Merge branch 'master' into enh/index_setops

pandas-dev · TomAugspurger · May 21, 2019 · Nov 7, 2018 · Nov 8, 2018 · Nov 8, 2018
commit fbf3242e0f581a3419d93fe0b99ea9719aa96741
diff --git a/doc/source/whatsnew/v0.25.0.rst b/doc/source/whatsnew/v0.25.0.rst
@@ -67,6 +67,41 @@ is respected in indexing. (:issue:`24076`, :issue:`16785`)
     df = pd.DataFrame([0], index=pd.DatetimeIndex(['2019-01-01'], tz='US/Pacific'))
     df['2019-01-01 12:00:00+04:00':'2019-01-01 13:00:00+04:00']
 
+Concatenating Sparse Values
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When passed DataFrames whose values are sparse, :func:`concat` will now return a
+Series or DataFrame with sparse values, rather than a ``SparseDataFrame`` (:issue:`25702`).
+
+.. ipython:: python
+
+   df = pd.DataFrame({"A": pd.SparseArray([0, 1])})
+
+*Previous Behavior:*
+
+.. code-block:: ipython
+
+   In [2]: type(pd.concat([df, df]))
+   pandas.core.sparse.frame.SparseDataFrame
+
+*New Behavior:*
+
+.. ipython:: python
+
+   type(pd.concat([df, df]))
+
+
+This now matches the existing behavior of :class:`concat` on ``Series`` with sparse values.
+:func:`concat` will continue to return a ``SparseDataFrame`` when all the values
+are instances of ``SparseDataFrame``.
+
+This change also affects routines using :func:`concat` internally, like :func:`get_dummies`,
+which now returns a :class:`DataFrame` in all cases (previously a ``SparseDataFrame`` was
+returned if all the columns were dummy encoded, and a :class:`DataFrame` otherwise).
+
+Providing any ``SparseSeries`` or ``SparseDataFrame`` to :func:`concat` will
+cause a ``SparseSeries`` or ``SparseDataFrame`` to be returned, as before.
+
 .. _whatsnew_0250.api_breaking.incompatible_index_unions
 
 Incompatible Index Type Unions