def test_subtract(self):
a = Quantity(1, 0.2, {'Newton': 1}, si_system)
b = Quantity(3, 0.4, {'Kilogram': 1, 'Meter': 1, 'Second': -2}, si_system)
c = Quantity(-2, 1 / math.sqrt(5), {'Newton': 1}, si_system)
d = Quantity(1, 0.2, {'Kilogram': 1}, si_system)
self.assert_quantity_equal(a - b, c.expand())
with self.assertRaises(TypeError): a - d
with self.assertRaises(TypeError): a - 1
def test_expand(self):
# Lorentz force
a = Quantity(1, 0,
{'Coulomb': 1, 'Meter': 1, 'Second': -1, 'Tesla': 1}, si_system)
b = Quantity(1, 0, {'Newton': 1}, si_system)
self.assert_quantity_equal(a.expand(), b.expand())
# Faraday's law
a = Quantity(1, 0, {'Weber': 1, 'Second': -1}, si_system)
b = Quantity(1, 0, {'Volt': 1}, si_system)
self.assert_quantity_equal(a.expand(), b.expand())
# torque of a motor
a = Quantity(1, 0, {'Ampere': 1, 'Tesla': 1, 'Meter': 2}, si_system)
b = Quantity(1, 0, {'Newton': 1, 'Meter': 1}, si_system)
self.assert_quantity_equal(a.expand(), b.expand())
# resonance frequency of an RLC circuit
a = Quantity(1, 0, {'Henry': -1/2, 'Farad': -1/2}, si_system)
b = Quantity(1, 0, {'Hertz': 1}, si_system)
self.assert_quantity_equal(a.expand(), b.expand())
