Sum of numbers in list

Define a function sum_of_numbers_in_list that receives a list of numbers
a_list of an unknown length and calculates the sum of those numbers using a
for loop. Do not use the sum function.

Examples:

>>> sum_of_numbers_in_list([])
0
>>> sum_of_numbers_in_list([4])
4
>>> sum_of_numbers_in_list([2, 3])
5
>>> sum_of_numbers_in_list([2, 3, 4])
9
import unittest


class SumOfNumbersInListTestCase(unittest.TestCase):

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

    def test_list_of_1(self):
        self.assertEqual(sum_of_numbers_in_list([4]), 4)

    def test_list_of_2(self):
        self.assertEqual(sum_of_numbers_in_list([2, 3]), 5)

    def test_list_of_3(self):
        self.assertEqual(sum_of_numbers_in_list([2, 3, 4]), 9)