Don't allow float(quantity) and int(quantity)

quantities/quantity.py

@@ -373,7 +373,7 @@ def __setitem__(self, key, value):
             value = Quantity(value)
         if self._dimensionality != value._dimensionality:
             value = value.rescale(self._dimensionality)
-        self.magnitude[key] = value
+        self.magnitude[key] = value.magnitude
 
     @with_doc(np.ndarray.__lt__)
     @wrap_comparison
@@ -413,6 +413,12 @@ def __ge__(self, other):
     def __gt__(self, other):
         return self.magnitude > other
 
+    def __float__(self):
+        raise TypeError("Quantities can't be converted to floats")
+
+    def __int__(self):
+        raise TypeError("Quantities can't be converted to ints")
+
     #I don't think this implementation is particularly efficient,
     #perhaps there is something better
     @with_doc(np.ndarray.tolist)

quantities/tests/test_conversion.py

@@ -32,6 +32,16 @@ def test_compound_reduction(self):
             delta=1e83
             )
 
+    def test_float_conversion(self):
+        with self.assertRaisesRegexp(
+                TypeError, "Quantities can't be converted to floats"):
+            float(3 * pq.um)
+
+    def test_int_conversion(self):
+        with self.assertRaisesRegexp(
+                TypeError, "Quantities can't be converted to ints"):
+            int(3 * pq.um)
+
 
 class TestDefaultUnits(TestCase):