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# What’s new in 2.1.1 (September 20, 2023)

These are the changes in pandas 2.1.1. See Release notes for a full changelog including other versions of pandas.

## Fixed regressions

- Fixed regression in `concat()` when `DataFrame` ‘s have two different extension dtypes (GH 54848)

- Fixed regression in `merge()` when merging over a PyArrow string index (GH 54894)

- Fixed regression in `read_csv()` when `usecols` is given and `dtypes` is a dict for `engine="python"` (GH 54868)

- Fixed regression in `read_csv()` when `delim_whitespace` is True (GH 54918, GH 54931)

- Fixed regression in `GroupBy.get_group()` raising for `axis=1` (GH 54858)

- Fixed regression in `DataFrame.__setitem__()` raising `AssertionError` when setting a `Series` with a partial `MultiIndex` (GH 54875)

- Fixed regression in `DataFrame.filter()` not respecting the order of elements for `filter` (GH 54980)

- Fixed regression in `DataFrame.to_sql()` not roundtripping datetime columns correctly for sqlite (GH 54877)

- Fixed regression in `DataFrameGroupBy.agg()` when aggregating a DataFrame with duplicate column names using a dictionary (GH 55006)

- Fixed regression in `MultiIndex.append()` raising when appending overlapping `IntervalIndex` levels (GH 54934)

- Fixed regression in `Series.drop_duplicates()` for PyArrow strings (GH 54904)

- Fixed regression in `Series.interpolate()` raising when `fill_value` was given (GH 54920)

- Fixed regression in `Series.value_counts()` raising for numeric data if `bins` was specified (GH 54857)

- Fixed regression in comparison operations for PyArrow backed columns not propagating exceptions correctly (GH 54944)

- Fixed regression when comparing a `Series` with `datetime64` dtype with `None` (GH 54870)

## Bug fixes

- Fixed bug for `ArrowDtype` raising `NotImplementedError` for fixed-size list (GH 55000)

- Fixed bug in `DataFrame.stack()` with `future_stack=True` and columns a non-`MultiIndex` consisting of tuples (GH 54948)

- Fixed bug in `Series.dt.tz()` with `ArrowDtype` where a string was returned instead of a `tzinfo` object (GH 55003)

- Fixed bug in `Series.pct_change()` and `DataFrame.pct_change()` showing unnecessary `FutureWarning` (GH 54981)

## Other

- Reverted the deprecation that disallowed `Series.apply()` returning a `DataFrame` when the passed-in callable returns a `Series` object (GH 52116)

## Contributors

For contributors, please see /usr/share/doc/contributors_list.txt or https://github.com/pandas-dev/pandas/graphs/contributors