REGR: registered accessors are no longer cached per DataFrame

[jorisvandenbossche]

Searching for it, this change was done on purpose in #58733 (triggered by #47667, #41357), and also mentioned in the whatsnew notes, although under the Performance section:

Eliminated circular reference in to original pandas object in accessor attributes (e.g. Series.str). However, accessor instantiation is no longer cached (GH 47667, GH 41357)

As a starter, we should certainly mention this in the breaking API changes section instead. But I am also wondering if we might not be underestimating the impact, and if there might be other options. For example, we could make this optional (e.g. a cached=True/False keyword in the register function). Or maybe using a weakref avoids the memory issues?

Small reproducer to illustrate:

@pd.api.extensions.register_dataframe_accessor("my_accessor")
class MyAccessor:
    def __init__(self, df):
        self._df = df

>>> df = pd.DataFrame({"a": [1, 2, 3]})
>>> df.my_accessor is df.my_accessor
False  # <-- this gives True with pandas < 3

Reason I am opening this is because I noticed that this breaks the usage pattern of the data_description accessor in pyjanitor.

[sanchit122006]

This #58733 change breaks the usual accessor pattern - df.my_accessor is df.my_accessor now returns False.

Quick repro:
@pd.api.extensions.register_dataframe_accessor("my_accessor")
class MyAccessor:
def init(self, data):
self._data = data

df = pd.DataFrame({"a": })​
df.my_accessor is df.my_accessor # False now

This breaks pyjanitor's data_description accessor.

Maybe add cached=True/False option to register_dataframe_accessor? Or weakref?

Should be in breaking changes whatsnew too, not just performance.

If this direction makes sense, happy to open a PR for it.

[mroeschke]

Repeating my comment during the 2025-12-17 meeting, I would prefer keeping this breaking change as is unless we receive more feedback of libraries relying on the caching behavior. The whatsnew note in #58733 should probably be moved to the Other API Changes section