how to get the same day of next month of a given day in python using datetime

Use dateutil module. It has relative time deltas:

import datetime
from dateutil import relativedelta
nextmonth = datetime.date.today() + relativedelta.relativedelta(months=1)

Beautiful.

Of course there isn't -- if today's January 31, what would be "the same day of the next month"?! Obviously there is no right solution, since February 31 does not exist, and the datetime module does not play at "guess what the user posing this impossible problem without a right solution thinks (wrongly) is the obvious solution";-).

I suggest:

try:
  nextmonthdate = x.replace(month=x.month+1)
except ValueError:
  if x.month == 12:
    nextmonthdate = x.replace(year=x.year+1, month=1)
  else:
    # next month is too short to have "same date"
    # pick your own heuristic, or re-raise the exception:
    raise