def test_sub_known_Values(self):
#subtraction should give known result with known input
#__rsub__ should give the negative of __sub__
#the units of the results should be the same as the units of the calling instance
known_sub_Values=(('1m', '5m',-4), ('1cm', '1cm',0), ('1cm', '5m',-499.0),
('7km', '1m',6.999))
for q1,q2, sum in known_sub_Values:
x = PhysicalQuantity(q1)
y = PhysicalQuantity(q2)
self.assertEqual((x-y).value, sum)
self.assertEqual((x-y).unit, x.unit)
self.assertEqual(x.__rsub__(y).value, -sum)
self.assertEqual(x.__rsub__(y).unit, x.unit)
#test for error if incompatible units
q1 = PhysicalQuantity('1cm')
q2 = PhysicalQuantity('1kg')
try:
q1 - q2
except TypeError as err:
self.assertEqual(str(err), "Incompatible units")
else:
self.fail("expecting TypeError")
#test offset units
q1 = PhysicalQuantity('1degK')
q2 = PhysicalQuantity('1degR')
q3 = PhysicalQuantity('1degC')
result = q1 - q2
self.assertAlmostEqual(result.value, .444,3)
self.assertEqual(result.unit, q1.unit)
try:
q3 - q2
except TypeError as err:
msg = "Unit conversion (degR to degC) cannot be expressed as a simple multiplicative factor"
self.assertEqual(str(err), msg)
else:
self.fail('expecting TypeError')
