validate result_type kwarg

Author: jreback

pandas/core/apply.py

@@ -39,6 +39,10 @@ def __init__(self, obj, func, broadcast, raw, reduce, result_type,
         self.args = args or ()
         self.kwds = kwds or {}
 
+        if result_type not in [None, 'reduce', 'broadcast', 'expand']:
+            raise ValueError("invalid value for result_type, must be one "
+                             "of {None, 'reduce', 'broadcast', 'expand'}")
+
         if broadcast is not None:
             warnings.warn("The broadcast argument is deprecated and will "
                           "be removed in a future version. You can specify "

pandas/io/formats/style.py

@@ -510,7 +510,7 @@ def _apply(self, func, axis=0, subset=None, **kwargs):
         data = self.data.loc[subset]
         if axis is not None:
             result = data.apply(func, axis=axis,
-                                result_type='infer', **kwargs)
+                                result_type='expand', **kwargs)
             result.columns = data.columns
         else:
             result = func(data, **kwargs)

pandas/tests/frame/test_apply.py

@@ -750,6 +750,18 @@ def test_result_type(self):
         expected.columns = columns
         assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize("result_type", ['foo', 1])
+    def test_result_type_error(self, result_type):
+        # allowed result_type
+        df = DataFrame(
+            np.tile(np.arange(3, dtype='int64'), 6).reshape(6, -1) + 1,
+            columns=['A', 'B', 'C'])
+
+        with pytest.raises(ValueError):
+            df.apply(lambda x: [1, 2, 3],
+                     axis=1,
+                     result_type=result_type)
+
     @pytest.mark.parametrize(
         "box",
         [lambda x: list(x),