BUG: Fix groupby with string columns.

The previous algorithm assumed that the group labels were integers. It
produced nonsense with LabelArrays (though sadly didn't crash because
numpy promotes None and void to object).

tests/pipeline/test_factor.py

@@ -23,6 +23,7 @@ from numpy import (
 from numpy.random import randn, seed
 
 from zipline.errors import UnknownRankMethod
+from zipline.lib.labelarray import LabelArray
 from zipline.lib.rank import masked_rankdata_2d
 from zipline.lib.normalize import naive_grouped_rowwise_apply as grouped_apply
 from zipline.pipeline import Classifier, Factor, Filter, TermGraph
@@ -38,6 +39,7 @@ from zipline.testing import (
 )
 from zipline.utils.functional import dzip_exact
 from zipline.utils.numpy_utils import (
+    categorical_dtype,
     datetime64ns_dtype,
     float64_dtype,
     int64_dtype,
@@ -442,6 +444,7 @@ class FactorTestCase(BasePipelineTestCase):
         f = self.f
         m = Mask()
         c = C()
+        str_c = C(dtype=categorical_dtype)
 
         factor_data = array(
             [[1.0, 2.0, 3.0, 4.0],
@@ -463,12 +466,18 @@ class FactorTestCase(BasePipelineTestCase):
              [1, 1, 2, 2]],
             dtype=int64_dtype,
         )
+        string_classifier_data = LabelArray(
+            classifier_data.astype(str).astype(object),
+            missing_value=None,
+        )
 
         terms = {
             'vanilla': f.demean(),
             'masked': f.demean(mask=m),
             'grouped': f.demean(groupby=c),
+            'grouped_str': f.demean(groupby=str_c),
             'grouped_masked': f.demean(mask=m, groupby=c),
+            'grouped_masked_str': f.demean(mask=m, groupby=str_c),
         }
         expected = {
             'vanilla': array(
@@ -496,6 +505,9 @@ class FactorTestCase(BasePipelineTestCase):
                  [-0.500,  0.500,  0.000,    nan]]
             )
         }
+        # Changing the classifier dtype shouldn't affect anything.
+        expected['grouped_str'] = expected['grouped']
+        expected['grouped_masked_str'] = expected['grouped_masked']
 
         graph = TermGraph(terms)
         results = self.run_graph(
@@ -503,6 +515,7 @@ class FactorTestCase(BasePipelineTestCase):
             initial_workspace={
                 f: factor_data,
                 c: classifier_data,
+                str_c: string_classifier_data,
                 m: filter_data,
             },
             mask=self.build_mask(self.ones_mask(shape=factor_data.shape)),