improved error message when concatenating an empty sequence of dataframes

Author: cel4

doc/source/whatsnew/v0.17.0.txt

@@ -526,6 +526,7 @@ Series with a ``CategoricalIndex`` (:issue:`10704`)
    ``return np.datetime64('NaT')``     ``to_datetime64`` (unchanged)
    ``raise ValueError``                All other public methods (names not beginning with underscores)
    ===============================     ===============================================================
+- Improved error message when concatenating an empty iterable of dataframes (:issue:`9157`)
 
 .. _whatsnew_0170.deprecations:
 

pandas/tools/merge.py

@@ -775,9 +775,14 @@ def __init__(self, objs, axis=0, join='outer', join_axes=None,
             if keys is None:
                 keys = sorted(objs)
             objs = [objs[k] for k in keys]
+        else:
+            objs = list(objs)
+
+        if len(objs) == 0:
+            raise ValueError('No objects to concatenate')
 
         if keys is None:
-            objs = [obj for obj in objs if obj is not None ]
+            objs = [obj for obj in objs if obj is not None]
         else:
             # #1649
             clean_keys = []

pandas/tools/tests/test_merge.py

@@ -2552,6 +2552,23 @@ def _constructor(self):
 
         tm.assertIsInstance(result, NotADataFrame)
 
+    def test_empty_sequence_concat(self):
+        # GH 9157
+        empty_pat = "[Nn]o objects"
+        none_pat = "objects.*None"
+        test_cases = [
+            ((), empty_pat),
+            ([], empty_pat),
+            ({}, empty_pat),
+            ([None], none_pat),
+            ([None, None], none_pat)
+        ]
+        for df_seq, pattern in test_cases:
+            assertRaisesRegexp(ValueError, pattern, pd.concat, df_seq)
+
+        pd.concat([pd.DataFrame()])
+        pd.concat([None, pd.DataFrame()])
+        pd.concat([pd.DataFrame(), None])
 
 if __name__ == '__main__':
     nose.runmodule(argv=[__file__, '-vvs', '-x', '--pdb', '--pdb-failure'],