DEPR: type argument in Index.view (#56421)

Author: jbrockmendel

doc/source/whatsnew/v2.2.0.rst

@@ -438,6 +438,7 @@ Other Deprecations
 - Deprecated :meth:`Series.view`, use :meth:`Series.astype` instead to change the dtype (:issue:`20251`)
 - Deprecated ``core.internals`` members ``Block``, ``ExtensionBlock``, and ``DatetimeTZBlock``, use public APIs instead (:issue:`55139`)
 - Deprecated ``year``, ``month``, ``quarter``, ``day``, ``hour``, ``minute``, and ``second`` keywords in the :class:`PeriodIndex` constructor, use :meth:`PeriodIndex.from_fields` instead (:issue:`55960`)
+- Deprecated accepting a type as an argument in :meth:`Index.view`, call without any arguments instead (:issue:`55709`)
 - Deprecated allowing non-integer ``periods`` argument in :func:`date_range`, :func:`timedelta_range`, :func:`period_range`, and :func:`interval_range` (:issue:`56036`)
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_clipboard`. (:issue:`54229`)
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_csv` except ``path_or_buf``. (:issue:`54229`)

pandas/core/indexes/base.py

@@ -1015,6 +1015,16 @@ def view(self, cls=None):
 
             result = self._data.view(cls)
         else:
+            if cls is not None:
+                warnings.warn(
+                    # GH#55709
+                    f"Passing a type in {type(self).__name__}.view is deprecated "
+                    "and will raise in a future version. "
+                    "Call view without any argument to retain the old behavior.",
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
+
             result = self._view()
         if isinstance(result, Index):
             result._id = self._id

pandas/tests/indexes/datetimes/test_setops.py

@@ -535,7 +535,7 @@ def test_intersection(self):
         assert isinstance(the_int, DatetimeIndex)
         assert the_int.freq == rng.freq
 
-        the_int = rng1.intersection(rng2.view(DatetimeIndex))
+        the_int = rng1.intersection(rng2)
         tm.assert_index_equal(the_int, expected)
 
         # non-overlapping

pandas/tests/indexes/numeric/test_numeric.py

@@ -318,7 +318,9 @@ def test_cant_or_shouldnt_cast(self, dtype):
 
     def test_view_index(self, simple_index):
         index = simple_index
-        index.view(Index)
+        msg = "Passing a type in .*Index.view is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            index.view(Index)
 
     def test_prevent_casting(self, simple_index):
         index = simple_index

pandas/tests/indexes/ranges/test_range.py

@@ -199,7 +199,9 @@ def test_view(self):
         i_view = i.view("i8")
         tm.assert_numpy_array_equal(i.values, i_view)
 
-        i_view = i.view(RangeIndex)
+        msg = "Passing a type in RangeIndex.view is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            i_view = i.view(RangeIndex)
         tm.assert_index_equal(i, i_view)
 
     def test_dtype(self, simple_index):
@@ -382,7 +384,9 @@ def test_cant_or_shouldnt_cast(self, start, stop, step):
 
     def test_view_index(self, simple_index):
         index = simple_index
-        index.view(Index)
+        msg = "Passing a type in RangeIndex.view is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            index.view(Index)
 
     def test_prevent_casting(self, simple_index):
         index = simple_index

pandas/tests/indexes/test_datetimelike.py

@@ -89,7 +89,9 @@ def test_view(self, simple_index):
         result = type(simple_index)(idx)
         tm.assert_index_equal(result, idx)
 
-        idx_view = idx.view(type(simple_index))
+        msg = "Passing a type in .*Index.view is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            idx_view = idx.view(type(simple_index))
         result = type(simple_index)(idx)
         tm.assert_index_equal(result, idx_view)
 

pandas/tests/indexes/test_old_base.py

@@ -960,7 +960,9 @@ def test_view(self, simple_index):
         idx_view = idx.view(dtype)
         tm.assert_index_equal(idx, index_cls(idx_view, name="Foo"), exact=True)
 
-        idx_view = idx.view(index_cls)
+        msg = "Passing a type in .*Index.view is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            idx_view = idx.view(index_cls)
         tm.assert_index_equal(idx, index_cls(idx_view, name="Foo"), exact=True)
 
     def test_format(self, simple_index):