Add tests for 16874

Author: hexgnu

doc/source/whatsnew/v0.23.0.txt

@@ -448,7 +448,7 @@ Reshaping
 - Bug in :func:`cut` which fails when using readonly arrays (:issue:`18773`)
 - Bug in :func:`Dataframe.pivot_table` which fails when the ``aggfunc`` arg is of type string.  The behavior is now consistent with other methods like ``agg`` and ``apply`` (:issue:`18713`)
 - Bug in :func:`DataFrame.merge` in which merging using ``Index`` objects as vectors raised an Exception (:issue:`19038`)
-- Bug in :func:`concat` when concatting sparse and dense series it returns only a SparseDataFrame. Should be a DataFrame. (:issue:`18914`, :issue:`18686`, and :issue:`16874`)
+- Bug in :func:`concat` when concatting sparse and dense series it returns only a ``SparseDataFrame``. Should be a ``DataFrame``. (:issue:`18914`, :issue:`18686`, and :issue:`16874`)
 
 Numeric
 ^^^^^^^

pandas/tests/sparse/test_combine_concat.py

@@ -338,6 +338,10 @@ def test_concat_sparse_dense(self, fill_value):
         # See GH18914 and #18686 for why this should be
         # A DataFrame
         assert isinstance(res, pd.DataFrame)
+        # See GH16874
+        assert res.isnull()
+        assert res[res.columns[0]]
+        assert res.iloc[0,0]
         for column in self.dense3.columns:
             tm.assert_series_equal(res[column], exp[column])
 
@@ -346,6 +350,10 @@ def test_concat_sparse_dense(self, fill_value):
         res = pd.concat([sparse, self.dense3], axis=1)
         exp = pd.concat([self.dense1, self.dense3], axis=1)
         assert isinstance(res, pd.DataFrame)
+        # See GH16874
+        assert res.isnull()
+        assert res[res.columns[0]]
+        assert res.iloc[0,0]
         for column in self.dense3.columns:
             tm.assert_series_equal(res[column], exp[column])
         tm.assert_frame_equal(res, exp)