed string primarily because some "tzinfo"
   subclasses will wish to return different names depending on the
   specific value of *dt* passed, especially if the "tzinfo" class is
   accounting for daylight time.

   The default implementation of "tzname()" raises
   "NotImplementedError".

These methods are called by a "datetime" or "time" object, in response
to their methods of the same names.  A "datetime" object passes itself
as the argument, and a "time" object passes "None" as the argument.  A
"tzinfo" subclass’s methods should therefore be prepared to accept a
*dt* argument of "None", or of class "datetime".

When "None" is passed, it’s up to the class designer to decide the
best response.  For example, returning "None" is appropriate if the
class wishes to say that time objects don’t participate in the
"tzinfo" protocols.  It may be more useful for "utcoffset(None)" to
return the standard UTC offset, as there is no other convention for
discovering the standard offset.

When a "datetime" object is passed in response to a "datetime" method,
"dt.tzinfo" is the same object as *self*.  "tzinfo" methods can rely
on this, unless user code calls "tzinfo" methods directly.  The intent
is that the "tzinfo" methods interpret *dt* as being in local time,
and not need worry about objects in other timezones.

There is one more "tzinfo" method that a subclass may wish to
override:

tzinfo.fromutc(self, dt)

   This is called from the default "datetime.astimezone()"
   implementation.  When called from that, "dt.tzinfo" is *self*, and
   *dt*’s date and time data are to be viewed as expressing a UTC
   time.  The purpose of "fromutc()" is to adjust the date and time
   data, returning an equivalent datetime in *self*’s local time.

   Most "tzinfo" subclasses should be able to inherit the default
   "fromutc()" implementation without problems.  It’s strong enough to
   handle fixed-offset time zones, and time zones accounting for both
   standard and daylight time, and the latter even if the DST
   transition times differ in different years.  An example of a time
   zone the default "fromutc()" implementation may not handle
   correctly in all cases is one where the standard offset (from UTC)
   depends on the specific date and time passed, which can happen for
   political reasons. The default implementations of "astimezone()"
   and "fromutc()" may not produce the result you want if the result
   is one of the hours straddling the moment the standard offset
   changes.

   Skipping code for error cases, the default "fromutc()"
   implementation acts like:

      def fromutc(self, dt):
          # raise ValueError error if dt.tzinfo is not self
          dtoff = dt.utcoffset()
          dtdst = dt.dst()
          # raise ValueError if dtoff is None or dtdst is None
          delta = dtoff - dtdst  # this is self's standard offset
          if delta:
              dt += delta   # convert to standard local time
              dtdst = dt.dst()
              # raise ValueError if dtdst is None
          if dtdst:
              return dt + dtdst
          else:
              return dt

Example "tzinfo" classes:

   from datetime import tzinfo, timedelta, datetime

   ZERO = timedelta(0)
   HOUR = timedelta(hours=1)

   # A UTC class.

   class UTC(tzinfo):
       """UTC"""

       def utcoffset(self, dt):
           return ZERO

       def tzname(self, dt):
           return "UTC"

       def dst(self, dt):
           return ZERO

   utc = UTC()

   # A class building tzinfo objects for fixed-offset time zones.
   # Note that FixedOffset(0, "UTC") is a different way to build a
   # UTC tzinfo object.

   class FixedOffset(tzinfo):
       """Fixed offset in minutes east from UTC."""

       def __init__(self, offset, name):
           self.__offset = timedelta(minutes = offset)
           self.__name = name

       def utcoffset(self, dt):
           return self.__offset

       def tzname(self, dt):
           return self.__name

       def dst(self, dt):
           return ZERO

   # A class capturing the platform's idea of local time.

   import time as _time

   STDOFFSET = timedelta(seconds = -_time.timezone)
   if _time.daylight:
       DSTOFFSET = timedelta(seconds = -_time.altzone)
   else:
       DSTOFFSET = STDOFFSET

   DSTDIFF = DSTOFFSET - STDOFFSET

   class LocalTimezone(tzinfo):

       def utcoffset(self, dt):
           if self._isdst(dt):
               return DSTOFFSET
           else:
               return STDOFFSET

       def dst(self, dt):
           if self._isdst(dt):
               return DSTDIFF
           else:
               return ZERO

       def tzname(self, dt):
           return _time.tzname[self._isdst(dt)]

       def _isdst(self, dt):
           tt = (dt.year, dt.month, dt.day,
                 dt.hour, dt.minute, dt.second,
                 dt.weekday(), 0, 0)
           stamp = _time.mktime(tt)
           tt = _time.localtime(stamp)
           return tt.tm_isdst > 0

   Local = LocalTimezone()


   # A complete implementation of current DST rules for major US time zones.

   def first_sunday_on_or_after(dt):
       days_to_go = 6 - dt.weekday()
       if days_to_go:
           dt += timedelta(days_to_go)
       return dt


   # US DST Rules
   #
   # This is a simplified (i.e., wrong for a few cases) set of rules for US
   # DST start and end times. For a complete and up-to-date set of DST rules
   # and timezone definitions, visit the Olson Database (or try pytz):
   # http://www.twinsun.com/tz/tz-link.htm
   # http://sourceforge.net/projects/pytz/ (might not be up-to-date)
   #
   # In the US, since 2007, DST starts at 2am (standard time) on the second
   # Sunday in March, which is the first Sunday on or after Mar 8.
   DSTSTART_2007 = datetime(1, 3, 8, 2)
   # and ends at 2am (DST time; 1am standard time) on the first Sunday of Nov.
   DSTEND_2007 = datetime(1, 11, 1, 1)
   # From 1987 to 2006, DST used to start at 2am (standard time) on the first
   # Sunday in April and to end at 2am (DST time; 1am standard time) on the last
   # Sunday of October, which is the first Sunday on or after Oct 25.
   DSTSTART_1987_2006 = datetime(1, 4, 1, 2)
   DSTEND_1987_2006 = datetime(1, 10, 25, 1)
   # From 1967 to 1986, DST used to start at 2am (standard time) on the last
   # Sunday in April (the one on or after April 24) and to end at 2am (DST time;
   # 1am standard time) on the last Sunday of October, which is the first Sunday
   # on or after Oct 25.
   DSTSTART_1967_1986 = datetime(1, 4, 24, 2)
   DSTEND_1967_1986 = DSTEND_1987_2006

   class USTimeZone(tzinfo):

       def __init__(self, hours, reprname, stdname, dstname):
           self.stdoffset = timedelta(hours=hours)
           self.reprname = reprname
           self.stdname = stdname
           self.dstname = dstname

       def __repr__(self):
           return self.reprname

       def tzname(self, dt):
           if self.dst(dt):
               return self.dstname
           else:
               return self.stdname

       def utcoffset(self, dt):
           return self.stdoffset + self.dst(dt)

       def dst(self, dt):
           if dt is None or dt.tzinfo is None:
               # An exception may be sensible here, in one or both cases.
               # It depends on how you want to treat them.  The default
               # fromutc() implementation (called by the default astimezone()
               # implementation) passes a datetime with dt.tzinfo is self.
               return ZERO
           assert dt.tzinfo is self

           # Find start and end times for US DST. For years before 1967, return
           # ZERO for no DST.
           if 2006 < dt.year:
               dststart, dstend = DSTSTART_2007, DSTEND_2007
           elif 1986 < dt.year < 2007:
               dststart, dstend = DSTSTART_1987_2006, DSTEND_1987_2006
           elif 1966 < dt.year < 1987:
               dststart, dstend = DSTSTART_1967_1986, DSTEND_1967_1986
           else:
               return ZERO

           start = first_sunday_on_or_after(dststart.replace(year=dt.year))
           end = first_sunday_on_or_after(dstend.replace(year=dt.year))

           # Can't compare naive to aware objects, so strip the timezone from
           # dt first.
           if start <= dt.replace(tzinfo=None) < end:
               return HOUR
           else:
               return ZERO

   Eastern  = USTimeZone(-5, "Eastern",  "EST", "EDT")
   Central  = USTimeZone(-6, "Central",  "CST", "CDT")
   Mountain = USTimeZone(-7, "Mountain", "MST", "MDT")
   Pacific  = USTimeZone(-8, "Pacific",  "PST", "PDT")

Note that there are unavoidable subtleties twice per year in a
"tzinfo" subclass accounting for both standard and daylight time, at
the DST transition points.  For concreteness, consider US Eastern (UTC
-0500), where EDT begins the minute after 1:59 (EST) on the second
Sunday in March, and ends the minute after 1:59 (EDT) on the first
Sunday in November:

     UTC   3:MM  4:MM  5:MM  6:MM  7:MM  8:MM
     EST  22:MM 23:MM  0:MM  1:MM  2:MM  3:MM
     EDT  23:MM  0:MM  1:MM  2:MM  3:MM  4:MM

   start  22:MM 23:MM  0:MM  1:MM  3:MM  4:MM

     end  23:MM  0:MM  1:MM  1:MM  2:MM  3:MM

When DST starts (the “start” line), the local wall clock leaps from
1:59 to 3:00.  A wall time of the form 2:MM doesn’t really make sense
on that day, so "astimezone(Eastern)" won’t deliver a result with
"hour == 2" on the day DST begins.  In order for "astimezone()" to
make this guarantee, the "rzinfo.dst()" method must consider times in
the “missing hour” (2:MM for Eastern) to be in daylight time.

When DST ends (the “end” line), there’s a potentially worse problem:
there’s an hour that can’t be spelled unambiguously in local wall
time: the last hour of daylight time.  In Eastern, that’s times of the
form 5:MM UTC on the day daylight time ends.  The local wall clock
leaps from 1:59 (daylight time) back to 1:00 (standard time) again.
Local times of the form 1:MM are ambiguous. "astimezone()" mimics the
local clock’s behavior by mapping two adjacent UTC hours into the same
local hour then.  In the Eastern example, UTC times of the form 5:MM
and 6:MM both map to 1:MM when converted to Eastern.  In order for
"astimezone()" to make this guarantee, the "tzinfo.dst()" method must
consider times in the “repeated hour” to be in standard time.  This is
easily arranged, as in the example, by expressing DST switch times in
the time zone’s standard local time.

Applications that can’t bear such ambiguities should avoid using
hybrid "tzinfo" subclasses; there are no ambiguities when using UTC,
or any other fixed-offset "tzinfo" subclass (such as a class
representing only EST (fixed offset -5 hours), or only EDT (fixed
offset -4 hours)).

参见:

  pytz
     The standard library has no "tzinfo" instances, but there exists
     a third-party library which brings the *IANA timezone database*
     (also known as the Olson database) to Python: *pytz*.

     *pytz* contains up-to-date information and its usage is
     recommended.

  IANA timezone database
     The Time Zone Database (often called tz or zoneinfo) contains
     code and data that represent the history of local time for many
     representative locations around the globe. It is updated
     periodically to reflect changes made by political bodies to time
     zone boundaries, UTC offsets, and daylight-saving rules.


8.1.7. "strftime()" and "strptime()" Behavior
=============================================

"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.

Conversely, the "datetime.strptime()" class method creates a
"datetime" object from a string representing a date and time and a
corresponding format string. "datetime.strptime(date_string, format)"
is equivalent to "datetime(*(time.strptime(date_string,
format)[0:6]))", except when the format includes sub-second components
or timezone offset information, which are supported in
"datetime.strptime" but are discarded by "time.strptime".

For "time" objects, the format codes for year, month, and day should
not be used, as time objects have no such values.  If they’re used
anyway, "1900" is substituted for the year, and "1" for the month and
day.

For "date" objects, the format codes for hours, minutes, seconds, and
microseconds should not be used, as "date" objects have no such
values.  If they’re used anyway, "0" is substituted for them.

The full set of format codes supported varies across platforms,
because Python calls the platform C library’s "strftime()" function,
and platform variations are common.  To see the full set of format
codes supported on your platform, consult the *strftime(3)*
documentation.

For the same reason, handling of format strings containing Unicode
code points that can’t be represented in the charset of the current
locale is also platform-dependent. On some platforms such code points
are preserved intact in the output, while on others "strftime" may
raise "UnicodeError" or return an empty string instead.

The following is a list of all the format codes that the C standard
(1989 version) requires, and these work on all platforms with a
standard C implementation.  Note that the 1999 version of the C
standard added additional format codes.

The exact range of years for which "strftime()" works also varies
across platforms.  Regardless of platform, years before 1900 cannot be
used.

+-------------+----------------------------------+--------------------------+---------+
| Directive   | 意义                             | 示例                     | 注释    |
+=============+==================================+==========================+=========+
| "%a"        | Weekday as locale’s abbreviated  | Sun, Mon, …, Sat         | (1)     |
|             | name.                            | (en_US); So, Mo, …, Sa   |         |
|             |                                  | (de_DE)                  |         |
+-------------+----------------------------------+--------------------------+---------+
| "%A"        | Weekday as locale’s full name.   | Sunday, Monday, …,       | (1)     |
|             |                                  | Saturday (en_US);        |         |
|             |                                  | Sonntag, Montag, …,      |         |
|             |                                  | Samstag (de_DE)          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%w"        | Weekday as a decimal number,     | 0, 1, …, 6               |         |
|             | where 0 is Sunday and 6 is       |                          |         |
|             | Saturday.                        |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%d"        | Day of the month as a zero-      | 01, 02, …, 31            |         |
|             | padded decimal number.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%b"        | Month as locale’s abbreviated    | Jan, Feb, …, Dec         | (1)     |
|             | name.                            | (en_US); Jan, Feb, …,    |         |
|             |                                  | Dez (de_DE)              |         |
+-------------+----------------------------------+--------------------------+---------+
| "%B"        | Month as locale’s full name.     | January, February, …,    | (1)     |
|             |                                  | December (en_US);        |         |
|             |                                  | Januar, Februar, …,      |         |
|             |                                  | Dezember (de_DE)         |         |
+-------------+----------------------------------+--------------------------+---------+
| "%m"        | Month as a zero-padded decimal   | 01, 02, …, 12            |         |
|             | number.                          |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%y"        | Year without century as a zero-  | 00, 01, …, 99            |         |
|             | padded decimal number.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%Y"        | Year with century as a decimal   | 1970, 1988, 2001, 2013   |         |
|             | number.                          |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%H"        | Hour (24-hour clock) as a zero-  | 00, 01, …, 23            |         |
|             | padded decimal number.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%I"        | Hour (12-hour clock) as a zero-  | 01, 02, …, 12            |         |
|             | padded decimal number.           |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%p"        | Locale’s equivalent of either AM | AM, PM (en_US); am, pm   | (1),    |
|             | or PM.                           | (de_DE)                  | (2)     |
+-------------+----------------------------------+--------------------------+---------+
| "%M"        | Minute as a zero-padded decimal  | 00, 01, …, 59            |         |
|             | number.                          |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%S"        | Second as a zero-padded decimal  | 00, 01, …, 59            | (3)     |
|             | number.                          |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%f"        | Microsecond as a decimal number, | 000000, 000001, …,       | (4)     |
|             | zero-padded on the left.         | 999999                   |         |
+-------------+----------------------------------+--------------------------+---------+
| "%z"        | UTC offset in the form +HHMM or  | (empty), +0000, -0400,   | (5)     |
|             | -HHMM (empty string if the the   | +1030                    |         |
|             | object is naive).                |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%Z"        | Time zone name (empty string if  | (empty), UTC, EST, CST   |         |
|             | the object is naive).            |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%j"        | Day of the year as a zero-padded | 001, 002, …, 366         |         |
|             | decimal number.                  |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%U"        | Week number of the year (Sunday  | 00, 01, …, 53            | (6)     |
|             | as the first day of the week) as |                          |         |
|             | a zero padded decimal number.    |                          |         |
|             | All days in a new year preceding |                          |         |
|             | the first Sunday are considered  |                          |         |
|             | to be in week 0.                 |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%W"        | Week number of the year (Monday  | 00, 01, …, 53            | (6)     |
|             | as the first day of the week) as |                          |         |
|             | a decimal number. All days in a  |                          |         |
|             | new year preceding the first     |                          |         |
|             | Monday are considered to be in   |                          |         |
|             | week 0.                          |                          |         |
+-------------+----------------------------------+--------------------------+---------+
| "%c"        | Locale’s appropriate date and    | Tue Aug 16 21:30:00 1988 | (1)     |
|             | time representation.             | (en_US); Di 16 Aug       |         |
|             |                                  | 21:30:00 1988 (de_DE)    |         |
+-------------+----------------------------------+--------------------------+---------+
| "%x"        | Locale’s appropriate date        | 08/16/88 (None);         | (1)     |
|             | representation.                  | 08/16/1988 (en_US);      |         |
|             |                                  | 16.08.1988 (de_DE)       |         |
+-------------+----------------------------------+--------------------------+---------+
| "%X"        | Locale’s appropriate time        | 21:30:00 (en_US);        | (1)     |
|             | representation.                  | 21:30:00 (de_DE)         |         |
+-------------+----------------------------------+--------------------------+---------+
| "%%"        | A literal "'%'" character.       | %                        |         |
+-------------+----------------------------------+--------------------------+---------+

注释:

1. Because the format depends on the current locale, care should be
   taken when making assumptions about the output value. Field
   orderings will vary (for example, “month/day/year” versus
   “day/month/year”), and the output may contain Unicode characters
   encoded using the locale’s default encoding (for example, if the
   current locale is "ja_JP", the default encoding could be any one of
   "eucJP", "SJIS", or "utf-8"; use "locale.getlocale()" to determine
   the current locale’s encoding).

2. When used with the "strptime()" method, the "%p" directive only
   affects the output hour field if the "%I" directive is used to
   parse the hour.

3. Unlike the "time" module, the "datetime" module does not support
   leap seconds.

4. "%f" is an extension to the set of format characters in the C
   standard (but implemented separately in datetime objects, and
   therefore always available).  When used with the "strptime()"
   method, the "%f" directive accepts from one to six digits and zero
   pads on the right.

   2.6 新版功能.

5. For a naive object, the "%z" and "%Z" format codes are replaced
   by empty strings.

   For an aware object:

   "%z"
      "utcoffset()" is transformed into a 5-character string of the
      form +HHMM or -HHMM, where HH is a 2-digit string giving the
      number of UTC offset hours, and MM is a 2-digit string giving
      the number of UTC offset minutes.  For example, if "utcoffset()"
      returns "timedelta(hours=-3, minutes=-30)", "%z" is replaced
      with the string "'-0330'".

   "%Z"
      If "tzname()" returns "None", "%Z" is replaced by an empty
      string.  Otherwise "%Z" is replaced by the returned value, which
      must be a string.

6. When used with the "strptime()" method, "%U" and "%W" are only
   used in calculations when the day of the week and the year are
   specified.

-[ 脚注 ]-

[1] If, that is, we ignore the effects of Relativity
