Add missing amount operations

2 files changed, 17 insertions, 0 deletions

diff --git a/portfolio.py b/portfolio.py
index 17a17ba..21a9834 100644
--- a/portfolio.py
+++ b/portfolio.py
@@ -124,6 +124,8 @@ class Amount:
         return Amount(self.currency, abs(self.value))
 
     def __add__(self, other):
+        if other == 0:
+            return self
         if other.currency != self.currency and other.value * self.value != 0:
             raise Exception("Summing amounts must be done with same currencies")
         return Amount(self.currency, self.value + other.value)
@@ -141,6 +143,12 @@ class Amount:
             raise Exception("Summing amounts must be done with same currencies")
         return Amount(self.currency, self.value - other.value)
 
+    def __rsub__(self, other):
+        if other == 0:
+            return -self
+        else:
+            return -self.__sub__(other)
+
     def __mul__(self, value):
         if not isinstance(value, (int, float, D)):
             raise TypeError("Amount may only be multiplied by numbers")
diff --git a/test.py b/test.py
index 1965521..c496e11 100644
--- a/test.py
+++ b/test.py
@@ -220,6 +220,8 @@ class AmountTest(WebMockTestCase):
         amount4 = portfolio.Amount("ETH", 0.0)
         self.assertEqual(amount1, amount1 + amount4)
 
+        self.assertEqual(amount1, amount1 + 0)
+
     def test__radd(self):
         amount = portfolio.Amount("XVG", "12.9")
 
@@ -241,6 +243,13 @@ class AmountTest(WebMockTestCase):
         amount4 = portfolio.Amount("ETH", 0.0)
         self.assertEqual(amount1, amount1 - amount4)
 
+    def test__rsub(self):
+        amount = portfolio.Amount("ETH", "1.6")
+        with self.assertRaises(Exception):
+            3 - amount
+
+        self.assertEqual(portfolio.Amount("ETH", "-1.6"), -amount)
+
     def test__mul(self):
         amount = portfolio.Amount("XEM", 11)