def subOneMonth(dt):
day = dt.day
res = dt.replace(day=1) - datetime.timedelta(days =1)
try:
res.replace(day= day)
except ValueError:
pass
return res
print subOneMonth(datetime.datetime(2016,07,11)).strftime('%d, %b %Y')
11, Jun 2016
print subOneMonth(datetime.datetime(2016,01,11)).strftime('%d, %b %Y')
11, Dec 2015
print subOneMonth(datetime.datetime(2016,3,31)).strftime('%d, %b %Y')
29, Feb 2016