test basics of division and multiplication

bia/test_units.py

@@ -18,6 +18,11 @@ def test_basic_conversions():
     assert g.as_unit('lb').scalar == a.as_unit('lb').scalar
 
 
+def test_complex_conversion():
+    a = Mass(2, ['kg', 'kg'])
+    b = a.as_unit('lb')
+
+
 def test_basic_cmp():
     assert Mass(1, 'kg') < Mass(2, 'kg')
     assert Mass(1, 'kg') <= Mass(2, 'kg')
@@ -41,3 +46,13 @@ def test_addition():
 def test_subtraction():
     assert Mass(2, 'kg') - Mass(1, 'kg') == Mass(1, 'kg')
     assert Mass(1, 'kg') - Mass(1, 'lb') < Mass(0.55, 'kg')
+
+
+def test_multiplication():
+    assert Mass(2, 'kg') * 2 == Mass(4, 'kg')
+    assert Mass(2, 'kg') * Mass(1, 'kg') == Mass(2, ['kg', 'kg'])
+
+
+def test_division():
+    assert Mass(8, 'kg') / 2 == Mass(4, 'kg')
+    assert Mass(2, 'kg') / Mass(1, 'kg') == Mass(2, [])

bia/units.py

@@ -12,11 +12,23 @@ class ConversionError(ValueError):
 class Unit(object):
     _mapping = {}
 
-    def __init__(self, n, unit):
+    def __init__(self, n, unit, denom=None):
         self.scalar = float(n)
-        if unit not in self._mapping:
+
-            raise ValueError('invalid unit {} for {}'.format(unit, self.__class__.__name__))
+        if isinstance(unit, str):
-        self.unit = unit
+            self.unit_numerator = [unit]
+        else:
+            self.unit_numerator = unit
+
+        self.unit_denominator = [] if denom is None else denom
+
+        for u in self.unit_numerator + self.unit_denominator:
+            if u not in self._mapping:
+                raise ValueError('invalid unit {} for {}'.format(unit, self.__class__.__name__))
+
+    @property
+    def unit(self):
+        return '*'.join(self.unit_numerator)
 
     def as_unit(self, unit):
         if self.unit == unit:
@@ -47,13 +59,37 @@ class Unit(object):
     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)
+        return self.__class__(self.scalar + 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)
+        return self.__class__(self.scalar - other.scalar, self.unit)
 
+    def __mul__(self, other):
+        if isinstance(other, Unit):
+            if self.unit != other.unit:
+                other = other.as_unit(self.unit)
+            return self.__class__(self.scalar * other.scalar,
+                                  self.unit_numerator + other.unit_numerator)
+        else:
+            return self.__class__(self.scalar * other, self.unit)
+
+    def __div__(self, other):
+        if isinstance(other, Unit):
+            if self.unit != other.unit:
+                other = other.as_unit(self.unit)
+            new_numerator = list(self.unit_numerator)
+            new_denominator = list(self.unit_denominator)
+            for u in other.unit_numerator:
+                if u in new_numerator:
+                    new_numerator.remove(u)
+                else:
+                    new_denominator.append(u)
+
+            return self.__class__(self.scalar / other.scalar, new_numerator, new_denominator)
+        else:
+            return self.__class__(self.scalar / other, self.unit)
 
 class Mass(Unit):
     _base = 'kg'