BUG: GH11847 Unstack with mixed dtypes coerces everything to object

Changed the way in which the original data frame is copied (dropped use of .values, since it does not preserve dtypes).

doc/source/whatsnew/v0.19.2.txt

@@ -68,3 +68,5 @@ Bug Fixes
 
 
 - Explicit check in ``to_stata`` and ``StataWriter`` for out-of-range values when writing doubles (:issue:`14618`)
+
+- Bug in ``unstack()`` if called with a list of column(s) as an argument, regardless of the dtypes of all columns, they get coerced to ``object`` (:issue:`11847`)

pandas/core/reshape.py

@@ -277,7 +277,8 @@ def _unstack_multiple(data, clocs):
                              verify_integrity=False)
 
     if isinstance(data, Series):
-        dummy = Series(data.values, index=dummy_index)
+        dummy = data.copy()
+        dummy.index = dummy_index
         unstacked = dummy.unstack('__placeholder__')
         new_levels = clevels
         new_names = cnames
@@ -292,7 +293,8 @@ def _unstack_multiple(data, clocs):
 
             return result
 
-        dummy = DataFrame(data.values, index=dummy_index, columns=data.columns)
+        dummy = data.copy()
+        dummy.index = dummy_index
 
         unstacked = dummy.unstack('__placeholder__')
         if isinstance(unstacked, Series):

pandas/tests/frame/test_reshape.py

@@ -282,6 +282,46 @@ class TestDataFrameReshape(tm.TestCase, TestData):
                              index=list('xyz'))
         assert_frame_equal(result, expected)
 
+    def test_unstack_preserve_dtypes(self):
+        # Checks fix for #11847
+        df = pd.DataFrame(dict(state=['IL', 'MI', 'NC'],
+                               index=['a', 'b', 'c'],
+                               some_categories=pd.Series(['a', 'b', 'c']
+                                                         ).astype('category'),
+                               A=np.random.rand(3),
+                               B=1,
+                               C='foo',
+                               D=pd.Timestamp('20010102'),
+                               E=pd.Series([1.0, 50.0, 100.0]
+                                           ).astype('float32'),
+                               F=pd.Series([3.0, 4.0, 5.0]).astype('float64'),
+                               G=False,
+                               H=pd.Series([1, 200, 923442], dtype='int8')))
+
+        def unstack_and_compare(df, column_name):
+            unstacked1 = df.unstack([column_name])
+            unstacked2 = df.unstack(column_name)
+            assert_frame_equal(unstacked1, unstacked2)
+
+        df1 = df.set_index(['state', 'index'])
+        unstack_and_compare(df1, 'index')
+
+        df1 = df.set_index(['state', 'some_categories'])
+        unstack_and_compare(df1, 'some_categories')
+
+        df1 = df.set_index(['F', 'C'])
+        unstack_and_compare(df1, 'F')
+
+        df1 = df.set_index(['G', 'B', 'state'])
+        unstack_and_compare(df1, 'B')
+
+        df1 = df.set_index(['E', 'A'])
+        unstack_and_compare(df1, 'E')
+
+        df1 = df.set_index(['state', 'index'])
+        s = df1['A']
+        unstack_and_compare(s, 'index')
+
     def test_stack_ints(self):
         columns = MultiIndex.from_tuples(list(itertools.product(range(3),
                                                                 repeat=3)))