ENH: Allow override of order amount rounding. (#1722)

* ENH: Use regular rounding to calculate order amounts.

We previously tried to prevent accidental over-ordering by truncating
orders down unless they were within 1e-4 of the next higher integer.
Unfortunately, this makes it easy for a sell order to be one share short
of the desired position.

Using regular rounding treats both buys and sells in the same way.

* ENH keep non-rounding behavior consistent, but leave code structured to make easier to override

* DOC make round_order public and describe behavior in docstring

tests/test_algorithm.py

@@ -907,6 +907,27 @@ def before_trading_start(context, data):
         )
         self.assertEqual(algo.sim_params.data_frequency, 'minute')
 
+    def test_order_rounding(self):
+        answer_key = [
+            (0, 0),
+            (10, 10),
+            (1.1, 1),
+            (1.5, 1),
+            (1.9998, 1),
+            (1.99991, 2),
+        ]
+
+        for input, answer in answer_key:
+            self.assertEqual(
+                answer,
+                TradingAlgorithm.round_order(input)
+            )
+
+            self.assertEqual(
+                -1 * answer,
+                TradingAlgorithm.round_order(-1 * input)
+            )
+
     @parameterized.expand([
         ('order', TestOrderAlgorithm,),
         ('order_value', TestOrderValueAlgorithm,),

zipline/algorithm.py

@@ -131,7 +131,7 @@ from zipline.utils.events import (
 from zipline.utils.factory import create_simulation_parameters
 from zipline.utils.math_utils import (
     tolerant_equals,
-    round_if_near_integer
+    round_if_near_integer,
 )
 from zipline.utils.pandas_utils import clear_dataframe_indexer_caches
 from zipline.utils.preprocess import preprocess
@@ -1422,10 +1422,7 @@ class TradingAlgorithm(object):
 
     def _calculate_order(self, asset, amount,
                          limit_price=None, stop_price=None, style=None):
-        # Truncate to the integer share count that's either within .0001 of
-        # amount or closer to zero.
-        # E.g. 3.9999 -> 4.0; 5.5 -> 5.0; -5.5 -> -5.0
-        amount = int(round_if_near_integer(amount))
+        amount = self.round_order(amount)
 
         # Raises a ZiplineError if invalid parameters are detected.
         self.validate_order_params(asset,
@@ -1441,6 +1438,18 @@ class TradingAlgorithm(object):
                                                         style)
         return amount, style
 
+    @staticmethod
+    def round_order(amount):
+        """
+        Convert number of shares to an integer.
+
+        By default, truncates to the integer share count that's either within
+        .0001 of amount or closer to zero.
+
+        E.g. 3.9999 -> 4.0; 5.5 -> 5.0; -5.5 -> -5.0
+        """
+        return int(round_if_near_integer(amount))
+
     def validate_order_params(self,
                               asset,
                               amount,