ENH: Added commission model PerDollar

Commissions will be calculated based on total dollar traded.

zipline/algorithm.py

@@ -38,7 +38,7 @@ from zipline.finance.slippage import (
     SlippageModel,
     transact_partial
 )
-from zipline.finance.commission import PerShare, PerTrade
+from zipline.finance.commission import PerShare, PerTrade, PerDollar
 from zipline.finance.blotter import Blotter
 from zipline.finance.constants import ANNUALIZER
 from zipline.finance import trading
@@ -419,7 +419,7 @@ class TradingAlgorithm(object):
         self.slippage = slippage
 
     def set_commission(self, commission):
-        if not isinstance(commission, (PerShare, PerTrade)):
+        if not isinstance(commission, (PerShare, PerTrade, PerDollar)):
             raise UnsupportedCommissionModel()
 
         if self.initialized:

zipline/errors.py

@@ -63,8 +63,8 @@ method.
 class UnsupportedCommissionModel(ZiplineError):
     """
     Raised if a user script calls the override_commission magic
-    with a commission object that isn't a PerShare or
-    PerTrade commission
+    with a commission object that isn't a PerShare, PerTrade or
+    PerDollar commission
     """
     msg = """
 You attempted to override commission with an unsupported class. \

zipline/finance/commission.py

@@ -66,3 +66,30 @@ class PerTrade(object):
             return 0.0, 0.0
 
         return abs(self.cost / transaction.amount), self.cost
+
+
+class PerDollar(object):
+    """
+    Calculates a commission for a transaction based on a per
+    dollar cost.
+    """
+
+    def __init__(self, cost=0.0015):
+        """
+        Cost parameter is the cost of a trade per-dollar. 0.0015
+        on $1 million means $1,500 commission (=1,000,000 x 0.0015)
+        """
+        self.cost = float(cost)
+
+    def __repr__(self):
+        return "{class_name}(cost={cost})".format(
+            class_name=self.__class__.__name__,
+            cost=self.cost)
+
+    def calculate(self, transaction):
+        """
+        returns a tuple of:
+        (per share commission, total transaction commission)
+        """
+        cost_per_share = transaction.price * self.cost
+        return cost_per_share, abs(transaction.amount) * cost_per_share