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DOC: document column attrs with Copy-on-Write #65645

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12 changes: 12 additions & 0 deletions doc/source/user_guide/migration.rst
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Expand Up @@ -149,6 +149,18 @@ A different alternative would be to not use ``inplace``:
df["foo"] = df["foo"].replace(1, 5)
df

**Column attributes cannot be used to store column-level metadata**

``DataFrame.attrs`` and ``Series.attrs`` remain available for metadata on the
object itself. With CoW, setting ``attrs`` on a ``Series`` returned by selecting
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@jbrockmendel jbrockmendel May 15, 2026

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This is unrelated to CoW, it’s the removal of the item_cache

a :class:`DataFrame` column is not a supported way to attach metadata to that
column. A future access to the same column may return a different ``Series``
object without those attributes.
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@rhshadrach rhshadrach May 16, 2026

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Suggested change
object itself. With CoW, setting ``attrs`` on a ``Series`` returned by selecting
a :class:`DataFrame` column is not a supported way to attach metadata to that
column. A future access to the same column may return a different ``Series``
object without those attributes.
object itself. Previously, setting ``attrs`` on a ``Series`` returned by selecting
a :class:`DataFrame` column would work in some cases due to
internal caching; such caching has been removed and is not a supported way
to attach metadata to that column. A future access to the same column will
now return a different ``Series`` object without those attributes.


If you need column-level metadata, store it on the parent :class:`DataFrame`
(for example in ``df.attrs["columns"]`` keyed by column name) or in a separate
metadata object that you manage alongside the :class:`DataFrame`.

**Constructors now copy NumPy arrays by default**

The Series and DataFrame constructors now copies a NumPy array by default when not
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