REGR: ensure DataFrame.select_dtypes() returns a copy (#48176)

Author: jorisvandenbossche

doc/source/whatsnew/v1.4.4.rst

@@ -26,6 +26,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrame.loc` setting a length-1 array like value to a single value in the DataFrame (:issue:`46268`)
 - Fixed regression when slicing with :meth:`DataFrame.loc` with :class:`DateOffset`-index (:issue:`46671`)
 - Fixed regression in setting ``None`` or non-string value into a ``string``-dtype Series using a mask (:issue:`47628`)
+- Fixed regression in :meth:`DataFrame.select_dtypes` returning a view on the original DataFrame (:issue:`48090`)
 - Fixed regression using custom Index subclasses (for example, used in xarray) with :meth:`~DataFrame.reset_index` or :meth:`Index.insert` (:issue:`47071`)
 - Fixed regression in :meth:`DatetimeIndex.intersection` when the :class:`DatetimeIndex` has dates crossing daylight savings time (:issue:`46702`)
 - Fixed regression in :func:`merge` throwing an error when passing a :class:`Series` with a multi-level name (:issue:`47946`)

pandas/core/frame.py

@@ -4740,7 +4740,7 @@ def predicate(arr: ArrayLike) -> bool:
 
             return True
 
-        mgr = self._mgr._get_data_subset(predicate)
+        mgr = self._mgr._get_data_subset(predicate).copy(deep=None)
         return type(self)(mgr).__finalize__(self)
 
     def insert(

pandas/tests/copy_view/test_methods.py

@@ -139,18 +139,16 @@ def test_select_dtypes(using_copy_on_write):
     df2 = df.select_dtypes("int64")
     df2._mgr._verify_integrity()
 
-    # currently this always returns a "view"
-    assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
 
     # mutating df2 triggers a copy-on-write for that column/block
     df2.iloc[0, 0] = 0
     if using_copy_on_write:
         assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
-        tm.assert_frame_equal(df, df_orig)
-    else:
-        # but currently select_dtypes() actually returns a view -> mutates parent
-        df_orig.iloc[0, 0] = 0
-        tm.assert_frame_equal(df, df_orig)
+    tm.assert_frame_equal(df, df_orig)
 
 
 def test_to_frame(using_copy_on_write):

pandas/tests/frame/methods/test_select_dtypes.py

@@ -456,3 +456,12 @@ def test_np_bool_ea_boolean_include_number(self):
         result = df.select_dtypes(include="number")
         expected = DataFrame({"a": [1, 2, 3]})
         tm.assert_frame_equal(result, expected)
+
+    def test_select_dtypes_no_view(self):
+        # https://github.com/pandas-dev/pandas/issues/48090
+        # result of this method is not a view on the original dataframe
+        df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+        df_orig = df.copy()
+        result = df.select_dtypes(include=["number"])
+        result.iloc[0, 0] = 0
+        tm.assert_frame_equal(df, df_orig)