ER: give concat a better error message for incompatible types

doc/source/release.rst

@@ -136,6 +136,8 @@ Improvements to existing features
   - Both ExcelFile and read_excel to accept an xlrd.Book for the io
     (formerly path_or_buf) argument; this requires engine to be set.
     (:issue:`4961`).
+  - ``concat`` now gives a more informative error message when passed objects
+    that cannot be concatenated (:issue:`4608`).
 
 API Changes
 ~~~~~~~~~~~

pandas/tools/merge.py

@@ -1245,7 +1245,11 @@ def _get_comb_axis(self, i):
         if self._is_series:
             all_indexes = [x.index for x in self.objs]
         else:
-            all_indexes = [x._data.axes[i] for x in self.objs]
+            try:
+                all_indexes = [x._data.axes[i] for x in self.objs]
+            except IndexError:
+                types = [type(x).__name__ for x in self.objs]
+                raise TypeError("Cannot concatenate list of %s" % types)
 
         return _get_combined_index(all_indexes, intersect=self.intersect)
 
@@ -1256,6 +1260,10 @@ def _get_concat_axis(self):
             elif self.keys is None:
                 names = []
                 for x in self.objs:
+                    if not isinstance(x, Series):
+                        raise TypeError("Cannot concatenate type 'Series' "
+                                        "with object of type "
+                                        "%r" % type(x).__name__)
                     if x.name is not None:
                         names.append(x.name)
                     else:

pandas/tools/tests/test_merge.py

@@ -1804,6 +1804,15 @@ def test_concat_invalid_first_argument(self):
         # generator ok though
         concat(DataFrame(np.random.rand(5,5)) for _ in range(3))
 
+    def test_concat_mixed_types_fails(self):
+        df = DataFrame(randn(10, 1))
+
+        with tm.assertRaisesRegexp(TypeError, "Cannot concatenate.+"):
+            concat([df[0], df], axis=1)
+
+        with tm.assertRaisesRegexp(TypeError, "Cannot concatenate.+"):
+            concat([df, df[0]], axis=1)
+
 class TestOrderedMerge(unittest.TestCase):
 
     def setUp(self):