BUG: utils.compute_forward_returns can raise out of bounds exception

alphalens/tests/test_utils.py

@@ -81,6 +81,27 @@ def test_compute_forward_returns(self):
 
         assert_frame_equal(fp, expected)
 
+    def test_compute_forward_returns_index_out_of_bound(self):
+        dr = date_range(start='2014-12-29', end='2015-1-3')
+        prices = DataFrame(index=dr, columns=['A', 'B'],
+                           data=[[nan, nan], [nan, nan], [nan, nan],
+                                 [1, 1], [1, 2], [2, 1]])
+
+        dr = date_range(start='2015-1-1', end='2015-1-3')
+        factor = DataFrame(index=dr, columns=['A', 'B'],
+                           data=[[1, 1], [1, 2], [2, 1]])
+        factor = factor.stack()
+
+        fp = compute_forward_returns(factor, prices, periods=[1, 2])
+
+        ix = MultiIndex.from_product([dr, ['A', 'B']],
+                                     names=['date', 'asset'])
+        expected = DataFrame(index=ix, columns=['1D', '2D'])
+        expected['1D'] = [0., 1., 1., -0.5, nan, nan]
+        expected['2D'] = [1., 0., nan, nan, nan, nan]
+
+        assert_frame_equal(fp, expected)
+
     def test_compute_forward_returns_non_cum(self):
         dr = date_range(start='2015-1-1', end='2015-1-3')
         prices = DataFrame(index=dr, columns=['A', 'B'],

alphalens/utils.py

@@ -299,19 +299,17 @@ def compute_forward_returns(factor,
 
         #
         # Find the period length, which will be the column name. We'll test
-        # several entries in order to find out the correct period length as
-        # there could be non-trading days which would make the computation
-        # wrong if made only one test
+        # several entries in order to find out the most likely period length
+        # (in case the user passed inconsinstent data)
         #
-        entries_to_test = min(
-            30,
-            len(forward_returns.index),
-            len(prices.index) - period
-        )
-
         days_diffs = []
-        for i in range(entries_to_test):
+        for i in range(30):
+            if i >= len(forward_returns.index):
+                break
             p_idx = prices.index.get_loc(forward_returns.index[i])
+            if p_idx is None or p_idx < 0 or (
+                    p_idx + period) >= len(prices.index):
+                continue
             start = prices.index[p_idx]
             end = prices.index[p_idx + period]
             period_len = diff_custom_calendar_timedeltas(start, end, freq)
@@ -619,7 +617,7 @@ def get_clean_factor(factor,
     print("Dropped %.1f%% entries from factor data: %.1f%% in forward "
           "returns computation and %.1f%% in binning phase "
           "(set max_loss=0 to see potentially suppressed Exceptions)." %
-          (tot_loss * 100, fwdret_loss * 100,  bin_loss * 100))
+          (tot_loss * 100, fwdret_loss * 100, bin_loss * 100))
 
     if tot_loss > max_loss:
         message = ("max_loss (%.1f%%) exceeded %.1f%%, consider increasing it."