v0.23.0.txt

.. _whatsnew_0230:

v0.23.0
-------

This is a major release from 0.22.0 and includes a number of API changes,
deprecations, new features, enhancements, and performance improvements along
with a large number of bug fixes. We recommend that all users upgrade to this
version.

.. _whatsnew_0230.enhancements:

New features
~~~~~~~~~~~~

.. _whatsnew_0210.enhancements.limit_area:

``DataFrame.interpolate`` has gained the ``limit_area`` kwarg
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

:meth:`DataFrame.interpolate` has gained a ``limit_area`` parameter to allow further control of which ``NaN`` s are replaced.
Use ``limit_area='inside'`` to fill only NaNs surrounded by valid values or use ``limit_area='outside'`` to fill only ``NaN`` s
outside the existing valid values while preserving those inside.  (:issue:`16284`) See the :ref:`full documentation here <missing_data.interp_limits>`.


.. ipython:: python

   ser = pd.Series([np.nan, np.nan, 5, np.nan, np.nan, np.nan, 13, np.nan, np.nan])
   ser

Fill one consecutive inside value in both directions

.. ipython:: python

   ser.interpolate(limit_direction='both', limit_area='inside', limit=1)

Fill all consecutive outside values backward

.. ipython:: python

   ser.interpolate(limit_direction='backward', limit_area='outside')

Fill all consecutive outside values in both directions

.. ipython:: python

   ser.interpolate(limit_direction='both', limit_area='outside')

.. _whatsnew_0210.enhancements.get_dummies_dtype:

``get_dummies`` now supports ``dtype`` argument
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :func:`get_dummies` now accepts a ``dtype`` argument, which specifies a dtype for the new columns. The default remains uint8. (:issue:`18330`)

.. ipython:: python

   df = pd.DataFrame({'a': [1, 2], 'b': [3, 4], 'c': [5, 6]})
   pd.get_dummies(df, columns=['c']).dtypes
   pd.get_dummies(df, columns=['c'], dtype=bool).dtypes


.. _whatsnew_0230.enhancements.merge_on_columns_and_levels:

Merging on a combination of columns and index levels
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Strings passed to :meth:`DataFrame.merge` as the ``on``, ``left_on``, and ``right_on``
parameters may now refer to either column names or index level names.
This enables merging ``DataFrame`` instances on a combination of index levels
and columns without resetting indexes. See the :ref:`Merge on columns and
levels <merging.merge_on_columns_and_levels>` documentation section.
(:issue:`14355`)

.. ipython:: python

   left_index = pd.Index(['K0', 'K0', 'K1', 'K2'], name='key1')

   left = pd.DataFrame({'A': ['A0', 'A1', 'A2', 'A3'],
                        'B': ['B0', 'B1', 'B2', 'B3'],
                        'key2': ['K0', 'K1', 'K0', 'K1']},
                       index=left_index)

   right_index = pd.Index(['K0', 'K1', 'K2', 'K2'], name='key1')

   right = pd.DataFrame({'C': ['C0', 'C1', 'C2', 'C3'],
                         'D': ['D0', 'D1', 'D2', 'D3'],
                         'key2': ['K0', 'K0', 'K0', 'K1']},
                        index=right_index)

   left.merge(right, on=['key1', 'key2'])

.. _whatsnew_0230.enhancements.sort_by_columns_and_levels:

Sorting by a combination of columns and index levels
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Strings passed to :meth:`DataFrame.sort_values` as the ``by`` parameter may
now refer to either column names or index level names.  This enables sorting
``DataFrame`` instances by a combination of index levels and columns without
resetting indexes. See the :ref:`Sorting by Indexes and Values
<basics.sort_indexes_and_values>` documentation section.
(:issue:`14353`)

.. ipython:: python

   # Build MultiIndex
   idx = pd.MultiIndex.from_tuples([('a', 1), ('a', 2), ('a', 2),
                                    ('b', 2), ('b', 1), ('b', 1)])
   idx.names = ['first', 'second']

   # Build DataFrame
   df_multi = pd.DataFrame({'A': np.arange(6, 0, -1)},
                           index=idx)
   df_multi

   # Sort by 'second' (index) and 'A' (column)
   df_multi.sort_values(by=['second', 'A'])

.. _whatsnew_0230.enhancements.timedelta_mod:

Timedelta mod method
^^^^^^^^^^^^^^^^^^^^

``mod`` (%) and ``divmod`` operations are now defined on ``Timedelta`` objects
when operating with either timedelta-like or with numeric arguments.
See the :ref:`documentation here <timedeltas.mod_divmod>`. (:issue:`19365`)

.. ipython:: python

    td = pd.Timedelta(hours=37)
    td % pd.Timedelta(minutes=45)

.. _whatsnew_0230.enhancements.ran_inf:

``.rank()`` handles ``inf`` values when ``NaN`` are present
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In previous versions, ``.rank()`` would assign ``inf`` elements ``NaN`` as their ranks. Now ranks are calculated properly. (:issue:`6945`)

.. ipython:: python

    s = pd.Series([-np.inf, 0, 1, np.nan, np.inf])
    s

Previous Behavior:

.. code-block:: ipython

    In [11]: s.rank()
    Out[11]:
    0    1.0
    1    2.0
    2    3.0
    3    NaN
    4    NaN
    dtype: float64

Current Behavior:

.. ipython:: python

    s.rank()

Furthermore, previously if you rank ``inf`` or ``-inf`` values together with ``NaN`` values, the calculation won't distinguish ``NaN`` from infinity when using 'top' or 'bottom' argument.

.. ipython:: python

    s = pd.Series([np.nan, np.nan, -np.inf, -np.inf])
    s

Previous Behavior:

.. code-block:: ipython

    In [15]: s.rank(na_option='top')
    Out[15]:
    0    2.5
    1    2.5
    2    2.5
    3    2.5
    dtype: float64

Current Behavior:

.. ipython:: python

    s.rank(na_option='top')

.. _whatsnew_0230.enhancements.round-trippable_json:

JSON read/write round-trippable with ``orient='table'``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A ``DataFrame`` can now be written to and subsequently read back via JSON while preserving metadata through usage of the ``orient='table'`` argument (see :issue:`18912` and :issue:`9146`). Previously, none of the available ``orient`` values guaranteed the preservation of dtypes and index names, amongst other metadata.

.. ipython:: python

   df = pd.DataFrame({'foo': [1, 2, 3, 4],
		      'bar': ['a', 'b', 'c', 'd'],
		      'baz': pd.date_range('2018-01-01', freq='d', periods=4),
		      'qux': pd.Categorical(['a', 'b', 'c', 'c'])
		      }, index=pd.Index(range(4), name='idx'))
   df
   df.dtypes
   df.to_json('test.json', orient='table')
   new_df = pd.read_json('test.json', orient='table')
   new_df
   new_df.dtypes

Please note that the string `index` is not supported with the round trip format, as it is used by default in ``write_json`` to indicate a missing index name.

.. ipython:: python
   :okwarning:

   df.index.name = 'index'

   df.to_json('test.json', orient='table')
   new_df = pd.read_json('test.json', orient='table')
   new_df
   new_df.dtypes

.. ipython:: python
   :suppress:

   import os
   os.remove('test.json')


.. _whatsnew_0230.enhancements.assign_dependent:


``.assign()`` accepts dependent arguments
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :func:`DataFrame.assign` now accepts dependent keyword arguments for python version later than 3.6 (see also `PEP 468
<https://www.python.org/dev/peps/pep-0468/>`_). Later keyword arguments may now refer to earlier ones if the argument is a callable. See the
:ref:`documentation here <dsintro.chained_assignment>` (:issue:`14207`)

.. ipython:: python

    df = pd.DataFrame({'A': [1, 2, 3]})
    df
    df.assign(B=df.A, C=lambda x:x['A']+ x['B'])

.. warning::

  This may subtly change the behavior of your code when you're
  using ``.assign()`` to update an existing column. Previously, callables
  referring to other variables being updated would get the "old" values

  Previous Behavior:

  .. code-block:: ipython

      In [2]: df = pd.DataFrame({"A": [1, 2, 3]})

      In [3]: df.assign(A=lambda df: df.A + 1, C=lambda df: df.A * -1)
      Out[3]:
         A  C
      0  2 -1
      1  3 -2
      2  4 -3

  New Behavior:

  .. ipython:: python

      df.assign(A=df.A+1, C= lambda df: df.A* -1)


.. _whatsnew_0230.enhancements.astype_category:

``DataFrame.astype`` performs column-wise conversion to ``Categorical``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

:meth:`DataFrame.astype` can now perform column-wise conversion to ``Categorical`` by supplying the string ``'category'`` or
a :class:`~pandas.api.types.CategoricalDtype`. Previously, attempting this would raise a ``NotImplementedError``. See the
:ref:`categorical.objectcreation` section of the documentation for more details and examples. (:issue:`12860`, :issue:`18099`)

Supplying the string ``'category'`` performs column-wise conversion, with only labels appearing in a given column set as categories:

.. ipython:: python

    df = pd.DataFrame({'A': list('abca'), 'B': list('bccd')})
    df = df.astype('category')
    df['A'].dtype
    df['B'].dtype


Supplying a ``CategoricalDtype`` will make the categories in each column consistent with the supplied dtype:

.. ipython:: python

    from pandas.api.types import CategoricalDtype
    df = pd.DataFrame({'A': list('abca'), 'B': list('bccd')})
    cdt = CategoricalDtype(categories=list('abcd'), ordered=True)
    df = df.astype(cdt)
    df['A'].dtype
    df['B'].dtype

.. _whatsnew_023.enhancements.interval:

Storing Interval Data in Series and DataFrame
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Interval data may now be stored in a Series or DataFrame, in addition to an
:class:`IntervalIndex` like before (:issue:`19453`).

.. ipython:: python

   ser = pd.Series(pd.interval_range(0, 5))
   ser
   ser.dtype

Previously, these would be cast to a NumPy array of Interval objects. In general,
this should result in better performance when storing an array of intervals in
a Series.

Note that the ``.values`` of a Series containing intervals is no longer a NumPy
array. Rather, it's an ``ExtensionArray``, composed of two arrays ``left`` and
``right``.

.. ipython:: python

   ser.values

To recover the NumPy array of Interval objects, use :func:`numpy.asarray`:

.. ipython:: python

   np.asarray(ser.values)

This is the same behavior as ``Series.values`` for categorical data. See
:ref:`whatsnew_0230.api_breaking.interval_values` for more.

.. _whatsnew_023.enhancements.extension:

Extending Pandas with Custom Types
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Pandas now supports storing array-like objects that aren't necessarily 1-D NumPy
arrays as columns in a DataFrame or values in a Series. This allows third-party
libraries to implement extensions to NumPy's types, similar to how pandas
implemented categoricals, datetimes with timezones, periods, and intervals.

As a demonstration, we'll use cyberpandas_, which provides an ``IPArray`` type
for storing ip addresses.

.. code-block:: ipython

   In [1]: from cyberpandas import IPArray

   In [2]: values = IPArray([
      ...:     0,
      ...:     3232235777,
      ...:     42540766452641154071740215577757643572
      ...: ])
      ...:
      ...:

``IPArray`` isn't a normal 1-D NumPy array, but because it's a pandas
``ExtensionArray``, it can be stored properly inside pandas' containers.

.. code-block:: ipython

   In [3]: ser = pd.Series(values)
      ...:

   In [4]: ser
   Out[4]:
   0                         0.0.0.0
   1                     192.168.1.1
   2    2001:db8:85a3::8a2e:370:7334
   dtype: ip

Notice that the dtype is ``ip``. The missing value semantics of the underlying
array are respected:

   In [5]: ser.isna()
      ...:
   Out[5]:
   0     True
   1    False
   2    False
   dtype: bool

For more, see the :ref:`extension types <extending.extension-types>`
documentation. If you build an extension array, publicize it on our
:ref:`ecosystem page <ecosystem.extensions>`.

.. _cyberpandas: https://cyberpandas.readthedocs.io/en/latest/

.. _whatsnew_0230.enhancements.other:

Other Enhancements
^^^^^^^^^^^^^^^^^^

- Unary ``+`` now permitted for ``Series`` and ``DataFrame`` as  numeric operator (:issue:`16073`)
- Better support for :func:`Dataframe.style.to_excel` output with the ``xlsxwriter`` engine. (:issue:`16149`)
- :func:`pandas.tseries.frequencies.to_offset` now accepts leading '+' signs e.g. '+1h'. (:issue:`18171`)
- :func:`MultiIndex.unique` now supports the ``level=`` argument, to get unique values from a specific index level (:issue:`17896`)
- :class:`pandas.io.formats.style.Styler` now has method ``hide_index()`` to determine whether the index will be rendered in output (:issue:`14194`)
- :class:`pandas.io.formats.style.Styler` now has method ``hide_columns()`` to determine whether columns will be hidden in output (:issue:`14194`)
- Improved wording of ``ValueError`` raised in :func:`to_datetime` when ``unit=`` is passed with a non-convertible value (:issue:`14350`)
- :func:`Series.fillna` now accepts a Series or a dict as a ``value`` for a categorical dtype (:issue:`17033`)
- :func:`pandas.read_clipboard` updated to use qtpy, falling back to PyQt5 and then PyQt4, adding compatibility with Python3 and multiple python-qt bindings (:issue:`17722`)
- Improved wording of ``ValueError`` raised in :func:`read_csv` when the ``usecols`` argument cannot match all columns. (:issue:`17301`)
- :func:`DataFrame.corrwith` now silently drops non-numeric columns when passed a Series. Before, an exception was raised (:issue:`18570`).
- :class:`IntervalIndex` now supports time zone aware ``Interval`` objects (:issue:`18537`, :issue:`18538`)
- :func:`Series` / :func:`DataFrame` tab completion also returns identifiers in the first level of a :func:`MultiIndex`. (:issue:`16326`)
- :func:`read_excel()` has gained the ``nrows`` parameter (:issue:`16645`)
- :func:``DataFrame.to_json`` and ``Series.to_json`` now accept an ``index`` argument which allows the user to exclude the index from the JSON output (:issue:`17394`)
- ``IntervalIndex.to_tuples()`` has gained the ``na_tuple`` parameter to control whether NA is returned as a tuple of NA, or NA itself (:issue:`18756`)
- ``Categorical.rename_categories``, ``CategoricalIndex.rename_categories`` and :attr:`Series.cat.rename_categories`
  can now take a callable as their argument (:issue:`18862`)
- :class:`Interval` and :class:`IntervalIndex` have gained a ``length`` attribute (:issue:`18789`)
- ``Resampler`` objects now have a functioning :attr:`~pandas.core.resample.Resampler.pipe` method.
  Previously, calls to ``pipe`` were diverted to  the ``mean`` method (:issue:`17905`).
- :func:`~pandas.api.types.is_scalar` now returns ``True`` for ``DateOffset`` objects (:issue:`18943`).
- :func:`DataFrame.pivot` now accepts a list for the ``values=`` kwarg (:issue:`17160`).
- Added :func:`pandas.api.extensions.register_dataframe_accessor`,
  :func:`pandas.api.extensions.register_series_accessor`, and
  :func:`pandas.api.extensions.register_index_accessor`, accessor for libraries downstream of pandas
  to register custom accessors like ``.cat`` on pandas objects. See
  :ref:`Registering Custom Accessors <extending.register-accessors>` for more (:issue:`14781`).

- ``IntervalIndex.astype`` now supports conversions between subtypes when passed an ``IntervalDtype`` (:issue:`19197`)
- :class:`IntervalIndex` and its associated constructor methods (``from_arrays``, ``from_breaks``, ``from_tuples``) have gained a ``dtype`` parameter (:issue:`19262`)
- Added :func:`SeriesGroupBy.is_monotonic_increasing` and :func:`SeriesGroupBy.is_monotonic_decreasing` (:issue:`17015`)
- For subclassed ``DataFrames``, :func:`DataFrame.apply` will now preserve the ``Series`` subclass (if defined) when passing the data to the applied function (:issue:`19822`)
- :func:`DataFrame.from_dict` now accepts a ``columns`` argument that can be used to specify the column names when ``orient='index'`` is used (:issue:`18529`)
- Added option ``display.html.use_mathjax`` so `MathJax <https://www.mathjax.org/>`_ can be disabled when rendering tables in ``Jupyter`` notebooks (:issue:`19856`, :issue:`19824`)
- :func:`DataFrame.replace` now supports the ``method`` parameter, which can be used to specify the replacement method when ``to_replace`` is a scalar, list or tuple and ``value`` is ``None`` (:issue:`19632`)
- :meth:`Timestamp.month_name`, :meth:`DatetimeIndex.month_name`, and :meth:`Series.dt.month_name` are now available (:issue:`12805`)
- :meth:`Timestamp.day_name` and :meth:`DatetimeIndex.day_name` are now available to return day names with a specified locale (:issue:`12806`)
- :meth:`DataFrame.to_sql` now performs a multivalue insert if the underlying connection supports itk rather than inserting row by row.
  ``SQLAlchemy`` dialects supporting multivalue inserts include: ``mysql``, ``postgresql``, ``sqlite`` and any dialect with ``supports_multivalues_insert``. (:issue:`14315`, :issue:`8953`)
- :func:`read_html` now accepts a ``displayed_only`` keyword argument to controls whether or not hidden elements are parsed (``True`` by default) (:issue:`20027`)
- zip compression is supported via ``compression=zip`` in :func:`DataFrame.to_pickle`, :func:`Series.to_pickle`, :func:`DataFrame.to_csv`, :func:`Series.to_csv`, :func:`DataFrame.to_json`, :func:`Series.to_json`. (:issue:`17778`)
- :class:`DataFrame` and :class:`Series` now support matrix multiplication (```@```) operator (:issue:`10259`) for Python>=3.5

.. _whatsnew_0230.api_breaking:

Backwards incompatible API changes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. _whatsnew_0230.api_breaking.deps:

Dependencies have increased minimum versions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We have updated our minimum supported versions of dependencies (:issue:`15184`).
If installed, we now require:

+-----------------+-----------------+----------+---------------+
| Package         | Minimum Version | Required |     Issue     |
+=================+=================+==========+===============+
| python-dateutil | 2.5.0           |    X     | :issue:`15184`|
+-----------------+-----------------+----------+---------------+
| openpyxl        | 2.4.0           |          | :issue:`15184`|
+-----------------+-----------------+----------+---------------+
| beautifulsoup4  | 4.2.1           |          | :issue:`20082`|
+-----------------+-----------------+----------+---------------+

.. _whatsnew_0230.api_breaking.dict_insertion_order:

Instantation from dicts preserves dict insertion order for python 3.6+
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Until Python 3.6, dicts in Python had no formally defined ordering. For Python
version 3.6 and later, dicts are ordered by insertion order, see
`PEP 468 <https://www.python.org/dev/peps/pep-0468/>`_.
Pandas will use the dict's insertion order, when creating a ``Series`` or
``DataFrame`` from a dict and you're using Python version 3.6 or
higher. (:issue:`19884`)

Previous Behavior (and current behavior if on Python < 3.6):

.. code-block:: ipython

   In [1]: pd.Series({'Income': 2000,
   ...                'Expenses': -1500,
   ...                'Taxes': -200,
   ...                'Net result': 300})
   Expenses     -1500
   Income        2000
   Net result     300
   Taxes         -200
   dtype: int64

Note the Series above is ordered alphabetically by the index values.

New Behavior (for Python >= 3.6):

.. ipython:: python

    pd.Series({'Income': 2000,
               'Expenses': -1500,
               'Taxes': -200,
               'Net result': 300})

Notice that the Series is now ordered by insertion order. This new behavior is
used for all relevant pandas types (``Series``, ``DataFrame``, ``SparseSeries``
and ``SparseDataFrame``).

If you wish to retain the old behavior while using Python >= 3.6, you can use
``.sort_index()``:

.. ipython:: python

    pd.Series({'Income': 2000,
               'Expenses': -1500,
               'Taxes': -200,
               'Net result': 300}).sort_index()

.. _whatsnew_0230.api_breaking.interval_values:

``IntervalIndex.values`` is now an ``IntervalArray``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``.values`` attribute of an :class:`IntervalIndex` now returns an
``IntervalArray``, rather than a NumPy array of :class:`Interval` objects
(:issue:`19453`).

Previous Behavior:

.. code-block:: ipython

   In [1]: idx = pd.interval_range(0, 4)

   In [2]: idx.values
   Out[2]:
   array([Interval(0, 1, closed='right'), Interval(1, 2, closed='right'),
          Interval(2, 3, closed='right'), Interval(3, 4, closed='right')],
         dtype=object)

New Behavior:

.. ipython:: python

   idx = pd.interval_range(0, 4)
   idx.values

This mirrors ``CateogricalIndex.values``, which returns a ``Categorical``.             

For situations where you need an ``ndarray`` of Interval objects, use
:meth:`numpy.asarray` or ``idx.astype(object)``.

.. ipython:: python

   idx.values.astype(object)

.. _whatsnew_0230.api_breaking.deprecate_panel:

Deprecate Panel
^^^^^^^^^^^^^^^

``Panel`` was deprecated in the 0.20.x release, showing as a ``DeprecationWarning``. Using ``Panel`` will now show a ``FutureWarning``. The recommended way to represent 3-D data are
with a ``MultiIndex`` on a ``DataFrame`` via the :meth:`~Panel.to_frame` or with the `xarray package <http://xarray.pydata.org/en/stable/>`__. Pandas
provides a :meth:`~Panel.to_xarray` method to automate this conversion. For more details see :ref:`Deprecate Panel <dsintro.deprecate_panel>` documentation. (:issue:`13563`, :issue:`18324`).

.. ipython:: python
   :okwarning:

   p = tm.makePanel()
   p

Convert to a MultiIndex DataFrame

.. ipython:: python

   p.to_frame()

Convert to an xarray DataArray

.. ipython:: python
   :okwarning:

   p.to_xarray()



.. _whatsnew_0230.api_breaking.core_common:

pandas.core.common removals
^^^^^^^^^^^^^^^^^^^^^^^^^^^

The following error & warning messages are removed from ``pandas.core.common`` (:issue:`13634`, :issue:`19769`):

- ``PerformanceWarning``
- ``UnsupportedFunctionCall``
- ``UnsortedIndexError``
- ``AbstractMethodError``

These are available from import from ``pandas.errors`` (since 0.19.0).


.. _whatsnew_0230.api_breaking.apply:

Changes to make output of ``DataFrame.apply`` consistent
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

:func:`DataFrame.apply` was inconsistent when applying an arbitrary user-defined-function that returned a list-like with ``axis=1``. Several bugs and inconsistencies
are resolved. If the applied function returns a Series, then pandas will return a DataFrame; otherwise a Series will be returned, this includes the case
where a list-like (e.g. ``tuple`` or ``list`` is returned) (:issue:`16353`, :issue:`17437`, :issue:`17970`, :issue:`17348`, :issue:`17892`, :issue:`18573`,
:issue:`17602`, :issue:`18775`, :issue:`18901`, :issue:`18919`).

.. ipython:: python

    df = pd.DataFrame(np.tile(np.arange(3), 6).reshape(6, -1) + 1, columns=['A', 'B', 'C'])
    df

Previous Behavior: if the returned shape happened to match the length of original columns, this would return a ``DataFrame``.
If the return shape did not match, a ``Series`` with lists was returned.

.. code-block:: python

   In [3]: df.apply(lambda x: [1, 2, 3], axis=1)
   Out[3]:
      A  B  C
   0  1  2  3
   1  1  2  3
   2  1  2  3
   3  1  2  3
   4  1  2  3
   5  1  2  3

   In [4]: df.apply(lambda x: [1, 2], axis=1)
   Out[4]:
   0    [1, 2]
   1    [1, 2]
   2    [1, 2]
   3    [1, 2]
   4    [1, 2]
   5    [1, 2]
   dtype: object


New Behavior: When the applied function returns a list-like, this will now *always* return a ``Series``.

.. ipython:: python

    df.apply(lambda x: [1, 2, 3], axis=1)
    df.apply(lambda x: [1, 2], axis=1)

To have expanded columns, you can use ``result_type='expand'``

.. ipython:: python

    df.apply(lambda x: [1, 2, 3], axis=1, result_type='expand')

To broadcast the result across the original columns (the old behaviour for
list-likes of the correct length), you can use ``result_type='broadcast'``.
The shape must match the original columns.

.. ipython:: python

    df.apply(lambda x: [1, 2, 3], axis=1, result_type='broadcast')

Returning a ``Series`` allows one to control the exact return structure and column names:

.. ipython:: python

    df.apply(lambda x: Series([1, 2, 3], index=['D', 'E', 'F']), axis=1)


.. _whatsnew_0230.api_breaking.build_changes:

Build Changes
^^^^^^^^^^^^^

- Building pandas for development now requires ``cython >= 0.24`` (:issue:`18613`)
- Building from source now explicitly requires ``setuptools`` in ``setup.py`` (:issue:`18113`)
- Updated conda recipe to be in compliance with conda-build 3.0+ (:issue:`18002`)

.. _whatsnew_0230.api_breaking.index_division_by_zero:

Index Division By Zero Fills Correctly
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Division operations on ``Index`` and subclasses will now fill division of positive numbers by zero with ``np.inf``, division of negative numbers by zero with ``-np.inf`` and `0 / 0` with ``np.nan``.  This matches existing ``Series`` behavior. (:issue:`19322`, :issue:`19347`)

Previous Behavior:

.. code-block:: ipython

    In [6]: index = pd.Int64Index([-1, 0, 1])

    In [7]: index / 0
    Out[7]: Int64Index([0, 0, 0], dtype='int64')

    # Previous behavior yielded different results depending on the type of zero in the divisor
    In [8]: index / 0.0
    Out[8]: Float64Index([-inf, nan, inf], dtype='float64')

    In [9]: index = pd.UInt64Index([0, 1])

    In [10]: index / np.array([0, 0], dtype=np.uint64)
    Out[10]: UInt64Index([0, 0], dtype='uint64')

    In [11]: pd.RangeIndex(1, 5) / 0
    ZeroDivisionError: integer division or modulo by zero

Current Behavior:

.. ipython:: python

    index = pd.Int64Index([-1, 0, 1])
    # division by zero gives -infinity where negative, +infinity where positive, and NaN for 0 / 0
    index / 0

    # The result of division by zero should not depend on whether the zero is int or float
    index / 0.0

    index = pd.UInt64Index([0, 1])
    index / np.array([0, 0], dtype=np.uint64)

    pd.RangeIndex(1, 5) / 0

.. _whatsnew_0230.api_breaking.extract:

Extraction of matching patterns from strings
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

By default, extracting matching patterns from strings with :func:`str.extract` used to return a
``Series`` if a single group was being extracted (a ``DataFrame`` if more than one group was
extracted``). As of Pandas 0.23.0 :func:`str.extract` always returns a ``DataFrame``, unless
``expand`` is set to ``False``. Finallay, ``None`` was an accepted value for
the ``expand`` parameter (which was equivalent to ``False``), but now raises a ``ValueError``. (:issue:`11386`)

Previous Behavior:

.. code-block:: ipython

    In [1]: s = pd.Series(['number 10', '12 eggs'])

    In [2]: extracted = s.str.extract('.*(\d\d).*')

    In [3]: extracted
    Out [3]:
    0    10
    1    12
    dtype: object

    In [4]: type(extracted)
    Out [4]:
    pandas.core.series.Series

New Behavior:

.. ipython:: python

    s = pd.Series(['number 10', '12 eggs'])
    extracted = s.str.extract('.*(\d\d).*')
    extracted
    type(extracted)

To restore previous behavior, simply set ``expand`` to ``False``:

.. ipython:: python

    s = pd.Series(['number 10', '12 eggs'])
    extracted = s.str.extract('.*(\d\d).*', expand=False)
    extracted
    type(extracted)

.. _whatsnew_0230.api_breaking.cdt_ordered:

Default value for the ``ordered`` parameter of ``CategoricalDtype``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The default value of the ``ordered`` parameter for :class:`~pandas.api.types.CategoricalDtype` has changed from ``False`` to ``None`` to allow updating of ``categories`` without impacting ``ordered``.  Behavior should remain consistent for downstream objects, such as :class:`Categorical` (:issue:`18790`)

In previous versions, the default value for the ``ordered`` parameter was ``False``.  This could potentially lead to the ``ordered`` parameter unintentionally being changed from ``True`` to ``False`` when users attempt to update ``categories`` if ``ordered`` is not explicitly specified, as it would silently default to ``False``.  The new behavior for ``ordered=None`` is to retain the existing value of ``ordered``.

New Behavior:

.. ipython:: python

    from pandas.api.types import CategoricalDtype
    cat = pd.Categorical(list('abcaba'), ordered=True, categories=list('cba'))
    cat
    cdt = CategoricalDtype(categories=list('cbad'))
    cat.astype(cdt)

Notice in the example above that the converted ``Categorical`` has retained ``ordered=True``.  Had the default value for ``ordered`` remained as ``False``, the converted ``Categorical`` would have become unordered, despite ``ordered=False`` never being explicitly specified.  To change the value of ``ordered``, explicitly pass it to the new dtype, e.g. ``CategoricalDtype(categories=list('cbad'), ordered=False)``.

Note that the unintenional conversion of ``ordered`` discussed above did not arise in previous versions due to separate bugs that prevented ``astype`` from doing any type of category to category conversion (:issue:`10696`, :issue:`18593`).  These bugs have been fixed in this release, and motivated changing the default value of ``ordered``.

.. _whatsnew_0230.api_breaking.pretty_printing:

Better pretty-printing of DataFrames in a terminal
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Previously, the default value for the maximum number of columns was
``pd.options.display.max_columns=20``. This meant that relatively wide data
frames would not fit within the terminal width, and pandas would introduce line
breaks to display these 20 columns. This resulted in an output that was
relatively difficult to read:

.. image:: _static/print_df_old.png

If Python runs in a terminal, the maximum number of columns is now determined
automatically so that the printed data frame fits within the current terminal
width (``pd.options.display.max_columns=0``) (:issue:`17023`). If Python runs
as a Jupyter kernel (such as the Jupyter QtConsole or a Jupyter notebook, as
well as in many IDEs), this value cannot be inferred automatically and is thus
set to `20` as in previous versions. In a terminal, this results in a much
nicer output:

.. image:: _static/print_df_new.png

Note that if you don't like the new default, you can always set this option
yourself. To revert to the old setting, you can run this line:

.. code-block:: python

  pd.options.display.max_columns = 20

.. _whatsnew_0230.api.datetimelike:

Datetimelike API Changes
^^^^^^^^^^^^^^^^^^^^^^^^

- The default ``Timedelta`` constructor now accepts an ``ISO 8601 Duration`` string as an argument (:issue:`19040`)
- Addition or subtraction of ``NaT`` from :class:`TimedeltaIndex` will return ``TimedeltaIndex`` instead of ``DatetimeIndex`` (:issue:`19124`)
- :func:`DatetimeIndex.shift` and :func:`TimedeltaIndex.shift` will now raise ``NullFrequencyError`` (which subclasses ``ValueError``, which was raised in older versions) when the index object frequency is ``None`` (:issue:`19147`)
- Addition and subtraction of ``NaN`` from a :class:`Series` with ``dtype='timedelta64[ns]'`` will raise a ``TypeError` instead of treating the ``NaN`` as ``NaT`` (:issue:`19274`)
- ``NaT`` division with :class:`datetime.timedelta` will now return ``NaN`` instead of raising (:issue:`17876`)
- :class:`Timestamp` will no longer silently ignore unused or invalid ``tz`` or ``tzinfo`` keyword arguments (:issue:`17690`)
- :class:`Timestamp` will no longer silently ignore invalid ``freq`` arguments (:issue:`5168`)
- :class:`CacheableOffset` and :class:`WeekDay` are no longer available in the ``pandas.tseries.offsets`` module (:issue:`17830`)
- ``pandas.tseries.frequencies.get_freq_group()`` and ``pandas.tseries.frequencies.DAYS`` are removed from the public API (:issue:`18034`)
- :func:`Series.truncate` and :func:`DataFrame.truncate` will raise a ``ValueError`` if the index is not sorted instead of an unhelpful ``KeyError`` (:issue:`17935`)
- Restricted ``DateOffset`` keyword arguments. Previously, ``DateOffset`` subclasses allowed arbitrary keyword arguments which could lead to unexpected behavior. Now, only valid arguments will be accepted. (:issue:`17176`, :issue:`18226`).
- Subtracting ``NaT`` from a :class:`Series` with ``dtype='datetime64[ns]'`` returns a ``Series`` with ``dtype='timedelta64[ns]'`` instead of ``dtype='datetime64[ns]'`` (:issue:`18808`)
- Operations between a :class:`Series` with dtype ``dtype='datetime64[ns]'`` and a :class:`PeriodIndex` will correctly raises ``TypeError`` (:issue:`18850`)
- Subtraction of :class:`Series` with timezone-aware ``dtype='datetime64[ns]'`` with mis-matched timezones will raise ``TypeError`` instead of ``ValueError`` (:issue:`18817`)
- :func:`pandas.merge` provides a more informative error message when trying to merge on timezone-aware and timezone-naive columns (:issue:`15800`)
- For :class:`DatetimeIndex` and :class:`TimedeltaIndex` with ``freq=None``, addition or subtraction of integer-dtyped array or ``Index`` will raise ``NullFrequencyError`` instead of ``TypeError`` (:issue:`19895`)
- :class:`Timestamp` constructor now accepts a `nanosecond` keyword or positional argument (:issue:`18898`)

.. _whatsnew_0230.api.other:

Other API Changes
^^^^^^^^^^^^^^^^^

- :func:`Series.astype` and :func:`Index.astype` with an incompatible dtype will now raise a ``TypeError`` rather than a ``ValueError`` (:issue:`18231`)
- ``Series`` construction with an ``object`` dtyped tz-aware datetime and ``dtype=object`` specified, will now return an ``object`` dtyped ``Series``, previously this would infer the datetime dtype (:issue:`18231`)
- A :class:`Series` of ``dtype=category`` constructed from an empty ``dict`` will now have categories of ``dtype=object`` rather than ``dtype=float64``, consistently with the case in which an empty list is passed (:issue:`18515`)
- All-NaN levels in a ``MultiIndex`` are now assigned ``float`` rather than ``object`` dtype, promoting consistency with ``Index`` (:issue:`17929`).
- Levels names of a ``MultiIndex`` (when not None) are now required to be unique: trying to create a ``MultiIndex`` with repeated names will raise a ``ValueError`` (:issue:`18872`)
- :func:`Index.map` can now accept ``Series`` and dictionary input objects (:issue:`12756`, :issue:`18482`, :issue:`18509`).
- :func:`DataFrame.unstack` will now default to filling with ``np.nan`` for ``object`` columns. (:issue:`12815`)
- :class:`IntervalIndex` constructor will raise if the ``closed`` parameter conflicts with how the input data is inferred to be closed (:issue:`18421`)
- Inserting missing values into indexes will work for all types of indexes and automatically insert the correct type of missing value (``NaN``, ``NaT``, etc.) regardless of the type passed in (:issue:`18295`)
- When created with duplicate labels, ``MultiIndex`` now raises a ``ValueError``. (:issue:`17464`)
- :func:`Series.fillna` now raises a ``TypeError`` instead of a ``ValueError`` when passed a list, tuple or DataFrame as a ``value`` (:issue:`18293`)
- :func:`pandas.DataFrame.merge` no longer casts a ``float`` column to ``object`` when merging on ``int`` and ``float`` columns (:issue:`16572`)
- :func:`pandas.merge` now raises a ``ValueError`` when trying to merge on incompatible data types (:issue:`9780`)
- The default NA value for :class:`UInt64Index` has changed from 0 to ``NaN``, which impacts methods that mask with NA, such as ``UInt64Index.where()`` (:issue:`18398`)
- Refactored ``setup.py`` to use ``find_packages`` instead of explicitly listing out all subpackages (:issue:`18535`)
- Rearranged the order of keyword arguments in :func:`read_excel()` to align with :func:`read_csv()` (:issue:`16672`)
- :func:`wide_to_long` previously kept numeric-like suffixes as ``object`` dtype. Now they are cast to numeric if possible (:issue:`17627`)
- In :func:`read_excel`, the ``comment`` argument is now exposed as a named parameter (:issue:`18735`)
- Rearranged the order of keyword arguments in :func:`read_excel()` to align with :func:`read_csv()` (:issue:`16672`)
- The options ``html.border`` and ``mode.use_inf_as_null`` were deprecated in prior versions, these will now show ``FutureWarning`` rather than a ``DeprecationWarning`` (:issue:`19003`)
- :class:`IntervalIndex` and ``IntervalDtype`` no longer support categorical, object, and string subtypes (:issue:`19016`)
- ``IntervalDtype`` now returns ``True`` when compared against ``'interval'`` regardless of subtype, and ``IntervalDtype.name`` now returns ``'interval'`` regardless of subtype (:issue:`18980`)
- ``KeyError`` now raises instead of ``ValueError`` in :meth:`~DataFrame.drop`, :meth:`~Panel.drop`, :meth:`~Series.drop`, :meth:`~Index.drop` when dropping a non-existent element in an axis with duplicates (:issue:`19186`)
- :func:`Series.to_csv` now accepts a ``compression`` argument that works in the same way as the ``compression`` argument in :func:`DataFrame.to_csv` (:issue:`18958`)
- Set operations (union, difference...) on :class:`IntervalIndex` with incompatible index types will now raise a ``TypeError`` rather than a ``ValueError`` (:issue:`19329`)
- :class:`DateOffset` objects render more simply, e.g. ``<DateOffset: days=1>`` instead of ``<DateOffset: kwds={'days': 1}>`` (:issue:`19403`)
- ``Categorical.fillna`` now validates its ``value`` and ``method`` keyword arguments. It now raises when both or none are specified, matching the behavior of :meth:`Series.fillna` (:issue:`19682`)
- ``pd.to_datetime('today')`` now returns a datetime, consistent with ``pd.Timestamp('today')``; previously ``pd.to_datetime('today')`` returned a ``.normalized()`` datetime (:issue:`19935`)
- :func:`Series.str.replace` now takes an optional `regex` keyword which, when set to ``False``, uses literal string replacement rather than regex replacement (:issue:`16808`)
- :func:`DatetimeIndex.strftime` and :func:`PeriodIndex.strftime` now return an ``Index`` instead of a numpy array to be consistent with similar accessors (:issue:`20127`)
- Constructing a Series from a list of length 1 no longer broadcasts this list when a longer index is specified (:issue:`19714`, :issue:`20391`).
- :func:`DataFrame.to_dict` with ``orient='index'`` no longer casts int columns to float for a DataFrame with only int and float columns (:issue:`18580`)

.. _whatsnew_0230.deprecations:

Deprecations
~~~~~~~~~~~~

- ``Series.from_array`` and ``SparseSeries.from_array`` are deprecated. Use the normal constructor ``Series(..)`` and ``SparseSeries(..)`` instead (:issue:`18213`).
- ``DataFrame.as_matrix`` is deprecated. Use ``DataFrame.values`` instead (:issue:`18458`).
- ``Series.asobject``, ``DatetimeIndex.asobject``, ``PeriodIndex.asobject`` and ``TimeDeltaIndex.asobject`` have been deprecated. Use ``.astype(object)`` instead (:issue:`18572`)
- Grouping by a tuple of keys now emits a ``FutureWarning`` and is deprecated.
  In the future, a tuple passed to ``'by'`` will always refer to a single key
  that is the actual tuple, instead of treating the tuple as multiple keys. To
  retain the previous behavior, use a list instead of a tuple (:issue:`18314`)
- ``Series.valid`` is deprecated. Use :meth:`Series.dropna` instead (:issue:`18800`).
- :func:`read_excel` has deprecated the ``skip_footer`` parameter. Use ``skipfooter`` instead (:issue:`18836`)
- The ``is_copy`` attribute is deprecated and will be removed in a future version (:issue:`18801`).
- ``IntervalIndex.from_intervals`` is deprecated in favor of the :class:`IntervalIndex` constructor (:issue:`19263`)
- ``DataFrame.from_items`` is deprecated. Use :func:`DataFrame.from_dict` instead, or ``DataFrame.from_dict(OrderedDict())`` if you wish to preserve the key order (:issue:`17320`, :issue:`17312`)

- The ``broadcast`` parameter of ``.apply()`` is deprecated in favor of ``result_type='broadcast'`` (:issue:`18577`)
- The ``reduce`` parameter of ``.apply()`` is deprecated in favor of ``result_type='reduce'`` (:issue:`18577`)
- The ``order`` parameter of :func:`factorize` is deprecated and will be removed in a future release (:issue:`19727`)
- :attr:`Timestamp.weekday_name`, :attr:`DatetimeIndex.weekday_name`, and :attr:`Series.dt.weekday_name` are deprecated in favor of :meth:`Timestamp.day_name`, :meth:`DatetimeIndex.day_name`, and :meth:`Series.dt.day_name` (:issue:`12806`)

- ``pandas.tseries.plotting.tsplot`` is deprecated. Use :func:`Series.plot` instead (:issue:`18627`)
- ``Index.summary()`` is deprecated and will be removed in a future version (:issue:`18217`)
- ``NDFrame.get_ftype_counts()`` is deprecated and will be removed in a future version (:issue:`18243`)

.. _whatsnew_0230.prior_deprecations:

Removal of prior version deprecations/changes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

- Warnings against the obsolete usage ``Categorical(codes, categories)``, which were emitted for instance when the first two arguments to ``Categorical()`` had different dtypes, and recommended the use of ``Categorical.from_codes``, have now been removed (:issue:`8074`)
- The ``levels`` and ``labels`` attributes of a ``MultiIndex`` can no longer be set directly (:issue:`4039`).
- ``pd.tseries.util.pivot_annual`` has been removed (deprecated since v0.19). Use ``pivot_table`` instead (:issue:`18370`)
- ``pd.tseries.util.isleapyear`` has been removed (deprecated since v0.19). Use ``.is_leap_year`` property in Datetime-likes instead (:issue:`18370`)
- ``pd.ordered_merge`` has been removed (deprecated since v0.19). Use ``pd.merge_ordered`` instead (:issue:`18459`)
- The ``SparseList`` class has been removed (:issue:`14007`)
- The ``pandas.io.wb`` and ``pandas.io.data`` stub modules have been removed (:issue:`13735`)
- ``Categorical.from_array`` has been removed (:issue:`13854`)
- The ``freq`` and ``how`` parameters have been removed from the ``rolling``/``expanding``/``ewm`` methods of DataFrame
  and Series (deprecated since v0.18). Instead, resample before calling the methods. (:issue:`18601` & :issue:`18668`)
- ``DatetimeIndex.to_datetime``, ``Timestamp.to_datetime``, ``PeriodIndex.to_datetime``, and ``Index.to_datetime`` have been removed (:issue:`8254`, :issue:`14096`, :issue:`14113`)
- :func:`read_csv` has dropped the ``skip_footer`` parameter (:issue:`13386`)
- :func:`read_csv` has dropped the ``as_recarray`` parameter (:issue:`13373`)
- :func:`read_csv` has dropped the ``buffer_lines`` parameter (:issue:`13360`)
- :func:`read_csv` has dropped the ``compact_ints`` and ``use_unsigned`` parameters (:issue:`13323`)
- The ``Timestamp`` class has dropped the ``offset`` attribute in favor of ``freq`` (:issue:`13593`)
- The ``Series``, ``Categorical``, and ``Index`` classes have dropped the ``reshape`` method (:issue:`13012`)
- ``pandas.tseries.frequencies.get_standard_freq`` has been removed in favor of ``pandas.tseries.frequencies.to_offset(freq).rule_code`` (:issue:`13874`)
- The ``freqstr`` keyword has been removed from ``pandas.tseries.frequencies.to_offset`` in favor of ``freq`` (:issue:`13874`)
- The ``Panel4D`` and ``PanelND`` classes have been removed (:issue:`13776`)
- The ``Panel`` class has dropped the ``to_long`` and ``toLong`` methods (:issue:`19077`)
- The options ``display.line_with`` and ``display.height`` are removed in favor of ``display.width`` and ``display.max_rows`` respectively (:issue:`4391`, :issue:`19107`)
- The ``labels`` attribute of the ``Categorical`` class has been removed in favor of :attr:`Categorical.codes` (:issue:`7768`)
- The ``flavor`` parameter have been removed from func:`to_sql` method (:issue:`13611`)
- The modules ``pandas.tools.hashing`` and ``pandas.util.hashing`` have been removed (:issue:`16223`)
- The top-level functions ``pd.rolling_*``, ``pd.expanding_*`` and ``pd.ewm*`` have been removed (Deprecated since v0.18).
  Instead, use the DataFrame/Series methods :attr:`~DataFrame.rolling`, :attr:`~DataFrame.expanding` and :attr:`~DataFrame.ewm` (:issue:`18723`)
- Imports from ``pandas.core.common`` for functions such as ``is_datetime64_dtype`` are now removed. These are located in ``pandas.api.types``. (:issue:`13634`, :issue:`19769`)
- The ``infer_dst`` keyword in :meth:`Series.tz_localize`, :meth:`DatetimeIndex.tz_localize`
  and :class:`DatetimeIndex` have been removed. ``infer_dst=True`` is equivalent to
  ``ambiguous='infer'``, and ``infer_dst=False`` to ``ambiguous='raise'`` (:issue:`7963`).


.. _whatsnew_0230.performance:

Performance Improvements
~~~~~~~~~~~~~~~~~~~~~~~~

- Indexers on ``Series`` or ``DataFrame`` no longer create a reference cycle (:issue:`17956`)
- Added a keyword argument, ``cache``, to :func:`to_datetime` that improved the performance of converting duplicate datetime arguments (:issue:`11665`)
- :class:`DateOffset` arithmetic performance is improved (:issue:`18218`)
- Converting a ``Series`` of ``Timedelta`` objects to days, seconds, etc... sped up through vectorization of underlying methods (:issue:`18092`)
- Improved performance of ``.map()`` with a ``Series/dict`` input (:issue:`15081`)
- The overridden ``Timedelta`` properties of days, seconds and microseconds have been removed, leveraging their built-in Python versions instead (:issue:`18242`)
- ``Series`` construction will reduce the number of copies made of the input data in certain cases (:issue:`17449`)
- Improved performance of :func:`Series.dt.date` and :func:`DatetimeIndex.date` (:issue:`18058`)
- Improved performance of :func:`Series.dt.time` and :func:`DatetimeIndex.time` (:issue:`18461`)
- Improved performance of :func:`IntervalIndex.symmetric_difference()` (:issue:`18475`)
- Improved performance of ``DatetimeIndex`` and ``Series`` arithmetic operations with Business-Month and Business-Quarter frequencies (:issue:`18489`)
- :func:`Series` / :func:`DataFrame` tab completion limits to 100 values, for better performance. (:issue:`18587`)
- Improved performance of :func:`DataFrame.median` with ``axis=1`` when bottleneck is not installed (:issue:`16468`)
- Improved performance of :func:`MultiIndex.get_loc` for large indexes, at the cost of a reduction in performance for small ones (:issue:`18519`)
- Improved performance of pairwise ``.rolling()`` and ``.expanding()`` with ``.cov()`` and ``.corr()`` operations (:issue:`17917`)
- Improved performance of :func:`pandas.core.groupby.GroupBy.rank` (:issue:`15779`)
- Improved performance of variable ``.rolling()`` on ``.min()`` and ``.max()`` (:issue:`19521`)
- Improved performance of :func:`pandas.core.groupby.GroupBy.ffill` and :func:`pandas.core.groupby.GroupBy.bfill` (:issue:`11296`)
- Improved performance of :func:`pandas.core.groupby.GroupBy.any` and :func:`pandas.core.groupby.GroupBy.all` (:issue:`15435`)
- Improved performance of :func:`pandas.core.groupby.GroupBy.pct_change` (:issue:`19165`)

.. _whatsnew_0230.docs:

Documentation Changes
~~~~~~~~~~~~~~~~~~~~~

Thanks to all of the contributors who participated in the Pandas Documentation
Sprint, which took place on March 10th. We had about 500 participants from over
30 locations across the world. You should notice that many of the
:ref:`API docstrings <api>` have greatly improved.

There were too many simultaneous contributions to include a release note for each
improvement, but this `GitHub search`_ should give you an idea of how many docstrings
were improved.

Special thanks to Marc Garcia for organizing the sprint. For more information,
read the `NumFOCUS blogpost`_ recapping the sprint.

.. _GitHub search: https://github.com/pandas-dev/pandas/pulls?utf8=%E2%9C%93&q=is%3Apr+label%3ADocs+created%3A2018-03-10..2018-03-15+
.. _NumFOCUS blogpost: https://www.numfocus.org/blog/worldwide-pandas-sprint/


- Changed spelling of "numpy" to "NumPy", and "python" to "Python". (:issue:`19017`)
- Consistency when introducing code samples, using either colon or period.
  Rewrote some sentences for greater clarity, added more dynamic references
  to functions, methods and classes.
  (:issue:`18941`, :issue:`18948`, :issue:`18973`, :issue:`19017`)
- Added a reference to :func:`DataFrame.assign` in the concatenate section of the merging documentation (:issue:`18665`)

.. _whatsnew_0230.bug_fixes:

Bug Fixes
~~~~~~~~~

Categorical
^^^^^^^^^^^

.. warning::

   A class of bugs were introduced in pandas 0.21 with ``CategoricalDtype`` that
   affects the correctness of operations like ``merge``, ``concat``, and
   indexing when comparing multiple unordered ``Categorical`` arrays that have
   the same categories, but in a different order. We highly recommend upgrading
   or manually aligning your categories before doing these operations.

- Bug in ``Categorical.equals`` returning the wrong result when comparing two
  unordered ``Categorical`` arrays with the same categories, but in a different
  order (:issue:`16603`)
- Bug in :func:`pandas.api.types.union_categoricals` returning the wrong result
  when for unordered categoricals with the categories in a different order.
  This affected :func:`pandas.concat` with Categorical data (:issue:`19096`).
- Bug in :func:`pandas.merge` returning the wrong result when joining on an
  unordered ``Categorical`` that had the same categories but in a different
  order (:issue:`19551`)
- Bug in :meth:`CategoricalIndex.get_indexer` returning the wrong result when
  ``target`` was an unordered ``Categorical`` that had the same categories as
  ``self`` but in a different order (:issue:`19551`)
- Bug in :meth:`Index.astype` with a categorical dtype where the resultant index is not converted to a :class:`CategoricalIndex` for all types of index (:issue:`18630`)
- Bug in :meth:`Series.astype` and ``Categorical.astype()`` where an existing categorical data does not get updated (:issue:`10696`, :issue:`18593`)
- Bug in :meth:`Series.str.split` with ``expand=True`` incorrectly raising an IndexError on empty strings (:issue:`20002`).
- Bug in :class:`Index` constructor with ``dtype=CategoricalDtype(...)`` where ``categories`` and ``ordered`` are not maintained (issue:`19032`)
- Bug in :class:`Series` constructor with scalar and ``dtype=CategoricalDtype(...)`` where ``categories`` and ``ordered`` are not maintained (issue:`19565`)
- Bug in ``Categorical.__iter__`` not converting to Python types (:issue:`19909`)
- Bug in :func:`pandas.factorize` returning the unique codes for the ``uniques``. This now returns a ``Categorical`` with the same dtype as the input (:issue:`19721`)
- Bug in :func:`pandas.factorize` including an item for missing values in the ``uniques`` return value (:issue:`19721`)

Datetimelike
^^^^^^^^^^^^

- Bug in :func:`Series.__sub__` subtracting a non-nanosecond ``np.datetime64`` object from a ``Series`` gave incorrect results (:issue:`7996`)
- Bug in :class:`DatetimeIndex`, :class:`TimedeltaIndex` addition and subtraction of zero-dimensional integer arrays gave incorrect results (:issue:`19012`)
- Bug in :class:`DatetimeIndex` and :class:`TimedeltaIndex` where adding or subtracting an array-like of ``DateOffset`` objects either raised (``np.array``, ``pd.Index``) or broadcast incorrectly (``pd.Series``) (:issue:`18849`)
- Bug in :func:`Series.__add__` adding Series with dtype ``timedelta64[ns]`` to a timezone-aware ``DatetimeIndex`` incorrectly dropped timezone information (:issue:`13905`)
- Adding a ``Period`` object to a ``datetime`` or ``Timestamp`` object will now correctly raise a ``TypeError`` (:issue:`17983`)
- Bug in :class:`Timestamp` where comparison with an array of ``Timestamp`` objects would result in a ``RecursionError`` (:issue:`15183`)
- Bug in :class:`Series` floor-division where operating on a scalar ``timedelta`` raises an exception (:issue:`18846`)
- Bug in :class:`DatetimeIndex` where the repr was not showing high-precision time values at the end of a day (e.g., 23:59:59.999999999) (:issue:`19030`)
- Bug in ``.astype()`` to non-ns timedelta units would hold the incorrect dtype (:issue:`19176`, :issue:`19223`, :issue:`12425`)
- Bug in subtracting :class:`Series` from ``NaT`` incorrectly returning ``NaT`` (:issue:`19158`)
- Bug in :func:`Series.truncate` which raises ``TypeError`` with a monotonic ``PeriodIndex`` (:issue:`17717`)
- Bug in :func:`~DataFrame.pct_change` using ``periods`` and ``freq`` returned different length outputs (:issue:`7292`)
- Bug in comparison of :class:`DatetimeIndex` against ``None`` or ``datetime.date`` objects raising ``TypeError`` for ``==`` and ``!=`` comparisons instead of all-``False`` and all-``True``, respectively (:issue:`19301`)
- Bug in :class:`Timestamp` and :func:`to_datetime` where a string representing a barely out-of-bounds timestamp would be incorrectly rounded down instead of raising ``OutOfBoundsDatetime`` (:issue:`19382`)
- Bug in :func:`Timestamp.floor` :func:`DatetimeIndex.floor` where time stamps far in the future and past were not rounded correctly (:issue:`19206`)
- Bug in :func:`to_datetime` where passing an out-of-bounds datetime with ``errors='coerce'`` and ``utc=True`` would raise ``OutOfBoundsDatetime`` instead of parsing to ``NaT`` (:issue:`19612`)
- Bug in :class:`DatetimeIndex` and :class:`TimedeltaIndex` addition and subtraction where name of the returned object was not always set consistently. (:issue:`19744`)
- Bug in :class:`DatetimeIndex` and :class:`TimedeltaIndex` addition and subtraction where operations with numpy arrays raised ``TypeError`` (:issue:`19847`)

Timedelta
^^^^^^^^^

- Bug in :func:`Timedelta.__mul__` where multiplying by ``NaT`` returned ``NaT`` instead of raising a ``TypeError`` (:issue:`19819`)
- Bug in :class:`Series` with ``dtype='timedelta64[ns]'`` where addition or subtraction of ``TimedeltaIndex`` had results cast to ``dtype='int64'`` (:issue:`17250`)
- Bug in :class:`Series` with ``dtype='timedelta64[ns]'`` where addition or subtraction of ``TimedeltaIndex`` could return a ``Series`` with an incorrect name (:issue:`19043`)
- Bug in :func:`Timedelta.__floordiv__` and :func:`Timedelta.__rfloordiv__` dividing by many incompatible numpy objects was incorrectly allowed (:issue:`18846`)
- Bug where dividing a scalar timedelta-like object with :class:`TimedeltaIndex` performed the reciprocal operation (:issue:`19125`)
- Bug in :class:`TimedeltaIndex` where division by a ``Series`` would return a ``TimedeltaIndex`` instead of a ``Series`` (:issue:`19042`)
- Bug in :func:`Timedelta.__add__`, :func:`Timedelta.__sub__` where adding or subtracting a ``np.timedelta64`` object would return another ``np.timedelta64`` instead of a ``Timedelta`` (:issue:`19738`)
- Bug in :func:`Timedelta.__floordiv__`, :func:`Timedelta.__rfloordiv__` where operating with a ``Tick`` object would raise a ``TypeError`` instead of returning a numeric value (:issue:`19738`)
- Bug in :func:`Period.asfreq` where periods near ``datetime(1, 1, 1)`` could be converted incorrectly (:issue:`19643`, :issue:`19834`)
- Bug in :func:`Timedelta.total_seconds()` causing precision errors i.e. ``Timedelta('30S').total_seconds()==30.000000000000004`` (:issue:`19458`)
- Bug in :func:`Timedelta.__rmod__` where operating with a ``numpy.timedelta64`` returned a ``timedelta64`` object instead of a ``Timedelta`` (:issue:`19820`)
- Multiplication of :class:`TimedeltaIndex` by ``TimedeltaIndex`` will now raise ``TypeError`` instead of raising ``ValueError`` in cases of length mis-match (:issue:`19333`)
- Bug in indexing a :class:`TimedeltaIndex` with a ``np.timedelta64`` object which was raising a ``TypeError`` (:issue:`20393`)


Timezones
^^^^^^^^^

- Bug in creating a ``Series`` from an array that contains both tz-naive and tz-aware values will result in a ``Series`` whose dtype is tz-aware instead of object (:issue:`16406`)
- Bug in comparison of timezone-aware :class:`DatetimeIndex` against ``NaT`` incorrectly raising ``TypeError`` (:issue:`19276`)
- Bug in :meth:`DatetimeIndex.astype` when converting between timezone aware dtypes, and converting from timezone aware to naive (:issue:`18951`)
- Bug in comparing :class:`DatetimeIndex`, which failed to raise ``TypeError`` when attempting to compare timezone-aware and timezone-naive datetimelike objects (:issue:`18162`)
- Bug in localization of a naive, datetime string in a ``Series`` constructor with a ``datetime64[ns, tz]`` dtype (:issue:`174151`)
- :func:`Timestamp.replace` will now handle Daylight Savings transitions gracefully (:issue:`18319`)
- Bug in tz-aware :class:`DatetimeIndex` where addition/subtraction with a :class:`TimedeltaIndex` or array with ``dtype='timedelta64[ns]'`` was incorrect (:issue:`17558`)
- Bug in :func:`DatetimeIndex.insert` where inserting ``NaT`` into a timezone-aware index incorrectly raised (:issue:`16357`)
- Bug in :class:`DataFrame` constructor, where tz-aware Datetimeindex and a given column name will result in an empty ``DataFrame`` (:issue:`19157`)
- Bug in :func:`Timestamp.tz_localize` where localizing a timestamp near the minimum or maximum valid values could overflow and return a timestamp with an incorrect nanosecond value (:issue:`12677`)
- Bug when iterating over :class:`DatetimeIndex` that was localized with fixed timezone offset that rounded nanosecond precision to microseconds (:issue:`19603`)
- Bug in :func:`DataFrame.diff` that raised an ``IndexError`` with tz-aware values (:issue:`18578`)
- Bug in :func:`melt` that converted tz-aware dtypes to tz-naive (:issue:`15785`)
- Bug in :func:`Dataframe.count` that raised an ``ValueError``, if :func:`Dataframe.dropna` was called for a single column with timezone-aware values. (:issue:`13407`)

Offsets
^^^^^^^

- Bug in :class:`WeekOfMonth` and :class:`Week` where addition and subtraction did not roll correctly (:issue:`18510`, :issue:`18672`, :issue:`18864`)
- Bug in :class:`WeekOfMonth` and :class:`LastWeekOfMonth` where default keyword arguments for constructor raised ``ValueError`` (:issue:`19142`)
- Bug in :class:`FY5253Quarter`, :class:`LastWeekOfMonth` where rollback and rollforward behavior was inconsistent with addition and subtraction behavior (:issue:`18854`)
- Bug in :class:`FY5253` where ``datetime`` addition and subtraction incremented incorrectly for dates on the year-end but not normalized to midnight (:issue:`18854`)
- Bug in :class:`FY5253` where date offsets could incorrectly raise an ``AssertionError`` in arithmetic operatons (:issue:`14774`)


Numeric
^^^^^^^
- Bug in :meth:`DataFrame.rank` and :meth:`Series.rank` when ``method='dense'`` and ``pct=True`` in which percentile ranks were not being used with the number of distinct observations (:issue:`15630`)
- Bug in :class:`Series` constructor with an int or float list where specifying ``dtype=str``, ``dtype='str'`` or ``dtype='U'`` failed to convert the data elements to strings (:issue:`16605`)
- Bug in :class:`Index` multiplication and division methods where operating with a ``Series`` would return an ``Index`` object instead of a ``Series`` object (:issue:`19042`)
- Bug in the :class:`DataFrame` constructor in which data containing very large positive or very large negative numbers was causing ``OverflowError`` (:issue:`18584`)
- Bug in :class:`Index` constructor with ``dtype='uint64'`` where int-like floats were not coerced to :class:`UInt64Index` (:issue:`18400`)
- Bug in :class:`DataFrame` flex arithmetic (e.g. ``df.add(other, fill_value=foo)``) with a ``fill_value`` other than ``None`` failed to raise ``NotImplementedError`` in corner cases where either the frame or ``other`` has length zero (:issue:`19522`)
- Multiplication and division of numeric-dtyped :class:`Index` objects with timedelta-like scalars returns ``TimedeltaIndex`` instead of raising ``TypeError`` (:issue:`19333`)
- Bug in :meth:`Series.rank` and :meth:`DataFrame.rank` when ``ascending='False'`` failed to return correct ranks for infinity if ``NaN`` were present (:issue:`19538`)
- Bug where ``NaN`` was returned instead of 0 by :func:`Series.pct_change` and :func:`DataFrame.pct_change` when ``fill_method`` is not ``None`` (:issue:`19873`)


Indexing
^^^^^^^^

- Bug in :class:`Index` construction from list of mixed type tuples (:issue:`18505`)
- Bug in :func:`Index.drop` when passing a list of both tuples and non-tuples (:issue:`18304`)
- Bug in :meth:`~DataFrame.drop`, :meth:`~Panel.drop`, :meth:`~Series.drop`, :meth:`~Index.drop` where no ``KeyError`` is raised when dropping a non-existent element from an axis that contains duplicates (:issue:`19186`)
- Bug in indexing a datetimelike ``Index`` that raised ``ValueError`` instead of ``IndexError`` (:issue:`18386`).
- :func:`Index.to_series` now accepts ``index`` and ``name`` kwargs (:issue:`18699`)
- :func:`DatetimeIndex.to_series` now accepts ``index`` and ``name`` kwargs (:issue:`18699`)
- Bug in indexing non-scalar value from ``Series`` having non-unique ``Index`` will return value flattened (:issue:`17610`)
- Bug in ``__setitem__`` when indexing a :class:`DataFrame` with a 2-d boolean ndarray (:issue:`18582`)
- Bug in ``str.extractall`` when there were no matches empty :class:`Index` was returned instead of appropriate :class:`MultiIndex` (:issue:`19034`)
- Bug in :class:`IntervalIndex` where empty and purely NA data was constructed inconsistently depending on the construction method (:issue:`18421`)
- Bug in :func:`IntervalIndex.symmetric_difference` where the symmetric difference with a non-``IntervalIndex`` did not raise (:issue:`18475`)
- Bug in :class:`IntervalIndex` where set operations that returned an empty ``IntervalIndex`` had the wrong dtype (:issue:`19101`)
- Bug in :meth:`DataFrame.drop_duplicates` where no ``KeyError`` is raised when passing in columns that don't exist on the ``DataFrame`` (issue:`19726`)
- Bug in ``Index`` subclasses constructors that ignore unexpected keyword arguments (:issue:`19348`)
- Bug in :meth:`Index.difference` when taking difference of an ``Index`` with itself (:issue:`20040`)
- Bug in :meth:`DataFrame.first_valid_index` and :meth:`DataFrame.last_valid_index` in presence of entire rows of NaNs in the middle of values (:issue:`20499`).
- Bug in the ``IntervalIndex`` repr missing a trailing comma at the end of the "data" section (:issue`20611`)

MultiIndex
^^^^^^^^^^

- Bug in :func:`MultiIndex.__contains__` where non-tuple keys would return ``True`` even if they had been dropped (:issue:`19027`)
- Bug in :func:`MultiIndex.set_labels` which would cause casting (and potentially clipping) of the new labels if the ``level`` argument is not 0 or a list like [0, 1, ... ]  (:issue:`19057`)
- Bug in :func:`MultiIndex.get_level_values` which would return an invalid index on level of ints with missing values (:issue:`17924`)
- Bug in :func:`MultiIndex.unique` when called on empty :class:`MultiIndex` (:issue:`20568`)
- Bug in :func:`MultiIndex.unique` which would not preserve level names (:issue:`20570`)
- Bug in :func:`MultiIndex.remove_unused_levels` which would fill nan values (:issue:`18417`)
- Bug in :func:`MultiIndex.from_tuples` which would fail to take zipped tuples in python3 (:issue:`18434`)
- Bug in :func:`MultiIndex.get_loc` which would fail to automatically cast values between float and int (:issue:`18818`, :issue:`15994`)
- Bug in :func:`MultiIndex.get_loc` which would cast boolean to integer labels (:issue:`19086`)
- Bug in :func:`MultiIndex.get_loc` which would fail to locate keys containing ``NaN`` (:issue:`18485`)
- Bug in :func:`MultiIndex.get_loc` in large :class:`MultiIndex`, would fail when levels had different dtypes (:issue:`18520`)
- Bug in indexing where nested indexers having only numpy arrays are handled incorrectly (:issue:`19686`)


I/O
^^^

- :func:`read_html` now rewinds seekable IO objects after parse failure, before attempting to parse with a new parser. If a parser errors and the object is non-seekable, an informative error is raised suggesting the use of a different parser (:issue:`17975`)
- :meth:`DataFrame.to_html` now has an option to add an id to the leading `<table>` tag (:issue:`8496`)
- Bug in :func:`read_msgpack` with a non existent file is passed in Python 2 (:issue:`15296`)
- Bug in :func:`read_csv` where a ``MultiIndex`` with duplicate columns was not being mangled appropriately (:issue:`18062`)
- Bug in :func:`read_csv` where missing values were not being handled properly when ``keep_default_na=False`` with dictionary ``na_values`` (:issue:`19227`)
- Bug in :func:`read_sas` where a file with 0 variables gave an ``AttributeError`` incorrectly. Now it gives an ``EmptyDataError`` (:issue:`18184`)
- Bug in :func:`DataFrame.to_latex()` where pairs of braces meant to serve as invisible placeholders were escaped (:issue:`18667`)
- Bug in :func:`read_json` where large numeric values were causing an ``OverflowError`` (:issue:`18842`)
- Bug in :func:`DataFrame.to_parquet` where an exception was raised if the write destination is S3 (:issue:`19134`)
- :class:`Interval` now supported in :func:`DataFrame.to_excel` for all Excel file types (:issue:`19242`)
- :class:`Timedelta` now supported in :func:`DataFrame.to_excel` for all Excel file types (:issue:`19242`, :issue:`9155`, :issue:`19900`)
- Bug in :meth:`pandas.io.stata.StataReader.value_labels` raising an ``AttributeError`` when called on very old files. Now returns an empty dict (:issue:`19417`)
- Bug in :func:`read_pickle` when unpickling objects with :class:`TimedeltaIndex` or :class:`Float64Index` created with pandas prior to version 0.20 (:issue:`19939`)
- Bug in :meth:`pandas.io.json.json_normalize` where subrecords are not properly normalized if any subrecords values are NoneType (:issue:`20030`)
- Bug in ``usecols`` parameter in :func:`pandas.io.read_csv` and :func:`pandas.io.read_table` where error is not raised correctly when passing a string. (:issue:`20529`)

Plotting
^^^^^^^^

- :func:`DataFrame.plot` now raises a ``ValueError`` when the ``x`` or ``y`` argument is improperly formed (:issue:`18671`)
- Bug in :func:`DataFrame.plot` when ``x`` and ``y`` arguments given as positions caused incorrect referenced columns for line, bar and area plots (:issue:`20056`)
- Bug in formatting tick labels with ``datetime.time()`` and fractional seconds (:issue:`18478`).
- :meth:`Series.plot.kde` has exposed the args ``ind`` and ``bw_method`` in the docstring (:issue:`18461`). The argument ``ind`` may now also be an integer (number of sample points).
- :func:`DataFrame.plot` now supports multiple columns to the ``y`` argument (:issue:`19699`)


Groupby/Resample/Rolling
^^^^^^^^^^^^^^^^^^^^^^^^

- Bug when grouping by a single column and aggregating with a class like ``list`` or ``tuple`` (:issue:`18079`)
- Fixed regression in :func:`DataFrame.groupby` which would not emit an error when called with a tuple key not in the index (:issue:`18798`)
- Bug in :func:`DataFrame.resample` which silently ignored unsupported (or mistyped) options for ``label``, ``closed`` and ``convention`` (:issue:`19303`)
- Bug in :func:`DataFrame.groupby` where tuples were interpreted as lists of keys rather than as keys (:issue:`17979`, :issue:`18249`)
- Bug in :func:`DataFrame.groupby` where aggregation by ``first``/``last``/``min``/``max`` was causing timestamps to lose precision (:issue:`19526`)
- Bug in :func:`DataFrame.transform` where particular aggregation functions were being incorrectly cast to match the dtype(s) of the grouped data (:issue:`19200`)
- Bug in :func:`DataFrame.groupby` passing the `on=` kwarg, and subsequently using ``.apply()`` (:issue:`17813`)
- Bug in :func:`DataFrame.resample().aggregate` not raising a ``KeyError`` when aggregating a non-existent column (:issue:`16766`, :issue:`19566`)
- Fixed a performance regression for ``GroupBy.nth`` and ``GroupBy.last`` with some object columns (:issue:`19283`)
- Bug in :func:`DataFrameGroupBy.cumsum` and :func:`DataFrameGroupBy.cumprod` when ``skipna`` was passed (:issue:`19806`)

Sparse
^^^^^^

- Bug in which creating a ``SparseDataFrame`` from a dense ``Series`` or an unsupported type raised an uncontrolled exception (:issue:`19374`)
- Bug in :class:`SparseDataFrame.to_csv` causing exception (:issue:`19384`)
- Bug in :class:`SparseSeries.memory_usage` which caused segfault by accessing non sparse elements (:issue:`19368`)
- Bug in constructing a ``SparseArray``: if ``data`` is a scalar and ``index`` is defined it will coerce to ``float64`` regardless of scalar's dtype. (:issue:`19163`)

Reshaping
^^^^^^^^^

- Bug in :func:`DataFrame.stack` which fails trying to sort mixed type levels under Python 3 (:issue:`18310`)
- Bug in :func:`DataFrame.unstack` which casts int to float if ``columns`` is a ``MultiIndex`` with unused levels (:issue:`17845`)
- Bug in :func:`DataFrame.unstack` which raises an error if ``index`` is a ``MultiIndex`` with unused labels on the unstacked level (:issue:`18562`)
- Fixed construction of a :class:`Series` from a ``dict`` containing ``NaN`` as key (:issue:`18480`)
- Fixed construction of a :class:`DataFrame` from a ``dict`` containing ``NaN`` as key (:issue:`18455`)
- Suppressed error in the construction of a :class:`DataFrame` from a ``dict`` containing scalar values when the corresponding keys are not included in the passed index (:issue:`18600`)

- Fixed (changed from ``object`` to ``float64``) dtype of :class:`DataFrame` initialized with axes, no data, and ``dtype=int`` (:issue:`19646`)
- Bug in :func:`Series.rank` where ``Series`` containing ``NaT`` modifies the ``Series`` inplace (:issue:`18521`)
- Bug in :func:`cut` which fails when using readonly arrays (:issue:`18773`)
- Bug in :func:`DataFrame.pivot_table` which fails when the ``aggfunc`` arg is of type string.  The behavior is now consistent with other methods like ``agg`` and ``apply`` (:issue:`18713`)
- Bug in :func:`DataFrame.merge` in which merging using ``Index`` objects as vectors raised an Exception (:issue:`19038`)
- Bug in :func:`DataFrame.stack`, :func:`DataFrame.unstack`, :func:`Series.unstack` which were not returning subclasses (:issue:`15563`)
- Bug in timezone comparisons, manifesting as a conversion of the index to UTC in ``.concat()`` (:issue:`18523`)
- Bug in :func:`concat` when concatting sparse and dense series it returns only a ``SparseDataFrame``. Should be a ``DataFrame``. (:issue:`18914`, :issue:`18686`, and :issue:`16874`)
- Improved error message for :func:`DataFrame.merge` when there is no common merge key (:issue:`19427`)
- Bug in :func:`DataFrame.join` which does an ``outer`` instead of a ``left`` join when being called with multiple DataFrames and some have non-unique indices (:issue:`19624`)
- :func:`Series.rename` now accepts ``axis`` as a kwarg (:issue:`18589`)
- Comparisons between :class:`Series` and :class:`Index` would return a ``Series`` with an incorrect name, ignoring the ``Index``'s name attribute (:issue:`19582`)
- Bug in :func:`qcut` where datetime and timedelta data with ``NaT`` present raised a ``ValueError`` (:issue:`19768`)
- Bug in :func:`DataFrame.iterrows`, which would infers strings not compliant to `ISO8601 <https://en.wikipedia.org/wiki/ISO_8601>`_ to datetimes (:issue:`19671`)
- Bug in :class:`Series` constructor with ``Categorical`` where a ```ValueError`` is not raised when an index of different length is given (:issue:`19342`)
- Bug in :meth:`DataFrame.astype` where column metadata is lost when converting to categorical or a dictionary of dtypes (:issue:`19920`)
- Bug in :func:`cut` and :func:`qcut` where timezone information was dropped (:issue:`19872`)
- Bug in :class:`Series` constructor with a ``dtype=str``, previously raised in some cases (:issue:`19853`)

Other
^^^^^

- Improved error message when attempting to use a Python keyword as an identifier in a ``numexpr`` backed query (:issue:`18221`)
- Bug in accessing a :func:`pandas.get_option`, which raised ``KeyError`` rather than ``OptionError`` when looking up a non-existant option key in some cases (:issue:`19789`)