BUG: fix issue with concat creating SparseFrame if not all series are sparse.

doc/source/whatsnew/v0.23.0.txt

@@ -537,6 +537,7 @@ Reshaping
 - Bug in :func:`DataFrame.merge` in which merging using ``Index`` objects as vectors raised an Exception (:issue:`19038`)
 - Bug in :func:`DataFrame.stack`, :func:`DataFrame.unstack`, :func:`Series.unstack` which were not returning subclasses (:issue:`15563`)
 - Bug in timezone comparisons, manifesting as a conversion of the index to UTC in ``.concat()`` (:issue:`18523`)
+- 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`)
 -
 
 

pandas/core/dtypes/concat.py

@@ -19,7 +19,7 @@
     _TD_DTYPE)
 from pandas.core.dtypes.generic import (
     ABCDatetimeIndex, ABCTimedeltaIndex,
-    ABCPeriodIndex, ABCRangeIndex)
+    ABCPeriodIndex, ABCRangeIndex, ABCSparseDataFrame)
 
 
 def get_dtype_kinds(l):
@@ -89,14 +89,16 @@ def _get_series_result_type(result, objs=None):
 def _get_frame_result_type(result, objs):
     """
     return appropriate class of DataFrame-like concat
-    if any block is SparseBlock, return SparseDataFrame
+    if all blocks are SparseBlock, return SparseDataFrame
     otherwise, return 1st obj
     """
-    if any(b.is_sparse for b in result.blocks):
+
+    if result.blocks and all(b.is_sparse for b in result.blocks):
         from pandas.core.sparse.api import SparseDataFrame
         return SparseDataFrame
     else:
-        return objs[0]
+        return next(obj for obj in objs if not isinstance(obj,
+                                                          ABCSparseDataFrame))
 
 
 def _concat_compat(to_concat, axis=0):

pandas/core/dtypes/generic.py

@@ -43,6 +43,8 @@ def _check(cls, inst):
 
 ABCSeries = create_pandas_abc_type("ABCSeries", "_typ", ("series", ))
 ABCDataFrame = create_pandas_abc_type("ABCDataFrame", "_typ", ("dataframe", ))
+ABCSparseDataFrame = create_pandas_abc_type("ABCSparseDataFrame", "_subtyp",
+                                            ("sparse_frame", ))
 ABCPanel = create_pandas_abc_type("ABCPanel", "_typ", ("panel",))
 ABCSparseSeries = create_pandas_abc_type("ABCSparseSeries", "_subtyp",
                                          ('sparse_series',

pandas/tests/dtypes/test_generic.py

@@ -18,6 +18,7 @@ class TestABCClasses(object):
     df = pd.DataFrame({'names': ['a', 'b', 'c']}, index=multi_index)
     sparse_series = pd.Series([1, 2, 3]).to_sparse()
     sparse_array = pd.SparseArray(np.random.randn(10))
+    sparse_frame = pd.SparseDataFrame({'a': [1, -1, None]})
 
     def test_abc_types(self):
         assert isinstance(pd.Index(['a', 'b', 'c']), gt.ABCIndex)
@@ -37,6 +38,7 @@ def test_abc_types(self):
             assert isinstance(self.df.to_panel(), gt.ABCPanel)
         assert isinstance(self.sparse_series, gt.ABCSparseSeries)
         assert isinstance(self.sparse_array, gt.ABCSparseArray)
+        assert isinstance(self.sparse_frame, gt.ABCSparseDataFrame)
         assert isinstance(self.categorical, gt.ABCCategorical)
         assert isinstance(pd.Period('2012', freq='A-DEC'), gt.ABCPeriod)
 

pandas/tests/reshape/test_reshape.py

@@ -454,6 +454,15 @@ def test_dataframe_dummies_preserve_categorical_dtype(self, dtype):
 
             tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize('sparse', [True, False])
+    def test_get_dummies_dont_sparsify_all_columns(self, sparse):
+        # GH18914
+        df = DataFrame.from_items([('GDP', [1, 2]), ('Nation', ['AB', 'CD'])])
+        df = get_dummies(df, columns=['Nation'], sparse=sparse)
+        df2 = df.reindex(columns=['GDP'])
+
+        tm.assert_frame_equal(df[['GDP']], df2)
+
 
 class TestCategoricalReshape(object):
 

pandas/tests/sparse/test_combine_concat.py

@@ -1,8 +1,10 @@
 # pylint: disable-msg=E1101,W0612
+import pytest
 
 import numpy as np
 import pandas as pd
 import pandas.util.testing as tm
+import itertools
 
 
 class TestSparseSeriesConcat(object):
@@ -317,37 +319,52 @@ def test_concat_axis1(self):
         assert isinstance(res, pd.SparseDataFrame)
         tm.assert_frame_equal(res.to_dense(), exp)
 
-    def test_concat_sparse_dense(self):
-        sparse = self.dense1.to_sparse()
-
-        res = pd.concat([sparse, self.dense2])
-        exp = pd.concat([self.dense1, self.dense2])
-        assert isinstance(res, pd.SparseDataFrame)
-        tm.assert_frame_equal(res.to_dense(), exp)
-
-        res = pd.concat([self.dense2, sparse])
-        exp = pd.concat([self.dense2, self.dense1])
-        assert isinstance(res, pd.SparseDataFrame)
-        tm.assert_frame_equal(res.to_dense(), exp)
-
-        sparse = self.dense1.to_sparse(fill_value=0)
-
-        res = pd.concat([sparse, self.dense2])
-        exp = pd.concat([self.dense1, self.dense2])
-        assert isinstance(res, pd.SparseDataFrame)
-        tm.assert_frame_equal(res.to_dense(), exp)
-
-        res = pd.concat([self.dense2, sparse])
-        exp = pd.concat([self.dense2, self.dense1])
-        assert isinstance(res, pd.SparseDataFrame)
-        tm.assert_frame_equal(res.to_dense(), exp)
-
-        res = pd.concat([self.dense3, sparse], axis=1)
-        exp = pd.concat([self.dense3, self.dense1], axis=1)
-        assert isinstance(res, pd.SparseDataFrame)
-        tm.assert_frame_equal(res, exp)
-
-        res = pd.concat([sparse, self.dense3], axis=1)
-        exp = pd.concat([self.dense1, self.dense3], axis=1)
-        assert isinstance(res, pd.SparseDataFrame)
-        tm.assert_frame_equal(res, exp)
+    @pytest.mark.parametrize('fill_value,sparse_idx,dense_idx',
+                             itertools.product([None, 0, 1, np.nan],
+                                               [0, 1],
+                                               [1, 0]))
+    def test_concat_sparse_dense_rows(self, fill_value, sparse_idx, dense_idx):
+        frames = [self.dense1, self.dense2]
+        sparse_frame = [frames[dense_idx],
+                        frames[sparse_idx].to_sparse(fill_value=fill_value)]
+        dense_frame = [frames[dense_idx], frames[sparse_idx]]
+
+        # This will try both directions sparse + dense and dense + sparse
+        for _ in range(2):
+            res = pd.concat(sparse_frame)
+            exp = pd.concat(dense_frame)
+
+            assert isinstance(res, pd.SparseDataFrame)
+            tm.assert_frame_equal(res.to_dense(), exp)
+
+            sparse_frame = sparse_frame[::-1]
+            dense_frame = dense_frame[::-1]
+
+    @pytest.mark.parametrize('fill_value,sparse_idx,dense_idx',
+                             itertools.product([None, 0, 1, np.nan],
+                                               [0, 1],
+                                               [1, 0]))
+    def test_concat_sparse_dense_cols(self, fill_value, sparse_idx, dense_idx):
+        # See GH16874, GH18914 and #18686 for why this should be a DataFrame
+
+        frames = [self.dense1, self.dense3]
+
+        sparse_frame = [frames[dense_idx],
+                        frames[sparse_idx].to_sparse(fill_value=fill_value)]
+        dense_frame = [frames[dense_idx], frames[sparse_idx]]
+
+        # This will try both directions sparse + dense and dense + sparse
+        for _ in range(2):
+            res = pd.concat(sparse_frame, axis=1)
+            exp = pd.concat(dense_frame, axis=1)
+
+            for column in frames[dense_idx].columns:
+                if dense_idx == sparse_idx:
+                    tm.assert_frame_equal(res[column], exp[column])
+                else:
+                    tm.assert_series_equal(res[column], exp[column])
+
+            tm.assert_frame_equal(res, exp)
+
+            sparse_frame = sparse_frame[::-1]
+            dense_frame = dense_frame[::-1]