API/CoW: take view of underlying Block values for shallow (deep=False) copies

pandas/core/internals/blocks.py

@@ -645,17 +645,17 @@ def copy(self, *, deep: bool) -> Self:
             values = values.copy()
             refs = None
         else:
+            values = values.view()
             refs = self.refs
         return type(self)(values, placement=self._mgr_locs, ndim=self.ndim, refs=refs)
 
     # ---------------------------------------------------------------------
     # Copy-on-Write Helpers
 
-    def _maybe_copy(self, inplace: bool) -> Self:
-        if inplace:
-            deep = self.refs.has_reference()
-            return self.copy(deep=deep)
-        return self.copy(deep=True)
+    def _maybe_copy(self, inplace: bool, deep: bool = True) -> Self:
+        if inplace and not self.refs.has_reference():
+            return self
+        return self.copy(deep=deep)
 
     @final
     def _get_refs_and_copy(self, inplace: bool):
@@ -695,14 +695,14 @@ def replace(
             #  replacing it is a no-op.
             # Note: If to_replace were a list, NDFrame.replace would call
             #  replace_list instead of replace.
-            return [self.copy(deep=False)]
+            return [self._maybe_copy(inplace, deep=False)]
 
         if mask is None:
             mask = missing.mask_missing(values, to_replace)
         if not mask.any():
             # Note: we get here with test_replace_extension_other incorrectly
             #  bc _can_hold_element is incorrect.
-            return [self.copy(deep=False)]
+            return [self._maybe_copy(inplace, deep=False)]
 
         elif self._can_hold_element(value) or (self.dtype == "string" and is_re(value)):
             # TODO(CoW): Maybe split here as well into columns where mask has True
@@ -867,7 +867,7 @@ def replace_list(
                     for b in result:
                         if b.refs is self.refs:
                             # We are still sharing memory with self
-                            if id(b) in self_blk_ids:
+                            if id(b) in self_blk_ids and b is not self:
                                 # Remove ourselves from the refs; we are temporary
                                 self.refs.referenced_blocks.pop(self_blk_ids[id(b)])
                         else:

pandas/tests/extension/date/array.py

@@ -167,6 +167,9 @@ def __repr__(self) -> str:
     def copy(self) -> DateArray:
         return DateArray((self._year.copy(), self._month.copy(), self._day.copy()))
 
+    def view(self, dtype: Dtype | None = None) -> DateArray:
+        return DateArray((self._year.view(), self._month.view(), self._day.view()))
+
     def isna(self) -> np.ndarray:
         return np.logical_and(
             np.logical_and(

pandas/tests/extension/decimal/array.py

@@ -180,7 +180,7 @@ def __getitem__(self, item):
         else:
             # array, slice.
             item = pd.api.indexers.check_array_indexer(self, item)
-            result = type(self)(self._data[item])
+            result = type(self)(self._data[item], context=self.dtype.context)
             if getitem_returns_view(self, item):
                 result._readonly = self._readonly
             return result

pandas/tests/frame/methods/test_select_dtypes.py

@@ -35,7 +35,7 @@ def __init__(self, data, dtype) -> None:
         self._dtype = dtype
 
     def __array__(self, dtype=None, copy=None):
-        return self.data
+        return np.asarray(self.data, dtype=dtype)
 
     @property
     def dtype(self):
@@ -50,6 +50,9 @@ def __getitem__(self, item):
     def copy(self):
         return self
 
+    def view(self):
+        return self
+
 
 class TestSelectDtypes:
     def test_select_dtypes_include_using_list_like(self, using_infer_string):

pandas/tests/internals/test_internals.py

@@ -489,7 +489,7 @@ def test_copy(self, mgr):
             # view assertion
             tm.assert_equal(cp_blk.values, blk.values)
             if isinstance(blk.values, np.ndarray):
-                assert cp_blk.values.base is blk.values.base
+                assert cp_blk.values.base.base is blk.values.base
             else:
                 # DatetimeTZBlock has DatetimeIndex values
                 assert cp_blk.values._ndarray.base is blk.values._ndarray.base