Fix TypeError when merging categorical dates

pandas/core/reshape/merge.py

@@ -24,6 +24,7 @@
     is_numeric_dtype,
     is_integer,
     is_int_or_datetime_dtype,
+    is_datetimelike,
     is_dtype_equal,
     is_bool,
     is_list_like,
@@ -877,7 +878,7 @@ def _get_merge_keys(self):
         return left_keys, right_keys, join_names
 
     def _maybe_coerce_merge_keys(self):
-        # we have valid mergee's but we may have to further
+        # we have valid mergees but we may have to further
         # coerce these if they are originally incompatible types
         #
         # for example if these are categorical, but are not dtype_equal
@@ -894,6 +895,13 @@ def _maybe_coerce_merge_keys(self):
             if is_categorical_dtype(lk) and is_categorical_dtype(rk):
                 if lk.is_dtype_equal(rk):
                     continue
+
+                # if we are dates with differing categories
+                # then allow them to proceed because
+                # coercing to object below results in integers.
+                if is_datetimelike(lk.categories) and is_datetimelike(rk.categories):
+                    continue
+
             elif is_categorical_dtype(lk) or is_categorical_dtype(rk):
                 pass
 
@@ -904,7 +912,7 @@ def _maybe_coerce_merge_keys(self):
             # kinds to proceed, eg. int64 and int8
             # further if we are object, but we infer to
             # the same, then proceed
-            if (is_numeric_dtype(lk) and is_numeric_dtype(rk)):
+            if is_numeric_dtype(lk) and is_numeric_dtype(rk):
                 if lk.dtype.kind == rk.dtype.kind:
                     continue
 

pandas/tests/reshape/test_merge.py

@@ -1,7 +1,7 @@
 # pylint: disable=E1103
 
 import pytest
-from datetime import datetime
+from datetime import datetime, date
 from numpy.random import randn
 from numpy import nan
 import numpy as np
@@ -1515,6 +1515,27 @@ def test_self_join_multiple_categories(self):
 
         assert_frame_equal(result, df)
 
+    def test_dtype_on_categorical_dates(self):
+        # GH 16900
+        # dates should not be coerced to ints
+
+        df = pd.DataFrame(
+            [[date(2001, 1, 1), 1.1],
+             [date(2001, 1, 2), 1.3]],
+            columns=['date', 'num2']
+        )
+        df['date'] = df['date'].astype('category')
+
+        df2 = pd.DataFrame(
+            [[date(2001, 1, 1), 1.3],
+             [date(2001, 1, 3), 1.4]],
+            columns=['date', 'num4']
+        )
+        df2['date'] = df2['date'].astype('category')
+
+        result = pd.merge(df, df2, how='outer', on=['date'])
+        assert result['date'].dtype == 'category'
+
 
 @pytest.fixture
 def left_df():