Sum if list only contains integers

Define a function sum_if_list_of_ints that receives a list and uses a loop
to make sure the list only contains integers. If so, it returns the sum of
the integers. If not, return 'not an int'.

Hint: Use isinstance again to determine the type.

Examples:

>>> sum_if_list_of_ints([1, 2, 3])
6
>>> sum_if_list_of_ints([1, 'a', 3])
'not an int'
>>> sum_if_list_of_ints([])
0
import unittest


class SumIfListOfIntsListTestCase(unittest.TestCase):

    def test_sum_of_ints(self):
        self.assertEqual(sum_if_list_of_ints([1, 2, 3]), 6)

    def test_mixed_list(self):
        self.assertEqual(sum_if_list_of_ints([1, 'a', 3]), 'not an int')

    def test_empty_list(self):
        self.assertEqual(sum_if_list_of_ints([]), 0)