云在天边看世界

最近在学习Google App Engine日期时间属性时，尝试对Micolog这个基于Google App Engine的Blog后台的评论做了一个简单的修改页面，这样就可以使我们省下了去App Engine的后台去搜寻相关数据库再做修改的时间。编辑代码的过程倒是基本简单，新建一个编辑评论的页面（comment.html），再到admin.py中加入相关代码就可以了。不过，这个过程中倒是遇到一个小问题，python提交时间更新时会抛出一个异常来：

BadValueError: Property date must be a datetime

解决办法倒是很简单，将时间格式化成Datetime类型即可。

比如说：我在模板中对时间格式化为{{comment.date|datetz:"Y-m-d H:i:s"}}(如：2010-07-05-20:00:12这样的形式)，那么在程序代码中可以这样写做：

comment.date=datetime.strptime(commentdate,"%Y-%m-%d %H:%M:%S")

当然其他类型的格式化都是可取的。由于Python中对时间进行格式化的相关字符与Django模板中time过滤器所用的格式字符串略有区别，下面就分别对其进行简单介绍，希望对你有所帮助。

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])).

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.

New in version 2.6: time and datetime objects support a %f format code which expands to the number of microseconds in the object, zero-padded on the left to six places.

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.

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.

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.

Notes:

1. When used with the strptime() method, the %f directive accepts from one to six digits and zero pads on the right. %f is an extension to the set of format characters in the C standard (but implemented separately in datetime objects, and therefore always available).
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. The range really is 0 to 61; according to the Posix standard this accounts for leap seconds and the (very rare) double leap seconds. The time module may produce and does accept leap seconds since it is based on the Posix standard, but the datetime module does not accept leap seconds in strptime() input nor will it produce them in strftime() output.
4. 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.
5. For example, if utcoffset() returns timedelta(hours=-3, minutes=-30), %z is replaced with the string '-0330'.