CoW: Set refs properly for non CoW mode (#54118)

Author: phofl

pandas/core/generic.py

@@ -802,11 +802,7 @@ def swapaxes(self, axis1: Axis, axis2: Axis, copy: bool_t | None = None) -> Self
 
         new_axes = [self._get_axis(mapping.get(k, k)) for k in range(self._AXIS_LEN)]
         new_values = self._values.swapaxes(i, j)  # type: ignore[union-attr]
-        if (
-            using_copy_on_write()
-            and self._mgr.is_single_block
-            and isinstance(self._mgr, BlockManager)
-        ):
+        if self._mgr.is_single_block and isinstance(self._mgr, BlockManager):
             # This should only get hit in case of having a single block, otherwise a
             # copy is made, we don't have to set up references.
             new_mgr = ndarray_to_mgr(
@@ -823,10 +819,10 @@ def swapaxes(self, axis1: Axis, axis2: Axis, copy: bool_t | None = None) -> Self
             new_mgr.blocks[0].refs.add_reference(
                 new_mgr.blocks[0]  # type: ignore[arg-type]
             )
-            return self._constructor(new_mgr).__finalize__(self, method="swapaxes")
+            if not using_copy_on_write() and copy is not False:
+                new_mgr = new_mgr.copy(deep=True)
 
-        elif (copy or copy is None) and self._mgr.is_single_block:
-            new_values = new_values.copy()
+            return self._constructor(new_mgr).__finalize__(self, method="swapaxes")
 
         return self._constructor(
             new_values,

pandas/core/internals/blocks.py

@@ -486,7 +486,7 @@ def _downcast_2d(self, dtype, using_cow: bool = False) -> list[Block]:
         """
         new_values = maybe_downcast_to_dtype(self.values, dtype=dtype)
         new_values = maybe_coerce_values(new_values)
-        refs = self.refs if using_cow and new_values is self.values else None
+        refs = self.refs if new_values is self.values else None
         return [self.make_block(new_values, refs=refs)]
 
     @final
@@ -529,7 +529,7 @@ def convert(
         refs = None
         if copy and res_values is values:
             res_values = values.copy()
-        elif res_values is values and using_cow:
+        elif res_values is values:
             refs = self.refs
 
         res_values = ensure_block_shape(res_values, self.ndim)
@@ -577,7 +577,7 @@ def astype(
         new_values = maybe_coerce_values(new_values)
 
         refs = None
-        if using_cow and astype_is_view(values.dtype, new_values.dtype):
+        if (using_cow or not copy) and astype_is_view(values.dtype, new_values.dtype):
             refs = self.refs
 
         newb = self.make_block(new_values, refs=refs)
@@ -914,7 +914,7 @@ def _replace_coerce(
                     nb = self.astype(np.dtype(object), copy=False, using_cow=using_cow)
                     if (nb is self or using_cow) and not inplace:
                         nb = nb.copy()
-                    elif inplace and has_ref and nb.refs.has_reference():
+                    elif inplace and has_ref and nb.refs.has_reference() and using_cow:
                         # no copy in astype and we had refs before
                         nb = nb.copy()
                     putmask_inplace(nb.values, mask, value)

pandas/core/series.py

@@ -893,7 +893,7 @@ def view(self, dtype: Dtype | None = None) -> Series:
         #  implementation
         res_values = self.array.view(dtype)
         res_ser = self._constructor(res_values, index=self.index, copy=False)
-        if isinstance(res_ser._mgr, SingleBlockManager) and using_copy_on_write():
+        if isinstance(res_ser._mgr, SingleBlockManager):
             blk = res_ser._mgr._block
             blk.refs = cast("BlockValuesRefs", self._references)
             blk.refs.add_reference(blk)  # type: ignore[arg-type]