What's new in 0.23.1 (June 12, 2018)

**Comparing Series with datetime.date**

We've reverted a 0.23.0 change to comparing a :class:`Series` holding datetimes and a ``datetime.date`` object (:issue:`21152`).
In pandas 0.22 and earlier, comparing a Series holding datetimes and ``datetime.date`` objects would coerce the ``datetime.date`` to a datetime before comparing.
This was inconsistent with Python, NumPy, and :class:`DatetimeIndex`, which never consider a datetime and ``datetime.date`` equal.

In 0.23.0, we unified operations between DatetimeIndex and Series, and in the process changed comparisons between a Series of datetimes and ``datetime.date`` without warning.

We've temporarily restored the 0.22.0 behavior, so datetimes and dates may again compare equal, but restore the 0.23.0 behavior in a future release.

To summarize, here's the behavior in 0.22.0, 0.23.0, 0.23.1:

.. code-block:: python

   # 0.22.0... Silently coerce the datetime.date
   >>> import datetime
   >>> pd.Series(pd.date_range('2017', periods=2)) == datetime.date(2017, 1, 1)
   0     True
   1    False
   dtype: bool

   # 0.23.0... Do not coerce the datetime.date
   >>> pd.Series(pd.date_range('2017', periods=2)) == datetime.date(2017, 1, 1)
   0    False
   1    False
   dtype: bool

   # 0.23.1... Coerce the datetime.date with a warning
   >>> pd.Series(pd.date_range('2017', periods=2)) == datetime.date(2017, 1, 1)
   /bin/python:1: FutureWarning: Comparing Series of datetimes with 'datetime.date'.  Currently, the
   'datetime.date' is coerced to a datetime. In the future pandas will
   not coerce, and the values not compare equal to the 'datetime.date'.
   To retain the current behavior, convert the 'datetime.date' to a
   datetime with 'pd.Timestamp'.
     #!/bin/python3
   0     True
   1    False
   dtype: bool

In addition, ordering comparisons will raise a ``TypeError`` in the future.

**Other fixes**

- Reverted the ability of :func:`~DataFrame.to_sql` to perform multivalue
  inserts as this caused regression in certain cases (:issue:`21103`).
  In the future this will be made configurable.
- Fixed regression in the :attr:`DatetimeIndex.date` and :attr:`DatetimeIndex.time`
  attributes in case of timezone-aware data: :attr:`DatetimeIndex.time` returned
  a tz-aware time instead of tz-naive (:issue:`21267`) and :attr:`DatetimeIndex.date`
  returned incorrect date when the input date has a non-UTC timezone (:issue:`21230`).
- Fixed regression in :meth:`pandas.io.json.json_normalize` when called with ``None`` values
  in nested levels in JSON, and to not drop keys with value as ``None`` (:issue:`21158`, :issue:`21356`).
- Bug in :meth:`~DataFrame.to_csv` causes encoding error when compression and encoding are specified (:issue:`21241`, :issue:`21118`)
- Bug preventing pandas from being importable with -OO optimization (:issue:`21071`)
- Bug in :meth:`Categorical.fillna` incorrectly raising a ``TypeError`` when ``value`` the individual categories are iterable and ``value`` is an iterable (:issue:`21097`, :issue:`19788`)
- Fixed regression in constructors coercing NA values like ``None`` to strings when passing ``dtype=str`` (:issue:`21083`)
- Regression in :func:`pivot_table` where an ordered ``Categorical`` with missing
  values for the pivot's ``index`` would give a mis-aligned result (:issue:`21133`)
- Fixed regression in merging on boolean index/columns (:issue:`21119`).

.. _whatsnew_0231.performance:

Performance improvements

**Groupby/resample/rolling**

- Bug in :func:`DataFrame.agg` where applying multiple aggregation functions to a :class:`DataFrame` with duplicated column names would cause a stack overflow (:issue:`21063`)
- Bug in :func:`.GroupBy.ffill` and :func:`.GroupBy.bfill` where the fill within a grouping would not always be applied as intended due to the implementations' use of a non-stable sort (:issue:`21207`)
- Bug in :func:`.GroupBy.rank` where results did not scale to 100% when specifying ``method='dense'`` and ``pct=True``
- Bug in :func:`pandas.DataFrame.rolling` and :func:`pandas.Series.rolling` which incorrectly accepted a 0 window size rather than raising (:issue:`21286`)

**Data-type specific**

- Bug in :meth:`Series.str.replace` where the method throws ``TypeError`` on Python 3.5.2 (:issue:`21078`)
- Bug in :class:`Timedelta` where passing a float with a unit would prematurely round the float precision (:issue:`14156`)
- Bug in :func:`pandas.testing.assert_index_equal` which raised ``AssertionError`` incorrectly, when comparing two :class:`CategoricalIndex` objects with param ``check_categorical=False`` (:issue:`19776`)

**Sparse**

- Bug in :attr:`SparseArray.shape` which previously only returned the shape :attr:`SparseArray.sp_values` (:issue:`21126`)

**Indexing**

- Bug in :meth:`Series.reset_index` where appropriate error was not raised with an invalid level name (:issue:`20925`)
- Bug in :func:`interval_range` when ``start``/``periods`` or ``end``/``periods`` are specified with float ``start`` or ``end`` (:issue:`21161`)
- Bug in :meth:`MultiIndex.set_names` where error raised for a ``MultiIndex`` with ``nlevels == 1`` (:issue:`21149`)
- Bug in :class:`IntervalIndex` constructors where creating an ``IntervalIndex`` from categorical data was not fully supported (:issue:`21243`, :issue:`21253`)
- Bug in :meth:`MultiIndex.sort_index` which was not guaranteed to sort correctly with ``level=1``; this was also causing data misalignment in particular :meth:`DataFrame.stack` operations (:issue:`20994`, :issue:`20945`, :issue:`21052`)

**Plotting**

- New keywords (sharex, sharey) to turn on/off sharing of x/y-axis by subplots generated with pandas.DataFrame().groupby().boxplot() (:issue:`20968`)

**I/O**

- Bug in IO methods specifying ``compression='zip'`` which produced uncompressed zip archives (:issue:`17778`, :issue:`21144`)
- Bug in :meth:`DataFrame.to_stata` which prevented exporting DataFrames to buffers and most file-like objects (:issue:`21041`)
- Bug in :meth:`read_stata` and :class:`StataReader` which did not correctly decode utf-8 strings on Python 3 from Stata 14 files (dta version 118) (:issue:`21244`)
- Bug in IO JSON :func:`read_json` reading empty JSON schema with ``orient='table'`` back to :class:`DataFrame` caused an error (:issue:`21287`)

**Reshaping**

- Bug in :func:`concat` where error was raised in concatenating :class:`Series` with numpy scalar and tuple names (:issue:`21015`)
- Bug in :func:`concat` warning message providing the wrong guidance for future behavior (:issue:`21101`)

**Other**

- Tab completion on :class:`Index` in IPython no longer outputs deprecation warnings (:issue:`21125`)
- Bug preventing pandas being used on Windows without C++ redistributable installed (:issue:`21106`)

.. _whatsnew_0.23.1.contributors:

Contributors