some basic math operators

bia/test_units.py

@@ -16,3 +16,23 @@ def test_basic_conversions():
     assert g.unit == 'g'
 
     assert g.as_unit('lb').scalar == a.as_unit('lb').scalar
+
+
+def test_basic_cmp():
+    assert Mass(1, 'kg') < Mass(2, 'kg')
+    assert Mass(1, 'kg') <= Mass(2, 'kg')
+    assert Mass(2, 'kg') <= Mass(2, 'kg')
+    assert Mass(2, 'kg') == Mass(2, 'kg')
+    assert Mass(2, 'kg') >= Mass(2, 'kg')
+    assert Mass(2, 'kg') > Mass(1, 'kg')
+    assert Mass(2, 'kg') >= Mass(1, 'kg')
+
+
+def test_conversion_cmp():
+    assert Mass(1, 'kg') < Mass(100, 'lb')
+    assert Mass(10000000, 'g') > Mass(100, 'lb')
+
+
+def test_add_sub():
+    assert Mass(1, 'kg') + Mass(2, 'kg') == Mass(3, 'kg')
+    assert Mass(2, 'kg') - Mass(1, 'kg') == Mass(1, 'kg')

bia/units.py

@@ -38,8 +38,20 @@ class Unit(object):
     def __repr__(self):
         return 'U({}, {!r})'.format(self.scalar, self.unit)
 
-    def __cmp__(self):
+    def __cmp__(self, other):
-        pass
+        if self.unit != other.unit:
+            other = other.as_unit(self.unit)
+        return self.scalar - other.scalar
+
+    def __add__(self, other):
+        if self.unit != other.unit:
+            other = other.as_unit(self.unit)
+        return self.__class__(self.scalar.__add__(other.scalar), self.unit)
+
+    def __sub__(self, other):
+        if self.unit != other.unit:
+            other = other.as_unit(self.unit)
+        return self.__class__(self.scalar.__sub__(other.scalar), self.unit)
 
 
 class Mass(Unit):