API: remove the copy kw from .xs to prevent an expectation of a view (which may not be possible)

doc/source/api.rst

@@ -78,7 +78,7 @@ SQL
 
 .. autosummary::
    :toctree: generated/
-    
+
    read_sql_table
    read_sql_query
    read_sql

doc/source/release.rst

@@ -176,6 +176,10 @@ API Changes
 - ``.quantile`` on a ``datetime[ns]`` series now returns ``Timestamp`` instead
   of ``np.datetime64`` objects (:issue:`6810`)
 
+- Remove the ``copy`` keyword from ``DataFrame.xs``,``Panel.major_xs``,``Panel.minor_xs``. A view will be
+  returned if possible, otherwise a copy will be made. Previously the user could think that ``copy=False`` would
+  ALWAYS return a view. (:issue:`6894`)
+
 Deprecations
 ~~~~~~~~~~~~
 

doc/source/v0.14.0.txt

@@ -214,6 +214,10 @@ API changes
   (and numpy defaults)
 - add ``inplace`` keyword to ``Series.order/sort`` to make them inverses (:issue:`6859`)
 
+- Remove the ``copy`` keyword from ``DataFrame.xs``,``Panel.major_xs``,``Panel.minor_xs``. A view will be
+  returned if possible, otherwise a copy will be made. Previously the user could think that ``copy=False`` would
+  ALWAYS return a view. (:issue:`6894`)
+
 .. _whatsnew_0140.sql:
 
 SQL

pandas/core/frame.py

@@ -1564,7 +1564,7 @@ def irow(self, i, copy=False):
     def icol(self, i):
         return self._ixs(i, axis=1)
 
-    def _ixs(self, i, axis=0, copy=False):
+    def _ixs(self, i, axis=0):
         """
         i : int, slice, or sequence of integers
         axis : int
@@ -1588,7 +1588,10 @@ def _ixs(self, i, axis=0, copy=False):
                     result = self.take(i, axis=axis)
                     copy=True
                 else:
-                    new_values, copy = self._data.fast_xs(i, copy=copy)
+                    new_values = self._data.fast_xs(i)
+
+                    # if we are a copy, mark as such
+                    copy = isinstance(new_values,np.ndarray) and new_values.base is None
                     result = Series(new_values, index=self.columns,
                                     name=self.index[i], dtype=new_values.dtype)
                 result._set_is_copy(self, copy=copy)

pandas/core/generic.py

@@ -1240,7 +1240,7 @@ def take(self, indices, axis=0, convert=True, is_copy=True):
 
         return result
 
-    def xs(self, key, axis=0, level=None, copy=True, drop_level=True):
+    def xs(self, key, axis=0, level=None, copy=None, drop_level=True):
         """
         Returns a cross-section (row(s) or column(s)) from the Series/DataFrame.
         Defaults to cross-section on the rows (axis=0).
@@ -1254,7 +1254,7 @@ def xs(self, key, axis=0, level=None, copy=True, drop_level=True):
         level : object, defaults to first n levels (n=1 or len(key))
             In case of a key partially contained in a MultiIndex, indicate
             which levels are used. Levels can be referred by label or position.
-        copy : boolean, default True
+        copy : boolean [deprecated]
             Whether to make a copy of the data
         drop_level : boolean, default True
             If False, returns object with same levels as self.
@@ -1276,14 +1276,6 @@ def xs(self, key, axis=0, level=None, copy=True, drop_level=True):
         b    9
         c    3
         Name: C
-        >>> s = df.xs('a', copy=False)
-        >>> s['A'] = 100
-        >>> df
-             A  B  C
-        a  100  5  2
-        b    4  0  9
-        c    9  7  3
-
 
         >>> df
                             A  B  C  D
@@ -1310,16 +1302,24 @@ def xs(self, key, axis=0, level=None, copy=True, drop_level=True):
         -------
         xs : Series or DataFrame
 
+        Notes
+        -----
+        xs is only for getting, not setting values.
+
+        MultiIndex Slicers is a generic way to get/set values on any level or levels
+        it is a superset of xs functionality, see :ref:`MultiIndex Slicers <indexing.mi_slicers>`
+
         """
+        if copy is not None:
+            warnings.warn("copy keyword is deprecated, "
+                          "default is to return a copy or a view if possible")
+
         axis = self._get_axis_number(axis)
         labels = self._get_axis(axis)
         if level is not None:
             loc, new_ax = labels.get_loc_level(key, level=level,
                                                drop_level=drop_level)
 
-            if not copy and not isinstance(loc, slice):
-                raise ValueError('Cannot retrieve view (copy=False)')
-
             # convert to a label indexer if needed
             if isinstance(loc, slice):
                 lev_num = labels._get_level_number(level)
@@ -1336,10 +1336,7 @@ def xs(self, key, axis=0, level=None, copy=True, drop_level=True):
             return result
 
         if axis == 1:
-            data = self[key]
-            if copy:
-                data = data.copy()
-            return data
+            return self[key]
 
         self._consolidate_inplace()
 
@@ -1362,7 +1359,7 @@ def xs(self, key, axis=0, level=None, copy=True, drop_level=True):
 
         if np.isscalar(loc):
             from pandas import Series
-            new_values, copy = self._data.fast_xs(loc, copy=copy)
+            new_values = self._data.fast_xs(loc)
 
             # may need to box a datelike-scalar
             #

pandas/core/indexing.py

@@ -76,17 +76,14 @@ def _get_label(self, label, axis=0):
             # but will fail when the index is not present
             # see GH5667
             try:
-                return self.obj._xs(label, axis=axis, copy=False)
+                return self.obj._xs(label, axis=axis)
             except:
                 return self.obj[label]
         elif (isinstance(label, tuple) and
                 isinstance(label[axis], slice)):
             raise IndexingError('no slices here, handle elsewhere')
 
-        try:
-            return self.obj._xs(label, axis=axis, copy=False)
-        except Exception:
-            return self.obj._xs(label, axis=axis, copy=True)
+        return self.obj._xs(label, axis=axis)
 
     def _get_loc(self, key, axis=0):
         return self.obj._ixs(key, axis=axis)

pandas/core/internals.py

@@ -2861,9 +2861,7 @@ def xs(self, key, axis=1, copy=True, takeable=False):
 
         new_blocks = []
         if len(self.blocks) > 1:
-            if not copy:
-                raise Exception('cannot get view of mixed-type or '
-                                'non-consolidated DataFrame')
+            # we must copy here as we are mixed type
             for blk in self.blocks:
                 newb = make_block(blk.values[slicer],
                                   blk.items,
@@ -2884,18 +2882,16 @@ def xs(self, key, axis=1, copy=True, takeable=False):
 
         return self.__class__(new_blocks, new_axes)
 
-    def fast_xs(self, loc, copy=False):
+    def fast_xs(self, loc):
         """
         get a cross sectional for a given location in the
         items ; handle dups
 
-        return the result and a flag if a copy was actually made
+        return the result, is *could* be a view in the case of a
+        single block
         """
         if len(self.blocks) == 1:
-            result = self.blocks[0].values[:, loc]
-            if copy:
-                result = result.copy()
-            return result, copy
+            return self.blocks[0].values[:, loc]
 
         items = self.items
 
@@ -2904,7 +2900,7 @@ def fast_xs(self, loc, copy=False):
             result = self._interleave(items)
             if self.ndim == 2:
                 result = result.T
-            return result[loc], True
+            return result[loc]
 
         # unique
         dtype = _interleaved_dtype(self.blocks)
@@ -2915,7 +2911,7 @@ def fast_xs(self, loc, copy=False):
                 i = items.get_loc(item)
                 result[i] = blk._try_coerce_result(blk.iget((j, loc)))
 
-        return result, True
+        return result
 
     def consolidate(self):
         """
@@ -3829,12 +3825,12 @@ def _consolidate_check(self):
     def _consolidate_inplace(self):
         pass
 
-    def fast_xs(self, loc, copy=False):
+    def fast_xs(self, loc):
         """
         fast path for getting a cross-section
+        return a view of the data
         """
-        result = self._block.values[loc]
-        return result, False
+        return self._block.values[loc]
 
 def construction_error(tot_items, block_shape, axes, e=None):
     """ raise a helpful message about our construction """

pandas/core/panel.py

@@ -688,43 +688,67 @@ def _combine_panel(self, other, func):
 
         return self._constructor(result_values, items, major, minor)
 
-    def major_xs(self, key, copy=True):
+    def major_xs(self, key, copy=None):
         """
         Return slice of panel along major axis
 
         Parameters
         ----------
         key : object
             Major axis label
-        copy : boolean, default True
-            Copy data
+        copy : boolean [deprecated]
+            Whether to make a copy of the data
 
         Returns
         -------
         y : DataFrame
             index -> minor axis, columns -> items
+
+        Notes
+        -----
+        major_xs is only for getting, not setting values.
+
+        MultiIndex Slicers is a generic way to get/set values on any level or levels
+        it is a superset of major_xs functionality, see :ref:`MultiIndex Slicers <indexing.mi_slicers>`
+
         """
-        return self.xs(key, axis=self._AXIS_LEN - 2, copy=copy)
+        if copy is not None:
+            warnings.warn("copy keyword is deprecated, "
+                          "default is to return a copy or a view if possible")
 
-    def minor_xs(self, key, copy=True):
+        return self.xs(key, axis=self._AXIS_LEN - 2)
+
+    def minor_xs(self, key, copy=None):
         """
         Return slice of panel along minor axis
 
         Parameters
         ----------
         key : object
             Minor axis label
-        copy : boolean, default True
-            Copy data
+        copy : boolean [deprecated]
+            Whether to make a copy of the data
 
         Returns
         -------
         y : DataFrame
             index -> major axis, columns -> items
+
+        Notes
+        -----
+        minor_xs is only for getting, not setting values.
+
+        MultiIndex Slicers is a generic way to get/set values on any level or levels
+        it is a superset of minor_xs functionality, see :ref:`MultiIndex Slicers <indexing.mi_slicers>`
+
         """
-        return self.xs(key, axis=self._AXIS_LEN - 1, copy=copy)
+        if copy is not None:
+            warnings.warn("copy keyword is deprecated, "
+                          "default is to return a copy or a view if possible")
+
+        return self.xs(key, axis=self._AXIS_LEN - 1)
 
-    def xs(self, key, axis=1, copy=True):
+    def xs(self, key, axis=1, copy=None):
         """
         Return slice of panel along selected axis
 
@@ -733,24 +757,36 @@ def xs(self, key, axis=1, copy=True):
         key : object
             Label
         axis : {'items', 'major', 'minor}, default 1/'major'
-        copy : boolean, default True
-            Copy data
+        copy : boolean [deprecated]
+            Whether to make a copy of the data
 
         Returns
         -------
         y : ndim(self)-1
+
+        Notes
+        -----
+        xs is only for getting, not setting values.
+
+        MultiIndex Slicers is a generic way to get/set values on any level or levels
+        it is a superset of xs functionality, see :ref:`MultiIndex Slicers <indexing.mi_slicers>`
+
         """
+        if copy is not None:
+            warnings.warn("copy keyword is deprecated, "
+                          "default is to return a copy or a view if possible")
+
         axis = self._get_axis_number(axis)
         if axis == 0:
-            data = self[key]
-            if copy:
-                data = data.copy()
-            return data
+            return self[key]
 
         self._consolidate_inplace()
         axis_number = self._get_axis_number(axis)
-        new_data = self._data.xs(key, axis=axis_number, copy=copy)
-        return self._construct_return_type(new_data)
+        new_data = self._data.xs(key, axis=axis_number, copy=False)
+        result = self._construct_return_type(new_data)
+        copy = new_data.is_mixed_type
+        result._set_is_copy(self, copy=copy)
+        return result
 
     _xs = xs
 

pandas/tests/test_frame.py

@@ -8371,12 +8371,8 @@ def test_xs(self):
         expected = self.frame['A']
         assert_series_equal(series, expected)
 
-        # no view by default
-        series[:] = 5
-        self.assert_((expected != 5).all())
-
-        # view
-        series = self.frame.xs('A', axis=1, copy=False)
+        # view is returned if possible
+        series = self.frame.xs('A', axis=1)
         series[:] = 5
         self.assert_((expected == 5).all())
 
@@ -11888,25 +11884,16 @@ def test_boolean_set_uncons(self):
         assert_almost_equal(expected, self.frame.values)
 
     def test_xs_view(self):
+        """
+        in 0.14 this will return a view if possible
+        a copy otherwise, but this is numpy dependent
+        """
+
         dm = DataFrame(np.arange(20.).reshape(4, 5),
                        index=lrange(4), columns=lrange(5))
 
-        dm.xs(2, copy=False)[:] = 5
-        self.assert_((dm.xs(2) == 5).all())
-
         dm.xs(2)[:] = 10
-        self.assert_((dm.xs(2) == 5).all())
-
-        # prior to chained assignment (GH5390)
-        # this would raise, but now just returns a copy (and sets is_copy)
-        # TODO (?): deal with mixed-type fiasco?
-        # with assertRaisesRegexp(TypeError, 'cannot get view of mixed-type'):
-        #    self.mixed_frame.xs(self.mixed_frame.index[2], copy=False)
-
-        # unconsolidated
-        dm['foo'] = 6.
-        dm.xs(3, copy=False)[:] = 10
-        self.assert_((dm.xs(3) == 10).all())
+        self.assert_((dm.xs(2) == 10).all())
 
     def test_boolean_indexing(self):
         idx = lrange(3)