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# What’s new in 1.5.1 (October 19, 2022)

These are the changes in pandas 1.5.1. See Release notes for a full changelog including other versions of pandas.

## Behavior of `groupby` with categorical groupers (GH 48645)

In versions of pandas prior to 1.5, `groupby` with `dropna=False` would still drop NA values when the grouper was a categorical dtype. A fix for this was attempted in 1.5, however it introduced a regression where passing `observed=False` and `dropna=False` to `groupby` would result in only observed categories. It was found that the patch fixing the `dropna=False` bug is incompatible with `observed=False`, and decided that the best resolution is to restore the correct `observed=False` behavior at the cost of reintroducing the `dropna=False` bug.

    In [1]: df = pd.DataFrame(
       ...:     {
       ...:         "x": pd.Categorical([1, None], categories=[1, 2, 3]),
       ...:         "y": [3, 4],
       ...:     }
       ...: )
       ...:

    In [2]: df
    Out[2]:
         x  y
    0    1  3
    1  NaN  4

1.5.0 behavior:

    In [3]: # Correct behavior, NA values are not dropped
            df.groupby("x", observed=True, dropna=False).sum()
    Out[3]:
         y
    x
    1    3
    NaN  4

    In [4]: # Incorrect behavior, only observed categories present
            df.groupby("x", observed=False, dropna=False).sum()
    Out[4]:
         y
    x
    1    3
    NaN  4

1.5.1 behavior:

    # Incorrect behavior, NA values are dropped
    In [3]: df.groupby("x", observed=True, dropna=False).sum()
    Out[3]:
         y
    x
    1    3
    NaN  4

    # Correct behavior, unobserved categories present (NA values still dropped)
    In [4]: df.groupby("x", observed=False, dropna=False).sum()
    Out[4]:
         y
    x
    1    3
    2    0
    3    0
    NaN  4

## Fixed regressions

- Fixed Regression in `Series.__setitem__()` casting `None` to `NaN` for object dtype (GH 48665)

- Fixed Regression in `DataFrame.loc()` when setting values as a `DataFrame` with all `True` indexer (GH 48701)

- Regression in `read_csv()` causing an `EmptyDataError` when using an UTF-8 file handle that was already read from (GH 48646)

- Regression in `to_datetime()` when `utc=True` and `arg` contained timezone naive and aware arguments raised a `ValueError` (GH 48678)

- Fixed regression in `DataFrame.loc()` raising `FutureWarning` when setting an empty `DataFrame` (GH 48480)

- Fixed regression in `DataFrame.describe()` raising `TypeError` when result contains `NA` (GH 48778)

- Fixed regression in `DataFrame.plot()` ignoring invalid `colormap` for `kind="scatter"` (GH 48726)

- Fixed regression in `MultiIndex.values()` resetting `freq` attribute of underlying `Index` object (GH 49054)

- Fixed performance regression in `factorize()` when `na_sentinel` is not `None` and `sort=False` (GH 48620)

- Fixed regression causing an `AttributeError` during warning emitted if the provided table name in `DataFrame.to_sql()` and the table name actually used in the database do not match (GH 48733)

- Fixed regression in `to_datetime()` when `arg` was a date string with nanosecond and `format` contained `%f` would raise a `ValueError` (GH 48767)

- Fixed regression in `testing.assert_frame_equal()` raising for `MultiIndex` with `Categorical` and `check_like=True` (GH 48975)

- Fixed regression in `DataFrame.fillna()` replacing wrong values for `datetime64[ns]` dtype and `inplace=True` (GH 48863)

- Fixed `DataFrameGroupBy.size()` not returning a Series when `axis=1` (GH 48738)

- Fixed Regression in `DataFrameGroupBy.apply()` when user defined function is called on an empty dataframe (GH 47985)

- Fixed regression in `DataFrame.apply()` when passing non-zero `axis` via keyword argument (GH 48656)

- Fixed regression in `Series.groupby()` and `DataFrame.groupby()` when the grouper is a nullable data type (e.g. `Int64`) or a PyArrow-backed string array, contains null values, and `dropna=False` (GH 48794)

- Fixed performance regression in `Series.isin()` with mismatching dtypes (GH 49162)

- Fixed regression in `DataFrame.to_parquet()` raising when file name was specified as `bytes` (GH 48944)

- Fixed regression in `ExcelWriter` where the `book` attribute could no longer be set; however setting this attribute is now deprecated and this ability will be removed in a future version of pandas (GH 48780)

- Fixed regression in `DataFrame.corrwith()` when computing correlation on tied data with `method="spearman"` (GH 48826)

## Bug fixes

- Bug in `Series.__getitem__()` not falling back to positional for integer keys and boolean `Index` (GH 48653)

- Bug in `DataFrame.to_hdf()` raising `AssertionError` with boolean index (GH 48667)

- Bug in `testing.assert_index_equal()` for extension arrays with non matching `NA` raising `ValueError` (GH 48608)

- Bug in `DataFrame.pivot_table()` raising unexpected `FutureWarning` when setting datetime column as index (GH 48683)

- Bug in `DataFrame.sort_values()` emitting unnecessary `FutureWarning` when called on `DataFrame` with boolean sparse columns (GH 48784)

- Bug in `arrays.ArrowExtensionArray` with a comparison operator to an invalid object would not raise a `NotImplementedError` (GH 48833)

## Other

- Avoid showing deprecated signatures when introspecting functions with warnings about arguments becoming keyword-only (GH 48692)

## Contributors

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