fixed minor ordering issue of combinedcost as visible through __repr__

cvxportfolio/costs.py

@@ -63,7 +63,7 @@ def __add__(self, other):
         by summing over costs.
         """
         if isinstance(other, CombinedCosts):
-            return other + self
+            return other.__radd__(self)
         return CombinedCosts([self, other], [1.0, 1.0])
 
     def __rmul__(self, other):
@@ -132,17 +132,27 @@ def __init__(self, costs, multipliers):
         self.do_convexity_check = True
 
     def __add__(self, other):
-        """Add other (combined) cost to self."""
+        """Add self to other (combined) cost."""
         if isinstance(other, CombinedCosts):
             return CombinedCosts(self.costs + other.costs,
                                  self.multipliers + other.multipliers)
         return CombinedCosts(self.costs + [other], self.multipliers + [1.0])
 
+    def __radd__(self, other):
+        """Add other (combined) cost to self."""
+        if isinstance(other, CombinedCosts):
+            return other + self # pragma: no cover
+        return CombinedCosts([other] + self.costs, [1.0] + self.multipliers)
+
     def __mul__(self, other):
         """Multiply by constant."""
         return CombinedCosts(self.costs,
                              [el * other for el in self.multipliers])
 
+    def __neg__(self):
+        """Take negative of cost."""
+        return self * -1
+
     def initialize_estimator_recursive(self, **kwargs):
         """Initialize iterating over constituent costs.
 

cvxportfolio/hyperparameters.py

@@ -129,21 +129,25 @@ def __repr__(self):
             expressions in general. It's not perfect but readable.
         """
 
+        # TODO this gives wrong string repr with nested expressions like
+        # ``-(cvx.Gamma() * -3 * (cvx.Gamma() - cvx.Gamma()))``
+        # internal algebra is correct, though
+
         def _minus_repr(obj):
             rawrepr = str(obj).lstrip()
             if rawrepr[0] == '-':
-                return ' +' + rawrepr[1:].lstrip()
+                return ' + ' + rawrepr[1:].lstrip()
             if rawrepr[0] == '+':
-                return ' -' + rawrepr[1:].lstrip() # pragma: no cover
-            return ' -' + rawrepr
+                return ' - ' + rawrepr[1:].lstrip() # pragma: no cover
+            return ' - ' + rawrepr
 
         def _plus_repr(obj):
             rawrepr = str(obj).lstrip()
             if rawrepr[0] == '-':
-                return ' -' + rawrepr[1:].lstrip()
+                return ' - ' + rawrepr[1:].lstrip()
             if rawrepr[0] == '+':
-                return ' +' + rawrepr[1:].lstrip()
-            return ' +' + str(obj)
+                return ' + ' + rawrepr[1:].lstrip()
+            return ' + ' + rawrepr
 
         result = ''
 

cvxportfolio/tests/test_hyperparameters.py

@@ -44,11 +44,11 @@ def test_repr(self):
             - cvx.Gamma() * cvx.StocksTransactionCost()
 
         self.assertTrue(str(obj) ==
-         '-Gamma(current_value=1.0) * '
-         + 'FullCovariance(Sigma=HistoricalFactorizedCovariance(kelly=True))'
-         + ' + ReturnsForecast(r_hat=HistoricalMeanReturn(), decay=1.0)'
-         + '-Gamma(current_value=1.0) * StocksTransactionCost(a=0.0,'
-         + ' pershare_cost=0.005, b=1.0, exponent=1.5)')
+            'ReturnsForecast(r_hat=HistoricalMeanReturn(), decay=1.0)'
+            + '- Gamma(current_value=1.0) * FullCovariance('
+            + 'Sigma=HistoricalFactorizedCovariance(kelly=True))'
+            + '- Gamma(current_value=1.0) * StocksTransactionCost('
+            + 'a=0.0, pershare_cost=0.005, b=1.0, exponent=1.5)')
 
         print(cvx.Gamma() * cvx.Gamma())
         print(cvx.Gamma() - cvx.Gamma())