dm.py

from dateutil.relativedelta import relativedelta


def three_months_from(date):
    return date + relativedelta(months=3)

test_dm.py

import unittest
import datetime
from dm import three_months_from


class ThreeMonthsFromDateTest(unittest.TestCase):
    def _test_date(self, input_params, output_params):
        target = datetime.date(*input_params)
        self.assertEqual(three_months_from(target), datetime.date(*output_params))

    def test_20130331(self):
        self._test_date((2013, 3, 31), (2013, 6, 30))

    def test_20130401(self):
        self._test_date((2013, 4, 1), (2013, 7, 1))

    def test_20131129(self):
        self._test_date((2013, 11, 29), (2014, 2, 28))

    def test_20131130(self):
        self._test_date((2013, 11, 30), (2014, 2, 28))

    def test_20151129(self):
        self._test_date((2015, 11, 29), (2016, 2, 29))

    def test_20151130(self):
        self._test_date((2015, 11, 30), (2016, 2, 29))


if __name__ == '__main__':
    unittest.main()
    # Result:
    # ......
    # ----------------------------------------------------------------------
    # Ran 6 tests in 0.001s
    #
    # OK