published_at = "{}".format(self.published_at)

str.join, может быть использован для построения строки.

d = datetime.now()
'/'.join(str(x) for x in (d.month, d.day, d.year))
'3/7/2016'

import datetime
now=datetime.datetime.now()
now.isoformat()
# ouptut --> '2016-03-09T08:18:20.860968'

date а такжеdatetime объекты (иtime а также) поддержатьмини-язык для указания выводаи есть два способа получить к нему доступ:

direct method call: dt.strftime('format here'); and new format method: '{:format here}'.format(dt)

Итак, ваш пример может выглядеть так:

dt.strftime('%m/%d/%Y')

или же

'{:%m/%d/%Y}'.format(dt)

Для полноты информации ради: вы также можете напрямую получить доступ к атрибутам объекта, но тогда вы получите только цифры:

'%s/%s/%s' % (dt.month, dt.day, dt.year)

Время изучения мини-языка того стоит.

Для справки вот коды, используемые на мини-языке:

%a Weekday as locale’s abbreviated name. %A Weekday as locale’s full name. %w Weekday as a decimal number, where 0 is Sunday and 6 is Saturday. %d Day of the month as a zero-padded decimal number. %b Month as locale’s abbreviated name. %B Month as locale’s full name. %m Month as a zero-padded decimal number. 01, ..., 12 %y Year without century as a zero-padded decimal number. 00, ..., 99 %Y Year with century as a decimal number. 1970, 1988, 2001, 2013 %H Hour (24-hour clock) as a zero-padded decimal number. 00, ..., 23 %I Hour (12-hour clock) as a zero-padded decimal number. 01, ..., 12 %p Locale’s equivalent of either AM or PM. %M Minute as a zero-padded decimal number. 00, ..., 59 %S Second as a zero-padded decimal number. 00, ..., 59 %f Microsecond as a decimal number, zero-padded on the left. 000000, ..., 999999 %z UTC offset in the form +HHMM or -HHMM (empty if naive), +0000, -0400, +1030 %Z Time zone name (empty if naive), UTC, EST, CST %j Day of the year as a zero-padded decimal number. 001, ..., 366 %U Week number of the year (Sunday is the first) as a zero padded decimal number. %W Week number of the year (Monday is first) as a decimal number. %c Locale’s appropriate date and time representation. %x Locale’s appropriate date representation. %X Locale’s appropriate time representation. %% A literal '%' character.

Можно преобразовать объект datetime в строку, работая напрямую с компонентами объекта datetime.

from datetime import date  

myDate = date.today()    
#print(myDate) would output 2017-05-23 because that is today
#reassign the myDate variable to myDate = myDate.month 
#then you could print(myDate.month) and you would get 5 as an integer
dateStr = str(myDate.month)+ "/" + str(myDate.day) + "/" + str(myDate.year)    
# myDate.month is equal to 5 as an integer, i use str() to change it to a 
# string I add(+)the "/" so now I have "5/" then myDate.day is 23 as
# an integer i change it to a string with str() and it is added to the "5/"   
# to get "5/23" and then I add another "/" now we have "5/23/" next is the 
# year which is 2017 as an integer, I use the function str() to change it to 
# a string and add it to the rest of the string.  Now we have "5/23/2017" as 
# a string. The final line prints the string.

print(dateStr)  

Выход - & gt; 5/23/2017

Вы можете использовать простые методы форматирования строк:

>>> dt = datetime.datetime(2012, 2, 23, 0, 0)
>>> '{0.month}/{0.day}/{0.year}'.format(dt)
'2/23/2012'
>>> '%s/%s/%s' % (dt.month, dt.day, dt.year)
'2/23/2012'

type-specific formatting также можно использовать:

t = datetime.datetime(2012, 2, 23, 0, 0)
"{:%m/%d/%Y}".format(t)

Выход:

'02/23/2012'