v0.23.0.txt

.. _whatsnew_0230:

v0.23.0
-------

This is a major release from 0.21.1 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.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.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

   df.index.name = 'index'
   df.to_json('test.json', orient='table')
   new_df = pd.read_json('test.json', orient='table')
   new_df
   print(new_df.index.name)

.. _whatsnew_0230.enhancements.other:

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

- 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`).
- 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 <developer.register-accessors>` for more (:issue:`14781`).


- ``IntervalIndex.astype`` now supports conversions between subtypes when passed an ``IntervalDtype`` (:issue:`19197`)

.. _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 |
   +=================+=================+==========+
   | python-dateutil | 2.5.0           |    X     |
   +-----------------+-----------------+----------+
   | openpyxl        | 2.4.0           |          |
   +-----------------+-----------------+----------+

.. _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()


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:

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`)
- ``NaT`` division with :class:`datetime.timedelta` will now return ``NaN`` instead of raising (:issue:`17876`)
- 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`)
- :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`)
- :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`)
- 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`).
- :func:`DataFrame.from_items` provides a more informative error message when passed scalar values (:issue:`17312`)
- 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`)
- 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:`pandas.merge` now raises a ``ValueError`` when trying to merge on incompatible data types (:issue:`9780`)
- :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`)
- 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`)
- :class:`IntervalIndex` and ``IntervalDtype`` no longer support categorical, object, and string subtypes (:issue:`19016`)
- The default ``Timedelta`` constructor now accepts an ``ISO 8601 Duration`` string as an argument (:issue:`19040`)
- ``IntervalDtype`` now returns ``True`` when compared against ``'interval'`` regardless of subtype, and ``IntervalDtype.name`` now returns ``'interval'`` regardless of subtype (:issue:`18980`)
- :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`)
- 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`)

.. _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`).


.. _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 :attribute:`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`)

.. _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`)

.. _whatsnew_0230.docs:

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

- 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`)
-

.. _whatsnew_0230.bug_fixes:

Bug Fixes
~~~~~~~~~


Conversion
^^^^^^^^^^

- Bug in :class:`Index` constructor with ``dtype='uint64'`` where int-like floats were not coerced to :class:`UInt64Index` (:issue:`18400`)
- Bug in the :class:`DataFrame` constructor in which data containing very large positive or very large negative numbers was causing ``OverflowError`` (:issue:`18584`)
- Fixed a bug where 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`)
- Adding a ``Period`` object to a ``datetime`` or ``Timestamp`` object will now correctly raise a ``TypeError`` (:issue:`17983`)
- Fixed a bug where ``FY5253`` date offsets could incorrectly raise an ``AssertionError`` in arithmetic operatons (:issue:`14774`)
- 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 :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:`Timestamp` where comparison with an array of ``Timestamp`` objects would result in a ``RecursionError`` (:issue:`15183`)
- Bug in :class:`WeekOfMonth` and class:`Week` where addition and subtraction did not roll correctly (:issue:`18510`,:issue:`18672`,:issue:`18864`)
- Bug in :meth:`DatetimeIndex.astype` when converting between timezone aware dtypes, and converting from timezone aware to naive (:issue:`18951`)
- 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:`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 :class:`Series` floor-division where operating on a scalar ``timedelta`` raises an exception (:issue:`18846`)
- Bug in :class:`FY5253Quarter`, :class:`LastWeekOfMonth` where rollback and rollforward behavior was inconsistent with addition and subtraction behavior (:issue:`18854`)
- Bug in :class:`Index` constructor with ``dtype=CategoricalDtype(...)`` where ``categories`` and ``ordered`` are not maintained (issue:`19032`)
- Bug in :class:`Index` constructor with ``dtype=CategoricalDtype(...)`` where ``categories`` and ``ordered`` are not maintained (issue:`19032`)
- 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:`TimedeltaIndex` where division by a ``Series`` would return a ``TimedeltaIndex`` instead of a ``Series`` (issue:`19042`)
- Bug in :class:`Series` with ``dtype='timedelta64[ns]`` where addition or subtraction of ``TimedeltaIndex`` could return a ``Series`` with an incorrect name (issue:`19043`)
- Fixed bug where comparing :class:`DatetimeIndex` failed to raise ``TypeError`` when attempting to compare timezone-aware and timezone-naive datetimelike objects (:issue:`18162`)
- 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 where dividing a scalar timedelta-like object with :class:`TimedeltaIndex` performed the reciprocal operation (:issue:`19125`)
- Bug in :class:`WeekOfMonth` and :class:`LastWeekOfMonth` where default keyword arguments for constructor raised ``ValueError`` (:issue:`19142`)
- 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 :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 ``.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`)


Indexing
^^^^^^^^

- Bug in :func:`Series.truncate` which raises ``TypeError`` with a monotonic ``PeriodIndex`` (:issue:`17717`)
- Bug in :func:`DataFrame.groupby` where tuples were interpreted as lists of keys rather than as keys (:issue:`17979`, :issue:`18249`)
- 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.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 :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 :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 indexing a datetimelike ``Index`` that raised ``ValueError`` instead of ``IndexError`` (:issue:`18386`).
- Bug in tz-aware :class:`DatetimeIndex` where addition/subtraction with a :class:`TimedeltaIndex` or array with ``dtype='timedelta64[ns]'`` was incorrect (:issue:`17558`)
- :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 :func:`DatetimeIndex.insert` where inserting ``NaT`` into a timezone-aware index incorrectly raised (:issue:`16357`)
- Bug in ``__setitem__`` when indexing a :class:`DataFrame` with a 2-d boolean ndarray (:issue:`18582`)
- 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 ``str.extractall`` when there were no matches empty :class:`Index` was returned instead of appropriate :class:`MultiIndex` (:issue:`19034`)
- Bug in :class:`IntervalIndex` where set operations that returned an empty ``IntervalIndex`` had the wrong dtype (:issue:`19101`)

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`)
- 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_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`)
-

Plotting
^^^^^^^^

- :func: `DataFrame.plot` now raises a ``ValueError`` when the ``x`` or ``y`` argument is improperly formed (:issue:`18671`)
- Bug in formatting tick labels with ``datetime.time()`` and fractional seconds (:issue:`18478`).
-
-

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`)
-
-

Sparse
^^^^^^

-
-
-

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`)
- 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`)
-

Numeric
^^^^^^^

- 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 :func:`Series.__add__` adding Series with dtype ``timedelta64[ns]`` to a timezone-aware ``DatetimeIndex`` incorrectly dropped timezone information (:issue:`13905`)
- Bug in :func:`Timedelta.__floordiv__` and :func:`Timedelta.__rfloordiv__` dividing by many incompatible numpy objects was incorrectly allowed (:issue:`18846`)
-

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

-
- Bug in :func:`pandas.api.types.union_categoricals` returning the wrong result
  when all the categoricals had the same categories, but in a different order.
  This affected :func:`pandas.concat` with Categorical data (:issue:`19096`).
- Bug in ``Categorical.equals`` between two unordered categories with the same categories, but in a different order (:issue:`16603`)
-

Other
^^^^^

- Improved error message when attempting to use a Python keyword as an identifier in a ``numexpr`` backed query (:issue:`18221`)