ENH: raise useful error message on invalid concat arguments

Author: cpcloud

pandas/tools/merge.py

@@ -888,6 +888,11 @@ class _Concatenator(object):
     def __init__(self, objs, axis=0, join='outer', join_axes=None,
                  keys=None, levels=None, names=None,
                  ignore_index=False, verify_integrity=False):
+        if not isinstance(objs, (tuple, list, dict)):
+            raise AssertionError('first argument must be a list of pandas '
+                                 'objects, you passed an object of type '
+                                 '"{0}"'.format(type(objs).__name__))
+
         if join == 'outer':
             self.intersect = False
         elif join == 'inner':

pandas/tools/tests/test_merge.py

@@ -13,7 +13,8 @@
 from pandas.tseries.index import DatetimeIndex
 from pandas.tools.merge import merge, concat, ordered_merge, MergeError
 from pandas.util.testing import (assert_frame_equal, assert_series_equal,
-                                 assert_almost_equal, rands)
+                                 assert_almost_equal, rands,
+                                 makeCustomDataframe as mkdf)
 import pandas.algos as algos
 import pandas.util.testing as tm
 
@@ -1689,6 +1690,11 @@ def test_concat_series_axis1_same_names_ignore_index(self):
         result = concat([s1, s2], axis=1, ignore_index=True)
         self.assertTrue(np.array_equal(result.columns, [0, 1]))
 
+    def test_concat_invalid_first_argument(self):
+        df1 = mkdf(10, 2)
+        df2 = mkdf(10, 2)
+        self.assertRaises(AssertionError, concat, df1, df2)
+
 class TestOrderedMerge(unittest.TestCase):
 
     def setUp(self):