from unittest import TestCase

class MyLibraryTest(TestCase):

    def test_dot_product(self):
        vector3 = ctypes.c_double * size
        a = vector3(1.0, 2.5, 3.5)
        b = vector3(-7, 4, -12.1)
        vector_ptr = ctypes.POINTER(ctypes.c_double)
        result = my_library.dot_product(
            3,
            ctypes.cast(a, vector_ptr),
            ctypes.cast(b, vector_ptr),
        )
        self.assertAlmostEqual(-39.35, result)

...
>>>
.
--------------------------------------------------------------
Ran 1 test in 0.000s
OK