v0.17.0.txt

.. _whatsnew_0170:

v0.17.0 (September ??, 2015)
----------------------------

This is a major release from 0.16.2 and includes a small number of API changes, several new features,
enhancements, and performance improvements along with a large number of bug fixes. We recommend that all
users upgrade to this version.

.. warning::

   pandas >= 0.17.0 will no longer support compatibility with Python version 3.2 (:issue:`9118`)

.. warning::

   The ``pandas.io.data`` package is deprecated and will be replaced by the
   `pandas-datareader package <https://github.com/pydata/pandas-datareader>`_.
   This will allow the data modules to be independently updated to your pandas
   installation. The API for ``pandas-datareader v0.1.1`` is exactly the same
   as in ``pandas v0.17.0`` (:issue:`8961`, :issue:`10861`).

   After installing pandas-datareader, you can easily change your imports:

   .. code-block:: Python

     from pandas.io import data, wb # becomes
     from pandas_datareader import data, wb

Highlights include:

- Release the Global Interpreter Lock (GIL) on some cython operations, see :ref:`here <whatsnew_0170.gil>`
- The sorting API has been revamped to remove some long-time inconsistencies, see :ref:`here <whatsnew_0170.api_breaking.sorting>`
- The default for ``to_datetime`` will now be to ``raise`` when presented with unparseable formats,
  previously this would return the original input, see :ref:`here <whatsnew_0170.api_breaking.to_datetime>`
- The default for ``dropna`` in ``HDFStore`` has changed to ``False``, to store by default all rows even
  if they are all ``NaN``, see :ref:`here <whatsnew_0170.api_breaking.hdf_dropna>`
- Support for ``Series.dt.strftime`` to generate formatted strings for datetime-likes, see :ref:`here <whatsnew_0170.strftime>`
- Development installed versions of pandas will now have ``PEP440`` compliant version strings (:issue:`9518`)
- Development support for benchmarking with the `Air Speed Velocity library <https://github.com/spacetelescope/asv/>`_ (:issue:`8316`)
- Support for reading SAS xport files, see :ref:`here <whatsnew_0170.enhancements.sas_xport>`
- Removal of the automatic TimeSeries broadcasting, deprecated since 0.8.0, see :ref:`here <whatsnew_0170.prior_deprecations>`

Check the :ref:`API Changes <whatsnew_0170.api>` and :ref:`deprecations <whatsnew_0170.deprecations>` before updating.

.. contents:: What's new in v0.17.0
    :local:
    :backlinks: none

.. _whatsnew_0170.enhancements:

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

- ``DataFrame`` has the ``nlargest`` and ``nsmallest`` methods (:issue:`10393`)
- SQL io functions now accept a SQLAlchemy connectable. (:issue:`7877`)
- Enable writing complex values to HDF stores when using table format (:issue:`10447`)
- Enable reading gzip compressed files via URL, either by explicitly setting the compression parameter or by inferring from the presence of the HTTP Content-Encoding header in the response (:issue:`8685`)
- Add a ``limit_direction`` keyword argument that works with ``limit`` to enable ``interpolate`` to fill ``NaN`` values forward, backward, or both (:issue:`9218` and :issue:`10420`)

  .. ipython:: python

     ser = pd.Series([np.nan, np.nan, 5, np.nan, np.nan, np.nan, 13])
     ser.interpolate(limit=1, limit_direction='both')

- Round DataFrame to variable number of decimal places (:issue:`10568`).

   .. ipython :: python

    df = pd.DataFrame(np.random.random([3, 3]), columns=['A', 'B', 'C'],
    index=['first', 'second', 'third'])
    df
    df.round(2)
    df.round({'A': 0, 'C': 2})
    
.. _whatsnew_0170.gil:

Releasing the GIL
^^^^^^^^^^^^^^^^^

We are releasing the global-interpreter-lock (GIL) on some cython operations.
This will allow other threads to run simultaneously during computation, potentially allowing performance improvements
from multi-threading. Notably ``groupby``, ``nsmallest`` and some indexing operations benefit from this. (:issue:`8882`)

For example the groupby expression in the following code will have the GIL released during the factorization step, e.g. ``df.groupby('key')``
as well as the ``.sum()`` operation.

.. code-block:: python

   N = 1000000
   ngroups = 10
   df = DataFrame({'key' : np.random.randint(0,ngroups,size=N),
                   'data' : np.random.randn(N) })
   df.groupby('key')['data'].sum()

Releasing of the GIL could benefit an application that uses threads for user interactions (e.g. QT_), or performaning multi-threaded computations. A nice example of a library that can handle these types of computation-in-parallel is the dask_ library.

.. _dask: https://dask.readthedocs.org/en/latest/
.. _QT: https://wiki.python.org/moin/PyQt

.. _whatsnew_0170.strftime:

Support strftime for Datetimelikes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We are now supporting a ``Series.dt.strftime`` method for datetime-likes to generate a formatted string (:issue:`10110`). Examples:

.. ipython:: python

   # DatetimeIndex
   s = pd.Series(pd.date_range('20130101', periods=4))
   s
   s.dt.strftime('%Y/%m/%d')

.. ipython:: python

   # PeriodIndex
   s = pd.Series(pd.period_range('20130101', periods=4))
   s
   s.dt.strftime('%Y/%m/%d')

The string format is as the python standard library and details can be found `here <https://docs.python.org/2/library/datetime.html#strftime-and-strptime-behavior>`_

.. _whatsnew_0170.periodfreq:

Period Frequency Enhancement
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``Period``, ``PeriodIndex`` and ``period_range`` can now accept multiplied freq. Also, ``Period.freq`` and ``PeriodIndex.freq`` are now stored as ``DateOffset`` instance like ``DatetimeIndex``, not ``str`` (:issue:`7811`)

Multiplied freq represents a span of corresponding length. Below example creates a period of 3 days. Addition and subtraction will shift the period by its span.

.. ipython:: python

   p = pd.Period('2015-08-01', freq='3D')
   p
   p + 1
   p - 2
   p.to_timestamp()
   p.to_timestamp(how='E')

You can use multiplied freq in ``PeriodIndex`` and ``period_range``.

.. ipython:: python

   idx = pd.period_range('2015-08-01', periods=4, freq='2D')
   idx
   idx + 1

.. _whatsnew_0170.enhancements.sas_xport:

Support for SAS XPORT files
^^^^^^^^^^^^^^^^^^^^^^^^^^^

:meth:`~pandas.io.read_sas` provides support for reading *SAS XPORT* format files. (:issue:`4052`).

.. code-block:: python

    df = pd.read_sas('sas_xport.xpt')

It is also possible to obtain an iterator and read an XPORT file
incrementally.

.. code-block:: python

    for df in pd.read_sas('sas_xport.xpt', chunksize=10000)
        do_something(df)

See the :ref:`docs <io.sas>` for more details.

.. _whatsnew_0170.enhancements.other:

Other enhancements
^^^^^^^^^^^^^^^^^^

- `read_sql` and `to_sql` can accept database URI as con parameter (:issue:`10214`)
- Enable `read_hdf` to be used without specifying a key when the HDF file contains a single dataset (:issue:`10443`)
- Enable writing Excel files in :ref:`memory <_io.excel_writing_buffer>` using StringIO/BytesIO (:issue:`7074`)
- Enable serialization of lists and dicts to strings in ExcelWriter (:issue:`8188`)
- Added functionality to use the ``base`` argument when resampling a ``TimeDeltaIndex`` (:issue:`10530`)
- ``DatetimeIndex`` can be instantiated using strings contains ``NaT`` (:issue:`7599`)
- The string parsing of ``to_datetime``, ``Timestamp`` and ``DatetimeIndex`` has been made consistent. (:issue:`7599`)

  Prior to v0.17.0, ``Timestamp`` and ``to_datetime`` may parse year-only datetime-string incorrectly using today's date, otherwise ``DatetimeIndex``
  uses the beginning of the year. ``Timestamp`` and ``to_datetime`` may raise ``ValueError`` in some types of datetime-string which ``DatetimeIndex``
  can parse, such as a quarterly string.

  Previous Behavior

  .. code-block:: python

     In [1]: Timestamp('2012Q2')
     Traceback
        ...
     ValueError: Unable to parse 2012Q2

     # Results in today's date.
     In [2]: Timestamp('2014')
     Out [2]: 2014-08-12 00:00:00

  v0.17.0 can parse them as below. It works on ``DatetimeIndex`` also.

  New Behaviour

  .. ipython:: python

     Timestamp('2012Q2')
     Timestamp('2014')
     DatetimeIndex(['2012Q2', '2014'])

  .. note:: If you want to perform calculations based on today's date, use ``Timestamp.now()`` and ``pandas.tseries.offsets``.

  .. ipython:: python

     import pandas.tseries.offsets as offsets
     Timestamp.now()
     Timestamp.now() + offsets.DateOffset(years=1)

- ``to_datetime`` can now accept ``yearfirst`` keyword (:issue:`7599`)

- ``pandas.tseries.offsets`` larger than the ``Day`` offset can now be used with with ``Series`` for addition/subtraction (:issue:`10699`).  See the :ref:`Documentation <timeseries.offsetseries>` for more details.

- ``pd.Series`` of type ``timedelta64`` has new method ``.dt.total_seconds()`` returning the duration of the timedelta in seconds (:issue: `10817`)

- ``pd.Timedelta.total_seconds()`` now returns Timedelta duration to ns precision (previously microsecond precision) (:issue: `10939`)

- ``.as_blocks`` will now take a ``copy`` optional argument to return a copy of the data, default is to copy (no change in behavior from prior versions), (:issue:`9607`)
- ``regex`` argument to ``DataFrame.filter`` now handles numeric column names instead of raising ``ValueError`` (:issue:`10384`).
- ``pd.read_stata`` will now read Stata 118 type files. (:issue:`9882`)

- ``pd.merge`` will now allow duplicate column names if they are not merged upon (:issue:`10639`).

- ``pd.pivot`` will now allow passing index as ``None`` (:issue:`3962`).

- ``read_sql_table`` will now allow reading from views (:issue:`10750`).

- ``msgpack`` submodule has been updated to 0.4.6 with backward compatibility (:issue:`10581`)

- ``DataFrame.to_dict`` now accepts the *index* option in ``orient`` keyword argument (:issue:`10844`).

- ``drop_duplicates`` and ``duplicated`` now accept ``keep`` keyword to target first, last, and all duplicates. ``take_last`` keyword is deprecated, see :ref:`deprecations <whatsnew_0170.deprecations>` (:issue:`6511`, :issue:`8505`)

  .. ipython :: python

     s = pd.Series(['A', 'B', 'C', 'A', 'B', 'D'])
     s.drop_duplicates()
     s.drop_duplicates(keep='last')
     s.drop_duplicates(keep=False)

- Reindex now has a ``tolerance`` argument that allows for finer control of :ref:`basics.limits_on_reindex_fill`:

  .. ipython:: python

     df = pd.DataFrame({'x': range(5), 't': pd.date_range('2000-01-01', periods=5)})
     df.reindex([0.1, 1.9, 3.5], method='nearest', tolerance=0.2)

  When used on a ``DatetimeIndex``, ``TimedeltaIndex`` or ``PeriodIndex``, ``tolerance`` will coerced into a ``Timedelta`` if possible. This allows you to specify tolerance with a string:

  .. ipython:: python

     df = df.set_index('t')
     df.reindex(pd.to_datetime(['1999-12-31']), method='nearest', tolerance='1 day')

  ``tolerance`` is also exposed by the lower level ``Index.get_indexer`` and ``Index.get_loc`` methods.

- Support pickling of ``Period`` objects (:issue:`10439`)

- ``DataFrame.apply`` will return a Series of dicts if the passed function returns a dict and ``reduce=True`` (:issue:`8735`).

- ``concat`` will now use existing Series names if provided (:issue:`10698`).

  .. ipython:: python

     foo = pd.Series([1,2], name='foo')
     bar = pd.Series([1,2])
     baz = pd.Series([4,5])

  Previous Behavior:

  .. code-block:: python

     In [1] pd.concat([foo, bar, baz], 1)
     Out[1]:
           0  1  2
        0  1  1  4
        1  2  2  5

  New Behavior:

  .. ipython:: python

    pd.concat([foo, bar, baz], 1)

.. _whatsnew_0170.api:

.. _whatsnew_0170.api_breaking:

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

.. _whatsnew_0170.api_breaking.sorting:

Changes to sorting API
^^^^^^^^^^^^^^^^^^^^^^

The sorting API has had some longtime inconsistencies. (:issue:`9816`, :issue:`8239`).

Here is a summary of the API **PRIOR** to 0.17.0:

- ``Series.sort`` is **INPLACE** while ``DataFrame.sort`` returns a new object.
- ``Series.order`` returns a new object
- It was possible to use ``Series/DataFrame.sort_index`` to sort by **values** by passing the ``by`` keyword.
- ``Series/DataFrame.sortlevel`` worked only on a ``MultiIndex`` for sorting by index.

To address these issues, we have revamped the API:

- We have introduced a new method, :meth:`DataFrame.sort_values`, which is the merger of ``DataFrame.sort()``, ``Series.sort()``,
  and ``Series.order()``, to handle sorting of **values**.
- The existing methods ``Series.sort()``, ``Series.order()``, and ``DataFrame.sort()`` has been deprecated and will be removed in a
  future version of pandas.
- The ``by`` argument of ``DataFrame.sort_index()`` has been deprecated and will be removed in a future version of pandas.
- The existing method ``.sort_index()`` will gain the ``level`` keyword to enable level sorting.

We now have two distinct and non-overlapping methods of sorting. A ``*`` marks items that
will show a ``FutureWarning``.

To sort by the **values**:

==================================    ====================================
Previous                              Replacement
==================================    ====================================
\* ``Series.order()``                 ``Series.sort_values()``
\* ``Series.sort()``                  ``Series.sort_values(inplace=True)``
\* ``DataFrame.sort(columns=...)``    ``DataFrame.sort_values(by=...)``
==================================    ====================================

To sort by the **index**:

==================================    ====================================
Previous                              Replacement
==================================    ====================================
``Series.sort_index()``               ``Series.sort_index()``
``Series.sortlevel(level=...)``       ``Series.sort_index(level=...``)
``DataFrame.sort_index()``            ``DataFrame.sort_index()``
``DataFrame.sortlevel(level=...)``    ``DataFrame.sort_index(level=...)``
\* ``DataFrame.sort()``                 ``DataFrame.sort_index()``
==================================    ====================================

We have also deprecated and changed similar methods in two Series-like classes, ``Index`` and ``Categorical``.

==================================    ====================================
Previous                              Replacement
==================================    ====================================
\* ``Index.order()``                  ``Index.sort_values()``
\* ``Categorical.order()``            ``Categorical.sort_values``
==================================    ====================================

.. _whatsnew_0170.api_breaking.to_datetime:

Changes to to_datetime and to_timedelta
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The default for ``pd.to_datetime`` error handling has changed to ``errors='raise'``. In prior versions it was ``errors='ignore'``.
Furthermore, the ``coerce`` argument has been deprecated in favor of ``errors='coerce'``. This means that invalid parsing will raise rather that return the original
input as in previous versions. (:issue:`10636`)

Previous Behavior:

.. code-block:: python

   In [2]: pd.to_datetime(['2009-07-31', 'asd'])
   Out[2]: array(['2009-07-31', 'asd'], dtype=object)

New Behavior:

.. code-block:: python

   In [3]: pd.to_datetime(['2009-07-31', 'asd'])
   ValueError: Unknown string format

.. ipython:: python

Of course you can coerce this as well.

.. ipython:: python

   to_datetime(['2009-07-31', 'asd'], errors='coerce')

To keep the previous behaviour, you can use ``errors='ignore'``:

.. ipython:: python

   to_datetime(['2009-07-31', 'asd'], errors='ignore')

Furthermore, ``pd.to_timedelta`` has gained a similar API, of ``errors='raise'|'ignore'|'coerce'``, and the ``coerce`` keyword
has been deprecated in favor of ``errors='coerce'``.

.. _whatsnew_0170.api_breaking.convert_objects:

Changes to convert_objects
^^^^^^^^^^^^^^^^^^^^^^^^^^

``DataFrame.convert_objects`` keyword arguments have been shortened. (:issue:`10265`)

=====================   =============
Previous                Replacement
=====================   =============
``convert_dates``       ``datetime``
``convert_numeric``     ``numeric``
``convert_timedelta``   ``timedelta``
=====================   =============

Coercing types with ``DataFrame.convert_objects`` is now implemented using the
keyword argument ``coerce=True``.  Previously types were coerced by setting a
keyword argument to ``'coerce'`` instead of ``True``, as in ``convert_dates='coerce'``.

.. ipython:: python

   df = pd.DataFrame({'i': ['1','2'],
                      'f': ['apple', '4.2'],
                      's': ['apple','banana']})
   df

The old usage of ``DataFrame.convert_objects`` used ``'coerce'`` along with the
type.

.. code-block:: python

   In [2]: df.convert_objects(convert_numeric='coerce')

Now the ``coerce`` keyword must be explicitly used.

.. ipython:: python

   df.convert_objects(numeric=True, coerce=True)

In earlier versions of pandas, ``DataFrame.convert_objects`` would not coerce
numeric types when there were no values convertible to a numeric type. This returns
the original DataFrame with no conversion.

.. code-block:: python

   In [1]: df = pd.DataFrame({'s': ['a','b']})
   In [2]: df.convert_objects(convert_numeric='coerce')
   Out[2]:
          s
       0  a
       1  b

THe new behavior will convert all non-number-like strings to ``NaN``,
when ``coerce=True`` is passed explicity.

.. ipython:: python

   pd.DataFrame({'s': ['a','b']})
   df.convert_objects(numeric=True, coerce=True)

In earlier versions of pandas, the default behavior was to try and convert
datetimes and timestamps. The new default is for ``DataFrame.convert_objects``
to do nothing, and so it is necessary to pass at least one conversion target
in the method call.

Changes to Index Comparisons
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Operator equal on ``Index`` should behavior similarly to ``Series`` (:issue:`9947`, :issue:`10637`)

Starting in v0.17.0, comparing ``Index`` objects of different lengths will raise
a ``ValueError``. This is to be consistent with the behavior of ``Series``.

Previous behavior:

.. code-block:: python

   In [2]: pd.Index([1, 2, 3]) == pd.Index([1, 4, 5])
   Out[2]: array([ True, False, False], dtype=bool)

   In [3]: pd.Index([1, 2, 3]) == pd.Index([2])
   Out[3]: array([False,  True, False], dtype=bool)

   In [4]: pd.Index([1, 2, 3]) == pd.Index([1, 2])
   Out[4]: False

New behavior:

.. code-block:: python

   In [8]: pd.Index([1, 2, 3]) == pd.Index([1, 4, 5])
   Out[8]: array([ True, False, False], dtype=bool)

   In [9]: pd.Index([1, 2, 3]) == pd.Index([2])
   ValueError: Lengths must match to compare

   In [10]: pd.Index([1, 2, 3]) == pd.Index([1, 2])
   ValueError: Lengths must match to compare

Note that this is different from the ``numpy`` behavior where a comparison can
be broadcast:

.. ipython:: python

   np.array([1, 2, 3]) == np.array([1])

or it can return False if broadcasting can not be done:

.. ipython:: python

   np.array([1, 2, 3]) == np.array([1, 2])

Changes to Boolean Comparisons vs. None
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Boolean comparisons of a ``Series`` vs ``None`` will now be equivalent to comparing with ``np.nan``, rather than raise ``TypeError``. xref (:issue:`1079`).

.. ipython:: python

   s = Series(range(3))
   s.iloc[1] = None
   s

Previous behavior:

.. code-block:: python

   In [5]: s==None
   TypeError: Could not compare <type 'NoneType'> type with Series

New behavior:

.. ipython:: python

   s==None

Usually you simply want to know which values are null.

.. ipython:: python

   s.isnull()

.. warning::

   You generally will want to use ``isnull/notnull`` for these types of comparisons, as ``isnull/notnull`` tells you which elements are null. One has to be
   mindful that ``nan's`` don't compare equal, but ``None's`` do. Note that Pandas/numpy uses the fact that ``np.nan != np.nan``, and treats ``None`` like ``np.nan``.

   .. ipython:: python

      None == None
      np.nan == np.nan

.. _whatsnew_0170.api_breaking.hdf_dropna:

HDFStore dropna behavior
^^^^^^^^^^^^^^^^^^^^^^^^

The default behavior for HDFStore write functions with ``format='table'`` is now to keep rows that are all missing. Previously, the behavior was to drop rows that were all missing save the index. The previous behavior can be replicated using the ``dropna=True`` option. (:issue:`9382`)

Previously:

.. ipython:: python

   df_with_missing = pd.DataFrame({'col1':[0, np.nan, 2],
                                   'col2':[1, np.nan, np.nan]})

   df_with_missing


.. code-block:: python

   In [28]:
   df_with_missing.to_hdf('file.h5',
                          'df_with_missing',
                          format='table',
                          mode='w')

   pd.read_hdf('file.h5', 'df_with_missing')

   Out [28]:
         col1  col2
     0     0     1
     2     2   NaN


New behavior:

.. ipython:: python
   :suppress:

   import os

.. ipython:: python

   df_with_missing.to_hdf('file.h5',
                          'df_with_missing',
                           format='table',
                           mode='w')

   pd.read_hdf('file.h5', 'df_with_missing')

.. ipython:: python
   :suppress:

   os.remove('file.h5')

See :ref:`documentation <io.hdf5>` for more details.

.. _whatsnew_0170.api_breaking.display_precision:

Changes to ``display.precision`` option
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``display.precision`` option has been clarified to refer to decimal places (:issue:`10451`).

Earlier versions of pandas would format floating point numbers to have one less decimal place than the value in
``display.precision``.

.. code-block:: python

  In [1]: pd.set_option('display.precision', 2)

  In [2]: pd.DataFrame({'x': [123.456789]})
  Out[2]:
         x
  0  123.5

If interpreting precision as "significant figures" this did work for scientific notation but that same interpretation
did not work for values with standard formatting. It was also out of step with how numpy handles formatting.

Going forward the value of ``display.precision`` will directly control the number of places after the decimal, for
regular formatting as well as scientific notation, similar to how numpy's ``precision`` print option works.

.. ipython:: python

  pd.set_option('display.precision', 2)
  pd.DataFrame({'x': [123.456789]})

To preserve output behavior with prior versions the default value of ``display.precision`` has been reduced to ``6``
from ``7``.

.. ipython:: python
  :suppress:

  pd.set_option('display.precision', 6)

.. _whatsnew_0170.api_breaking.categorical_unique:

Changes to ``Categorical.unique``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``Categorical.unique`` now returns new ``Categoricals`` with ``categories`` and ``codes`` that are unique, rather than returning ``np.array`` (:issue:`10508`)

- unordered category: values and categories are sorted by appearance order.
- ordered category: values are sorted by appearance order, categories keep existing order.

.. ipython :: python

   cat = pd.Categorical(['C', 'A', 'B', 'C'],
                        categories=['A', 'B', 'C'],
                        ordered=True)
   cat
   cat.unique()

   cat = pd.Categorical(['C', 'A', 'B', 'C'],
                        categories=['A', 'B', 'C'])
   cat
   cat.unique()


.. _whatsnew_0170.api_breaking.other:

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

- Line and kde plot with ``subplots=True`` now uses default colors, not all black. Specify ``color='k'`` to draw all lines in black (:issue:`9894`)
- Calling the ``.value_counts`` method on a Series with ``categorical`` dtype now returns a Series with a ``CategoricalIndex`` (:issue:`10704`)
- Allow passing `kwargs` to the interpolation methods (:issue:`10378`).
- The metadata properties of subclasses of pandas objects will now be serialized (:issue:`10553`).
- Allow ``DataFrame`` with ``MultiIndex`` columns to be written to Excel (:issue:`10564`). This was changed in 0.16.2 as the read-back method could not always guarantee perfect fidelity (:issue:`9794`).
- ``groupby`` using ``Categorical`` follows the same rule as ``Categorical.unique`` described above  (:issue:`10508`)
- Improved error message when concatenating an empty iterable of dataframes (:issue:`9157`)
- When constructing ``DataFrame`` with an array of ``complex64`` dtype that meant the corresponding column was automatically promoted to the ``complex128`` dtype. Pandas will now preserve the itemsize of the input for complex data (:issue:`10952`)

- ``NaT``'s methods now either raise ``ValueError``, or return ``np.nan`` or ``NaT`` (:issue:`9513`)

  ===============================     ===============================================================
  Behavior                            Methods
  ===============================     ===============================================================
  return ``np.nan``                   ``weekday``, ``isoweekday``
  return ``NaT``                      ``date``, ``now``, ``replace``, ``to_datetime``, ``today``
  return ``np.datetime64('NaT')``     ``to_datetime64`` (unchanged)
  raise ``ValueError``                All other public methods (names not beginning with underscores)
  ===============================     ===============================================================

.. _whatsnew_0170.deprecations:

Deprecations
^^^^^^^^^^^^

.. note:: These indexing function have been deprecated in the documentation since 0.11.0.

- For ``Series`` the following indexing functions are deprecated (:issue:`10177`).

  =====================  =================================
  Deprecated Function    Replacement
  =====================  =================================
  ``.irow(i)``           ``.iloc[i]`` or ``.iat[i]``
  ``.iget(i)``           ``.iloc[i]``
  ``.iget_value(i)``     ``.iloc[i]`` or ``.iat[i]``
  =====================  =================================

- For ``DataFrame`` the following indexing functions are deprecated (:issue:`10177`).

  =====================  =================================
  Deprecated Function    Replacement
  =====================  =================================
  ``.irow(i)``           ``.iloc[i]``
  ``.iget_value(i, j)``  ``.iloc[i, j]`` or ``.iat[i, j]``
  ``.icol(j)``           ``.iloc[:, j]``
  =====================  =================================

- ``Categorical.name`` was deprecated to make ``Categorical`` more ``numpy.ndarray`` like. Use ``Series(cat, name="whatever")`` instead (:issue:`10482`).
- Setting missing values (NaN) in a ``Categorical``'s ``categories`` will issue a warning (:issue:`10748`). You can still have missing values in the ``values``.
- ``drop_duplicates`` and ``duplicated``'s ``take_last`` keyword was deprecated in favor of ``keep``. (:issue:`6511`, :issue:`8505`)
- ``Series.nsmallest`` and ``nlargest``'s ``take_last`` keyword was deprecated in favor of ``keep``. (:issue:`10792`)
- ``DataFrame.combineAdd`` and ``DataFrame.combineMult`` are deprecated. They
  can easily be replaced by using the ``add`` and ``mul`` methods:
  ``DataFrame.add(other, fill_value=0)`` and ``DataFrame.mul(other, fill_value=1.)``
  (:issue:`10735`).
- ``TimeSeries`` deprecated in favor of ``Series`` (note that this has been alias since 0.13.0), (:issue:`10890`)
- Legacy offsets (like ``'A@JAN'``) listed in :ref:`here <timeseries.legacyaliases>` are deprecated (note that this has been alias since 0.8.0), (:issue:`10878`)
- ``WidePanel`` deprecated in favor of ``Panel``, ``LongPanel`` in favor of ``DataFrame`` (note these have been aliases since < 0.11.0), (:issue:`10892`)

.. _whatsnew_0170.prior_deprecations:

Removal of prior version deprecations/changes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

- Remove use of some deprecated numpy comparison operations, mainly in tests. (:issue:`10569`)
- Removal of ``na_last`` parameters from ``Series.order()`` and ``Series.sort()``, in favor of ``na_position``, xref (:issue:`5231`)
- Remove of ``percentile_width`` from ``.describe()``, in favor of ``percentiles``. (:issue:`7088`)
- Removal of ``colSpace`` parameter from ``DataFrame.to_string()``, in favor of ``col_space``, circa 0.8.0 version.
- Removal of automatic time-series broadcasting (:issue:`2304`)

  .. ipython :: python

     np.random.seed(1234)
     df = DataFrame(np.random.randn(5,2),columns=list('AB'),index=date_range('20130101',periods=5))
     df

  Previously

  .. code-block:: python

     In [3]: df + df.A
     FutureWarning: TimeSeries broadcasting along DataFrame index by default is deprecated.
     Please use DataFrame.<op> to explicitly broadcast arithmetic operations along the index

     Out[3]:
                         A         B
     2013-01-01  0.942870 -0.719541
     2013-01-02  2.865414  1.120055
     2013-01-03 -1.441177  0.166574
     2013-01-04  1.719177  0.223065
     2013-01-05  0.031393 -2.226989

  Current

  .. ipython :: python

     df.add(df.A,axis='index')


- Remove ``table`` keyword in ``HDFStore.put/append``, in favor of using ``format=`` (:issue:`4645`)
- Remove ``kind`` in ``read_excel/ExcelFile`` as its unused (:issue:`4712`)
- Remove ``infer_type`` keyword from ``pd.read_html`` as its unused (:issue:`4770`, :issue:`7032`)
- Remove ``offset`` and ``timeRule`` keywords from ``Series.tshift/shift``, in favor of ``freq`` (:issue:`4853`, :issue:`4864`)
- Remove ``pd.load/pd.save`` aliases in favor of ``pd.to_pickle/pd.read_pickle`` (:issue:`3787`)

.. _whatsnew_0170.performance:

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

- Development support for benchmarking with the `Air Speed Velocity library <https://github.com/spacetelescope/asv/>`_ (:issue:`8316`)
- Added vbench benchmarks for alternative ExcelWriter engines and reading Excel files (:issue:`7171`)
- Performance improvements in ``Categorical.value_counts`` (:issue:`10804`)
- Performance improvements in ``SeriesGroupBy.nunique`` and ``SeriesGroupBy.value_counts`` (:issue:`10820`)
- Performance improvements in ``DataFrame.drop_duplicates`` with integer dtypes (:issue:`10917`)
- 4x improvement in ``timedelta`` string parsing (:issue:`6755`, :issue:`10426`)
- 8x improvement in ``timedelta64`` and ``datetime64`` ops (:issue:`6755`)
- Significantly improved performance of indexing ``MultiIndex`` with slicers (:issue:`10287`)
- 8x improvement in ``iloc`` using list-like input (:issue:`10791`)
- Improved performance of ``Series.isin`` for datetimelike/integer Series (:issue:`10287`)
- 20x improvement in ``concat`` of Categoricals when categories are identical (:issue:`10587`)
- Improved performance of ``to_datetime`` when specified format string is ISO8601 (:issue:`10178`)
- 2x improvement of ``Series.value_counts`` for float dtype (:issue:`10821`)

.. _whatsnew_0170.bug_fixes:

Bug Fixes
~~~~~~~~~

- Bug in incorrection computation of ``.mean()`` on ``timedelta64[ns]`` because of overflow (:issue:`9442`)
- Bug in ``DataFrame.to_html(index=False)`` renders unnecessary ``name`` row (:issue:`10344`)
- Bug in ``DataFrame.to_latex()`` the ``column_format`` argument could not be passed (:issue:`9402`)
- Bug in ``DataFrame.apply`` when function returns categorical series. (:issue:`9573`)
- Bug in ``to_datetime`` with invalid dates and formats supplied (:issue:`10154`)
- Bug in ``Index.drop_duplicates`` dropping name(s) (:issue:`10115`)
- Bug in ``Series.quantile`` dropping name (:issue:`10881`)
- Bug in ``pd.Series`` when setting a value on an empty ``Series`` whose index has a frequency. (:issue:`10193`)
- Bug in ``pd.Series.interpolate`` with invalid ``order`` keyword values. (:issue:`10633`)
- Bug in ``DataFrame.plot`` raises ``ValueError`` when color name is specified by multiple characters (:issue:`10387`)
- Bug in ``Index`` construction with a mixed list of tuples (:issue:`10697`)
- Bug in ``DataFrame.reset_index`` when index contains ``NaT``. (:issue:`10388`)
- Bug in ``ExcelReader`` when worksheet is empty (:issue:`6403`)
- Bug in ``BinGrouper.group_info`` where returned values are not compatible with base class (:issue:`10914`)
- Bug in clearing the cache on ``DataFrame.pop`` and a subsequent inplace op (:issue:`10912`)
- Bug in indexing with a mixed-integer ``Index`` causing an ``ImportError`` (:issue:`10610`)
- Bug in ``Series.count`` when index has nulls (:issue:`10946`)

- Bug causing ``DataFrame.where`` to not respect the ``axis`` parameter when the frame has a symmetric shape. (:issue:`9736`)

- Bug in ``Table.select_column`` where name is not preserved (:issue:`10392`)
- Bug in ``offsets.generate_range`` where ``start`` and ``end`` have finer precision than ``offset`` (:issue:`9907`)
- Bug in ``pd.rolling_*`` where ``Series.name`` would be lost in the output (:issue:`10565`)
- Bug in ``stack`` when index or columns are not unique. (:issue:`10417`)
- Bug in setting a ``Panel`` when an axis has a multi-index (:issue:`10360`)
- Bug in ``USFederalHolidayCalendar`` where ``USMemorialDay`` and ``USMartinLutherKingJr`` were incorrect (:issue:`10278` and :issue:`9760` )
- Bug in ``.sample()`` where returned object, if set, gives unnecessary ``SettingWithCopyWarning`` (:issue:`10738`)
- Bug in ``.sample()`` where weights passed as ``Series`` were not aligned along axis before being treated positionally, potentially causing problems if weight indices were not aligned with sampled object. (:issue:`10738`)

- Regression fixed in (:issue:`9311`, :issue: `6620`, :issue:`9345`), where groupby with a datetime-like converting to float with certain aggregators (:issue:`10979`)

- Bug in ``DataFrame.interpolate`` with ``axis=1`` and ``inplace=True`` (:issue:`10395`)
- Bug in ``io.sql.get_schema`` when specifying multiple columns as primary
  key (:issue:`10385`).

- Bug in ``groupby(sort=False)`` with datetime-like ``Categorical`` raises ``ValueError`` (:issue:`10505`)

- Bug in ``test_categorical`` on big-endian builds (:issue:`10425`)
- Bug in ``Series.shift`` and ``DataFrame.shift`` not supporting categorical data (:issue:`9416`)
- Bug in ``Series.map`` using categorical ``Series`` raises ``AttributeError`` (:issue:`10324`)
- Bug in ``MultiIndex.get_level_values`` including ``Categorical`` raises ``AttributeError`` (:issue:`10460`)
- Bug in ``pd.get_dummies`` with `sparse=True` not returning ``SparseDataFrame`` (:issue:`10531`)
- Bug in ``Index`` subtypes (such as ``PeriodIndex``) not returning their own type for ``.drop`` and ``.insert`` methods (:issue:`10620`)
- Bug in ``algos.outer_join_indexer`` when ``right`` array is empty (:issue:`10618`)

- Bug in ``filter`` (regression from 0.16.0) and ``transform`` when grouping on multiple keys, one of which is datetime-like (:issue:`10114`)


- Bug in ``to_datetime`` and ``to_timedelta`` causing ``Index`` name to be lost (:issue:`10875`)


- Bug that caused segfault when resampling an empty Series (:issue:`10228`)
- Bug in ``DatetimeIndex`` and ``PeriodIndex.value_counts`` resets name from its result, but retains in result's ``Index``. (:issue:`10150`)
- Bug in ``pd.eval`` using ``numexpr`` engine coerces 1 element numpy array to scalar (:issue:`10546`)
- Bug in ``pd.concat`` with ``axis=0`` when column is of dtype ``category`` (:issue:`10177`)
- Bug in ``read_msgpack`` where input type is not always checked (:issue:`10369`, :issue:`10630`)
- Bug in ``pd.read_csv`` with kwargs ``index_col=False``, ``index_col=['a', 'b']`` or ``dtype``
  (:issue:`10413`, :issue:`10467`, :issue:`10577`)
- Bug in ``Series.from_csv`` with ``header`` kwarg not setting the ``Series.name`` or the ``Series.index.name`` (:issue:`10483`)
- Bug in ``groupby.var`` which caused variance to be inaccurate for small float values (:issue:`10448`)
- Bug in ``Series.plot(kind='hist')`` Y Label not informative (:issue:`10485`)
- Bug in ``read_csv`` when using a converter which generates a ``uint8`` type (:issue:`9266`)

- Bug causes memory leak in time-series line and area plot (:issue:`9003`)


- Bug in line and kde plot cannot accept multiple colors when ``subplots=True`` (:issue:`9894`)
- Bug in ``DataFrame.plot`` raises ``ValueError`` when color name is specified by multiple characters (:issue:`10387`)

- Bug in left and right ``align`` of ``Series`` with ``MultiIndex`` may be inverted (:issue:`10665`)
- Bug in left and right ``join`` of with ``MultiIndex`` may be inverted (:issue:`10741`)

- Bug in ``read_stata`` when reading a file with a different order set in ``columns`` (:issue:`10757`)
- Bug in ``Categorical`` may not representing properly when category contains ``tz`` or ``Period`` (:issue:`10713`)
- Bug in ``Categorical.__iter__`` may not returning correct ``datetime`` and ``Period`` (:issue:`10713`)

- Bug in ``read_csv`` with ``engine='c'``: EOF preceded by a comment, blank line, etc. was not handled correctly (:issue:`10728`, :issue:`10548`)

- Reading "famafrench" data via ``DataReader`` results in HTTP 404 error because of the website url is changed (:issue:`10591`).
- Bug in ``read_msgpack`` where DataFrame to decode has duplicate column names (:issue:`9618`)
- Bug in ``io.common.get_filepath_or_buffer`` which caused reading of valid S3 files to fail if the bucket also contained keys for which the user does not have read permission (:issue:`10604`)
- Bug in vectorised setting of timestamp columns with python ``datetime.date`` and numpy ``datetime64`` (:issue:`10408`, :issue:`10412`)
- Bug in ``Index.take`` may add unnecessary ``freq`` attribute (:issue:`10791`)
- Bug in ``merge`` with empty ``DataFrame`` may raise ``IndexError`` (:issue:`10824`)


- Bug in ``read_csv`` when using the ``nrows`` or ``chunksize`` parameters if file contains only a header line (:issue:`9535`)
- Bug in serialization of ``category`` types in HDF5 in presence of alternate encodings. (:issue:`10366`)
- Bug in ``pd.DataFrame`` when constructing an empty DataFrame with a string dtype (:issue:`9428`)
- Bug in ``pd.DataFrame.diff`` when DataFrame is not consolidated (:issue:`10907`)
- Bug in ``pd.unique`` for arrays with the ``datetime64`` or ``timedelta64`` dtype that meant an array with object dtype was returned instead the original dtype (:issue:`9431`)
- Bug in ``Timedelta`` raising error when slicing from 0s (:issue:`10583`)
- Bug in ``DatetimeIndex.take`` and ``TimedeltaIndex.take`` may not raise ``IndexError`` against invalid index (:issue:`10295`)
- Bug in ``Series([np.nan]).astype('M8[ms]')``, which now returns ``Series([pd.NaT])`` (:issue:`10747`)
- Bug in ``PeriodIndex.order`` reset freq (:issue:`10295`)
- Bug in ``date_range`` when ``freq`` divides ``end`` as nanos (:issue:`10885`)
- Bug in ``iloc`` allowing memory outside bounds of a Series to be accessed with negative integers (:issue:`10779`)
- Bug in ``read_msgpack`` where encoding is not respected (:issue:`10580`)
- Bug preventing access to the first index when using ``iloc`` with a list containing the appropriate negative integer (:issue:`10547`, :issue:`10779`)
- Bug in ``TimedeltaIndex`` formatter causing error while trying to save ``DataFrame`` with ``TimedeltaIndex`` using ``to_csv`` (:issue:`10833`)
- Bug in ``DataFrame.where`` when handling Series slicing (:issue:`10218`, :issue:`9558`)
- Bug where ``pd.read_gbq`` throws ``ValueError`` when Bigquery returns zero rows (:issue:`10273`)
- Bug in ``to_json`` which was causing segmentation fault when serializing 0-rank ndarray (:issue:`9576`)
- Bug in plotting functions may raise ``IndexError`` when plotted on ``GridSpec`` (:issue:`10819`)
- Bug in plot result may show unnecessary minor ticklabels (:issue:`10657`)
- Bug in ``groupby`` incorrect computation for aggregation on ``DataFrame`` with ``NaT`` (E.g ``first``, ``last``, ``min``). (:issue:`10590`)