v0.19.0.txt

.. _whatsnew_0190:

v0.19.0 (August ??, 2016)
-------------------------

This is a major release from 0.18.1 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.

Highlights include:

- :func:`merge_asof` for asof-style time-series joining, see :ref:`here <whatsnew_0190.enhancements.asof_merge>`
- ``.rolling()`` are now time-series aware, see :ref:`here <whatsnew_0190.enhancements.rolling_ts>`
- pandas development api, see :ref:`here <whatsnew_0190.dev_api>`

.. contents:: What's new in v0.19.0
    :local:
    :backlinks: none

.. _whatsnew_0190.new_features:

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

.. _whatsnew_0190.dev_api:

pandas development API
^^^^^^^^^^^^^^^^^^^^^^

As part of making pandas APi more uniform and accessible in the future, we have created a standard
sub-package of pandas, ``pandas.api`` to hold public API's. We are starting by exposing type
introspection functions in ``pandas.api.types``. More sub-packages and officially sanctioned API's
will be published in future versions of pandas.

The following are now part of this API:

.. ipython:: python

   import pprint
   from pandas.api import types
   funcs = [ f for f in dir(types) if not f.startswith('_') ]
   pprint.pprint(funcs)

.. _whatsnew_0190.enhancements.asof_merge:

:func:`merge_asof` for asof-style time-series joining
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A long-time requested feature has been added through the :func:`merge_asof` function, to
support asof style joining of time-series. (:issue:`1870`, :issue:`13695`, :issue:`13709`). Full documentation is
:ref:`here <merging.merge_asof>`

The :func:`merge_asof` performs an asof merge, which is similar to a left-join
except that we match on nearest key rather than equal keys.

.. ipython:: python

   left = pd.DataFrame({'a': [1, 5, 10],
                        'left_val': ['a', 'b', 'c']})
   right = pd.DataFrame({'a': [1, 2, 3, 6, 7],
                        'right_val': [1, 2, 3, 6, 7]})

   left
   right

We typically want to match exactly when possible, and use the most
recent value otherwise.

.. ipython:: python

   pd.merge_asof(left, right, on='a')

We can also match rows ONLY with prior data, and not an exact match.

.. ipython:: python

   pd.merge_asof(left, right, on='a', allow_exact_matches=False)


In a typical time-series example, we have ``trades`` and ``quotes`` and we want to ``asof-join`` them.
This also illustrates using the ``by`` parameter to group data before merging.

.. ipython:: python

   trades = pd.DataFrame({
       'time': pd.to_datetime(['20160525 13:30:00.023',
                               '20160525 13:30:00.038',
                               '20160525 13:30:00.048',
                               '20160525 13:30:00.048',
                               '20160525 13:30:00.048']),
       'ticker': ['MSFT', 'MSFT',
                  'GOOG', 'GOOG', 'AAPL'],
       'price': [51.95, 51.95,
                 720.77, 720.92, 98.00],
       'quantity': [75, 155,
                    100, 100, 100]},
       columns=['time', 'ticker', 'price', 'quantity'])

   quotes = pd.DataFrame({
       'time': pd.to_datetime(['20160525 13:30:00.023',
                               '20160525 13:30:00.023',
                               '20160525 13:30:00.030',
                               '20160525 13:30:00.041',
                               '20160525 13:30:00.048',
                               '20160525 13:30:00.049',
                               '20160525 13:30:00.072',
                               '20160525 13:30:00.075']),
       'ticker': ['GOOG', 'MSFT', 'MSFT',
                  'MSFT', 'GOOG', 'AAPL', 'GOOG',
                  'MSFT'],
       'bid': [720.50, 51.95, 51.97, 51.99,
               720.50, 97.99, 720.50, 52.01],
       'ask': [720.93, 51.96, 51.98, 52.00,
               720.93, 98.01, 720.88, 52.03]},
       columns=['time', 'ticker', 'bid', 'ask'])

.. ipython:: python

   trades
   quotes

An asof merge joins on the ``on``, typically a datetimelike field, which is ordered, and
in this case we are using a grouper in the ``by`` field. This is like a left-outer join, except
that forward filling happens automatically taking the most recent non-NaN value.

.. ipython:: python

   pd.merge_asof(trades, quotes,
                 on='time',
                 by='ticker')

This returns a merged DataFrame with the entries in the same order as the original left
passed DataFrame (``trades`` in this case), with the fields of the ``quotes`` merged.

.. _whatsnew_0190.enhancements.rolling_ts:

``.rolling()`` are now time-series aware
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``.rolling()`` objects are now time-series aware and can accept a time-series offset (or convertible) for the ``window`` argument (:issue:`13327`, :issue:`12995`)
See the full documentation :ref:`here <stats.moments.ts>`.

.. ipython:: python

   dft = pd.DataFrame({'B': [0, 1, 2, np.nan, 4]},
                      index=pd.date_range('20130101 09:00:00', periods=5, freq='s'))
   dft

This is a regular frequency index. Using an integer window parameter works to roll along the window frequency.

.. ipython:: python

   dft.rolling(2).sum()
   dft.rolling(2, min_periods=1).sum()

Specifying an offset allows a more intuitive specification of the rolling frequency.

.. ipython:: python

   dft.rolling('2s').sum()

Using a non-regular, but still monotonic index, rolling with an integer window does not impart any special calculation.

.. ipython:: python


   dft = DataFrame({'B': [0, 1, 2, np.nan, 4]},
                   index = pd.Index([pd.Timestamp('20130101 09:00:00'),
                                     pd.Timestamp('20130101 09:00:02'),
                                     pd.Timestamp('20130101 09:00:03'),
                                     pd.Timestamp('20130101 09:00:05'),
                                     pd.Timestamp('20130101 09:00:06')],
                                    name='foo'))

   dft
   dft.rolling(2).sum()

Using the time-specification generates variable windows for this sparse data.

.. ipython:: python

   dft.rolling('2s').sum()

Furthermore, we now allow an optional ``on`` parameter to specify a column (rather than the
default of the index) in a DataFrame.

.. ipython:: python

   dft = dft.reset_index()
   dft
   dft.rolling('2s', on='foo').sum()

.. _whatsnew_0190.enhancements.read_csv_dupe_col_names_support:

:func:`read_csv` has improved support for duplicate column names
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

:ref:`Duplicate column names <io.dupe_names>` are now supported in :func:`read_csv` whether
they are in the file or passed in as the ``names`` parameter (:issue:`7160`, :issue:`9424`)

.. ipython :: python

   data = '0,1,2\n3,4,5'
   names = ['a', 'b', 'a']

Previous behaviour:

.. code-block:: ipython

   In [2]: pd.read_csv(StringIO(data), names=names)
   Out[2]:
      a  b  a
   0  2  1  2
   1  5  4  5

The first 'a' column contains the same data as the second 'a' column, when it should have
contained the array ``[0, 3]``.

New behaviour:

.. ipython :: python

   In [2]: pd.read_csv(StringIO(data), names=names)

.. _whatsnew_0190.enhancements.semi_month_offsets:

Semi-Month Offsets
^^^^^^^^^^^^^^^^^^

Pandas has gained new frequency offsets, ``SemiMonthEnd`` ('SM') and ``SemiMonthBegin`` ('SMS').
These provide date offsets anchored (by default) to the 15th and end of month, and 15th and 1st of month respectively.
(:issue:`1543`)

.. ipython:: python

    from pandas.tseries.offsets import SemiMonthEnd, SemiMonthBegin

SemiMonthEnd:

.. ipython:: python

    Timestamp('2016-01-01') + SemiMonthEnd()

    pd.date_range('2015-01-01', freq='SM', periods=4)

SemiMonthBegin:

.. ipython:: python

    Timestamp('2016-01-01') + SemiMonthBegin()

    pd.date_range('2015-01-01', freq='SMS', periods=4)

Using the anchoring suffix, you can also specify the day of month to use instead of the 15th.

.. ipython:: python

    pd.date_range('2015-01-01', freq='SMS-16', periods=4)

    pd.date_range('2015-01-01', freq='SM-14', periods=4)

.. _whatsnew_0190.enhancements.index:

New Index methods
^^^^^^^^^^^^^^^^^

Following methods and options are added to ``Index`` to be more consistent with ``Series`` and ``DataFrame``.

``Index`` now supports the ``.where()`` function for same shape indexing (:issue:`13170`)

.. ipython:: python

   idx = pd.Index(['a', 'b', 'c'])
   idx.where([True, False, True])


``Index`` now supports ``.dropna`` to exclude missing values (:issue:`6194`)

.. ipython:: python

   idx = pd.Index([1, 2, np.nan, 4])
   idx.dropna()

For ``MultiIndex``, values are dropped if any level is missing by default. Specifying
``how='all'`` only drops values where all levels are missing.

.. ipython:: python

   midx = pd.MultiIndex.from_arrays([[1, 2, np.nan, 4],
                                       [1, 2, np.nan, np.nan]])
   midx
   midx.dropna()
   midx.dropna(how='all')

``Index`` now supports ``.str.extractall()`` which returns a ``DataFrame``, the see :ref:`docs here <text.extractall>` (:issue:`10008`, :issue:`13156`)

.. ipython:: python

   idx = pd.Index(["a1a2", "b1", "c1"])
   idx.str.extractall("[ab](?P<digit>\d)")

``Index.astype()`` now accepts an optional boolean argument ``copy``, which allows optional copying if the requirements on dtype are satisfied (:issue:`13209`)

.. _whatsnew_0170.gbq:

Google BigQuery Enhancements
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
- The :func:`pandas.io.gbq.read_gbq` method has gained the ``dialect`` argument to allow users to specify whether to use BigQuery's legacy SQL or BigQuery's standard SQL. See the :ref:`docs <io.bigquery_reader>` for more details (:issue:`13615`).

.. _whatsnew_0190.enhancements.other:

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

- The ``.tz_localize()`` method of ``DatetimeIndex`` and ``Timestamp`` has gained the ``errors`` keyword, so you can potentially coerce nonexistent timestamps to ``NaT``. The default behaviour remains to raising a ``NonExistentTimeError`` (:issue:`13057`)
- ``pd.to_numeric()`` now accepts a ``downcast`` parameter, which will downcast the data if possible to smallest specified numerical dtype (:issue:`13352`)

  .. ipython:: python

     s = ['1', 2, 3]
     pd.to_numeric(s, downcast='unsigned')
     pd.to_numeric(s, downcast='integer')

- ``.to_hdf/read_hdf()`` now accept path objects (e.g. ``pathlib.Path``, ``py.path.local``) for the file path (:issue:`11773`)

- ``Timestamp`` can now accept positional and keyword parameters similar to :func:`datetime.datetime` (:issue:`10758`, :issue:`11630`)

  .. ipython:: python

    pd.Timestamp(2012, 1, 1)

    pd.Timestamp(year=2012, month=1, day=1, hour=8, minute=30)

- The ``pd.read_csv()`` with ``engine='python'`` has gained support for the ``decimal`` option (:issue:`12933`)
- The ``pd.read_csv()`` with ``engine='python'`` has gained support for the ``na_filter`` option (:issue:`13321`)
- The ``pd.read_csv()`` with ``engine='python'`` has gained support for the ``memory_map`` option (:issue:`13381`)

- The ``pd.read_html()`` has gained support for the ``na_values``, ``converters``, ``keep_default_na``  options (:issue:`13461`)

- ``Categorical.astype()`` now accepts an optional boolean argument ``copy``, effective when dtype is categorical (:issue:`13209`)
- ``DataFrame`` has gained the ``.asof()`` method to return the last non-NaN values according to the selected subset (:issue:`13358`)
- Consistent with the Python API, ``pd.read_csv()`` will now interpret ``+inf`` as positive infinity (:issue:`13274`)
- The ``DataFrame`` constructor will now respect key ordering if a list of ``OrderedDict`` objects are passed in (:issue:`13304`)
- ``pd.read_html()`` has gained support for the ``decimal`` option (:issue:`12907`)
- A function :func:`union_categorical` has been added for combining categoricals, see :ref:`Unioning Categoricals<categorical.union>` (:issue:`13361`)
- ``Series`` has gained the properties ``.is_monotonic``, ``.is_monotonic_increasing``, ``.is_monotonic_decreasing``, similar to ``Index`` (:issue:`13336`)
- ``DataFrame.to_sql()`` now allows a single value as the SQL type for all columns (:issue:`11886`).
- ``Series.append`` now supports the ``ignore_index`` option (:issue:`13677`)
- ``.to_stata()`` and ``StataWriter`` can now write variable labels to Stata dta files using a dictionary to make column names to labels (:issue:`13535`, :issue:`13536`)
- ``.to_stata()`` and ``StataWriter`` will automatically convert ``datetime64[ns]`` columns to Stata format ``%tc``, rather than raising a ``ValueError`` (:issue:`12259`)
- ``DataFrame`` has gained support to re-order the columns based on the values
  in a row using ``df.sort_values(by='...', axis=1)`` (:issue:`10806`)

  .. ipython:: python

     df = pd.DataFrame({'A': [2, 7], 'B': [3, 5], 'C': [4, 8]},
                       index=['row1', 'row2'])
     df.sort_values(by='row2', axis=1)

- Added documentation to :ref:`I/O<io.dtypes>` regarding the perils of reading in columns with mixed dtypes and how to handle it (:issue:`13746`)

.. _whatsnew_0190.api:


API changes
~~~~~~~~~~~


- ``Panel.to_sparse`` will raise a ``NotImplementedError`` exception when called (:issue:`13778`)
- ``Index.reshape`` will raise a ``NotImplementedError`` exception when called (:issue:`12882`)
- Non-convertible dates in an excel date column will be returned without conversion and the column will be ``object`` dtype, rather than raising an exception  (:issue:`10001`)
- ``eval``'s upcasting rules for ``float32`` types have been updated to be more consistent with NumPy's rules.  New behavior will not upcast to ``float64`` if you multiply a pandas ``float32`` object by a scalar float64. (:issue:`12388`)
- An ``UnsupportedFunctionCall`` error is now raised if NumPy ufuncs like ``np.mean`` are called on groupby or resample objects (:issue:`12811`)
- Calls to ``.sample()`` will respect the random seed set via ``numpy.random.seed(n)`` (:issue:`13161`)
- ``Styler.apply`` is now more strict about the outputs your function must return. For ``axis=0`` or ``axis=1``, the output shape must be identical. For ``axis=None``, the output must be a DataFrame with identical columns and index labels. (:issue:`13222`)
- ``Float64Index.astype(int)`` will now raise ``ValueError`` if ``Float64Index`` contains ``NaN`` values (:issue:`13149`)
- ``TimedeltaIndex.astype(int)`` and ``DatetimeIndex.astype(int)`` will now return ``Int64Index`` instead of ``np.array`` (:issue:`13209`)
- ``.filter()`` enforces mutual exclusion of the keyword arguments. (:issue:`12399`)
- ``PeridIndex`` can now accept ``list`` and ``array`` which contains ``pd.NaT`` (:issue:`13430`)
- ``__setitem__`` will no longer apply a callable rhs as a function instead of storing it. Call ``where`` directly to get the previous behavior. (:issue:`13299`)
- Passing ``Period`` with multiple frequencies to normal ``Index`` now returns ``Index`` with ``object`` dtype (:issue:`13664`)
- ``PeriodIndex.fillna`` with ``Period`` has different freq now coerces to ``object`` dtype (:issue:`13664`)
- More informative exceptions are passed through the csv parser. The exception type would now be the original exception type instead of ``CParserError``. (:issue:`13652`)
- ``astype()`` will now accept a dict of column name to data types mapping as the ``dtype`` argument. (:issue:`12086`)
- The ``pd.read_json`` and ``DataFrame.to_json`` has gained support for reading and writing json lines with ``lines`` option see :ref:`Line delimited json <io.jsonl>` (:issue:`9180`)
- ``pd.Timedelta(None)`` is now accepted and will return ``NaT``, mirroring ``pd.Timestamp`` (:issue:`13687`)
- ``Timestamp``, ``Period``, ``DatetimeIndex``, ``PeriodIndex`` and ``.dt`` accessor have gained a ``.is_leap_year`` property to check whether the date belongs to a leap year. (:issue:`13727`)
- ``pd.read_hdf`` will now raise a ``ValueError`` instead of ``KeyError``, if a mode other than ``r``, ``r+`` and ``a`` is supplied. (:issue:`13623`)
- Subclassed ``SparseDataFrame`` and ``SparseSeries`` now preserve class types when slicing or transposing. (:issue:`13787`)



.. _whatsnew_0190.api.tolist:

``Series.tolist()`` will now return Python types
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``Series.tolist()`` will now return Python types in the output, mimicking NumPy ``.tolist()`` behaviour (:issue:`10904`)


.. ipython:: python

   s = pd.Series([1,2,3])
   type(s.tolist()[0])

Previous Behavior:

.. code-block:: ipython

   In [7]: type(s.tolist()[0])
   Out[7]:
    <class 'numpy.int64'>

New Behavior:

.. ipython:: python

   type(s.tolist()[0])

.. _whatsnew_0190.api.promote:

``Series`` type promotion on assignment
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A ``Series`` will now correctly promote its dtype for assignment with incompat values to the current dtype (:issue:`13234`)


.. ipython:: python

   s = pd.Series()

Previous Behavior:

.. code-block:: ipython

   In [2]: s["a"] = pd.Timestamp("2016-01-01")

   In [3]: s["b"] = 3.0
   TypeError: invalid type promotion

New Behavior:

.. ipython:: python

   s["a"] = pd.Timestamp("2016-01-01")
   s["b"] = 3.0
   s
   s.dtype

.. _whatsnew_0190.api.to_datetime_coerce:

``.to_datetime()`` when coercing
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A bug is fixed in ``.to_datetime()`` when passing integers or floats, and no ``unit`` and ``errors='coerce'`` (:issue:`13180`).
Previously if ``.to_datetime()`` encountered mixed integers/floats and strings, but no datetimes with ``errors='coerce'`` it would convert all to ``NaT``.

Previous Behavior:

.. code-block:: ipython

   In [2]: pd.to_datetime([1, 'foo'], errors='coerce')
   Out[2]: DatetimeIndex(['NaT', 'NaT'], dtype='datetime64[ns]', freq=None)

This will now convert integers/floats with the default unit of ``ns``.

.. ipython:: python

   pd.to_datetime([1, 'foo'], errors='coerce')

.. _whatsnew_0190.api.merging:

Merging changes
^^^^^^^^^^^^^^^

Merging will now preserve the dtype of the join keys (:issue:`8596`)

.. ipython:: python

   df1 = pd.DataFrame({'key': [1], 'v1': [10]})
   df1
   df2 = pd.DataFrame({'key': [1, 2], 'v1': [20, 30]})
   df2

Previous Behavior:

.. code-block:: ipython

   In [5]: pd.merge(df1, df2, how='outer')
   Out[5]:
      key    v1
   0  1.0  10.0
   1  1.0  20.0
   2  2.0  30.0

   In [6]: pd.merge(df1, df2, how='outer').dtypes
   Out[6]:
   key    float64
   v1     float64
   dtype: object

New Behavior:

We are able to preserve the join keys

.. ipython:: python

   pd.merge(df1, df2, how='outer')
   pd.merge(df1, df2, how='outer').dtypes

Of course if you have missing values that are introduced, then the
resulting dtype will be upcast, which is unchanged from previous.

.. ipython:: python

   pd.merge(df1, df2, how='outer', on='key')
   pd.merge(df1, df2, how='outer', on='key').dtypes

.. _whatsnew_0190.api.describe:

``.describe()`` changes
^^^^^^^^^^^^^^^^^^^^^^^

Percentile identifiers in the index of a ``.describe()`` output will now be rounded to the least precision that keeps them distinct (:issue:`13104`)

.. ipython:: python

   s = pd.Series([0, 1, 2, 3, 4])
   df = pd.DataFrame([0, 1, 2, 3, 4])

Previous Behavior:

The percentiles were rounded to at most one decimal place, which could raise ``ValueError`` for a data frame if the percentiles were duplicated.

.. code-block:: ipython

   In [3]: s.describe(percentiles=[0.0001, 0.0005, 0.001, 0.999, 0.9995, 0.9999])
   Out[3]:
   count     5.000000
   mean      2.000000
   std       1.581139
   min       0.000000
   0.0%      0.000400
   0.1%      0.002000
   0.1%      0.004000
   50%       2.000000
   99.9%     3.996000
   100.0%    3.998000
   100.0%    3.999600
   max       4.000000
   dtype: float64

   In [4]: df.describe(percentiles=[0.0001, 0.0005, 0.001, 0.999, 0.9995, 0.9999])
   Out[4]:
   ...
   ValueError: cannot reindex from a duplicate axis

New Behavior:

.. ipython:: python

   s.describe(percentiles=[0.0001, 0.0005, 0.001, 0.999, 0.9995, 0.9999])
   df.describe(percentiles=[0.0001, 0.0005, 0.001, 0.999, 0.9995, 0.9999])

Furthermore:

- Passing duplicated ``percentiles`` will now raise a ``ValueError``.
- Bug in ``.describe()`` on a DataFrame with a mixed-dtype column index, which would previously raise a ``TypeError`` (:issue:`13288`)

.. _whatsnew_0190.api.periodnat:

``Period('NaT')`` now returns ``pd.NaT``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Previously, ``Period`` has its own ``Period('NaT')`` representation different from ``pd.NaT``. Now ``Period('NaT')`` has been changed to return ``pd.NaT``. (:issue:`12759`, :issue:`13582`)

Previous Behavior:

.. code-block:: ipython

   In [5]: pd.Period('NaT', freq='D')
   Out[5]: Period('NaT', 'D')

New Behavior:

These result in ``pd.NaT`` without providing ``freq`` option.

.. ipython:: python

   pd.Period('NaT')
   pd.Period(None)


To be compat with ``Period`` addition and subtraction, ``pd.NaT`` now supports addition and subtraction with ``int``. Previously it raises ``ValueError``.

Previous Behavior:

.. code-block:: ipython

   In [5]: pd.NaT + 1
   ...
   ValueError: Cannot add integral value to Timestamp without freq.

New Behavior:

.. ipython:: python

   pd.NaT + 1
   pd.NaT - 1


.. _whatsnew_0190.api.difference:

``Index.difference`` and ``.symmetric_difference`` changes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``Index.difference`` and ``Index.symmetric_difference`` will now, more consistently, treat ``NaN`` values as any other values. (:issue:`13514`)

.. ipython:: python

   idx1 = pd.Index([1, 2, 3, np.nan])
   idx2 = pd.Index([0, 1, np.nan])

Previous Behavior:

.. code-block:: ipython

   In [3]: idx1.difference(idx2)
   Out[3]: Float64Index([nan, 2.0, 3.0], dtype='float64')

   In [4]: idx1.symmetric_difference(idx2)
   Out[4]: Float64Index([0.0, nan, 2.0, 3.0], dtype='float64')

New Behavior:

.. ipython:: python

   idx1.difference(idx2)
   idx1.symmetric_difference(idx2)

.. _whatsnew_0190.api.autogenerated_chunksize_index:

:func:`read_csv` called with ``chunksize`` will progressively enumerate chunks
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When :func:`read_csv` is called with ``chunksize='n'`` and without specifying an index,
each chunk used to have an independently generated index from `0`` to ``n-1``.
They are now given instead a progressive index, starting from ``0`` for the first chunk,
from ``n`` for the second, and so on, so that, when concatenated, they are identical to
the result of calling :func:`read_csv` without the ``chunksize=`` argument.
(:issue:`12185`)

.. ipython :: python

   data = 'A,B\n0,1\n2,3\n4,5\n6,7'

Previous behaviour:

.. code-block:: ipython

   In [2]: pd.concat(pd.read_csv(StringIO(data), chunksize=2))
   Out[2]:
      A  B
   0  0  1
   1  2  3
   0  4  5
   1  6  7

New behaviour:

.. ipython :: python

   pd.concat(pd.read_csv(StringIO(data), chunksize=2))

.. _whatsnew_0190.deprecations:

Deprecations
^^^^^^^^^^^^
- ``Categorical.reshape`` has been deprecated and will be removed in a subsequent release (:issue:`12882`)
- ``Series.reshape`` has been deprecated and will be removed in a subsequent release (:issue:`12882`)

- ``DataFrame.to_html()`` and ``DataFrame.to_latex()`` have dropped the ``colSpace`` parameter in favor of ``col_space`` (:issue:`13857`)
- ``DataFrame.to_sql()`` has deprecated the ``flavor`` parameter, as it is superfluous when SQLAlchemy is not installed (:issue:`13611`)
- ``compact_ints`` and ``use_unsigned`` have been deprecated in ``pd.read_csv()`` and will be removed in a future version (:issue:`13320`)
- ``buffer_lines`` has been deprecated in ``pd.read_csv()`` and will be removed in a future version (:issue:`13360`)
- ``as_recarray`` has been deprecated in ``pd.read_csv()`` and will be removed in a future version (:issue:`13373`)
- ``skip_footer`` has been deprecated in ``pd.read_csv()`` in favor of ``skipfooter`` and will be removed in a future version (:issue:`13349`)
- top-level ``pd.ordered_merge()`` has been renamed to ``pd.merge_ordered()`` and the original name will be removed in a future version (:issue:`13358`)
- ``Timestamp.offset`` property (and named arg in the constructor), has been deprecated in favor of ``freq`` (:issue:`12160`)
- ``pd.tseries.util.pivot_annual`` is deprecated. Use ``pivot_table`` as alternative, an example is :ref:`here <cookbook.pivot>` (:issue:`736`)
- ``pd.tseries.util.isleapyear`` has been deprecated and will be removed in a subsequent release. Datetime-likes now have a ``.is_leap_year`` property. (:issue:`13727`)
- ``Panel4D`` and ``PanelND`` constructors are deprecated and will be removed in a future version. The recommended way to represent these types of n-dimensional data are with the `xarray package <http://xarray.pydata.org/en/stable/>`__. Pandas provides a :meth:`~Panel4D.to_xarray` method to automate this conversion. (:issue:`13564`)

.. _whatsnew_0190.prior_deprecations:

Removal of prior version deprecations/changes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
- The ``SparsePanel`` class has been removed (:issue:`13778`)
- The ``pd.sandbox`` module has been removed in favor of the external library ``pandas-qt`` (:issue:`13670`)
- The ``pandas.io.data`` and ``pandas.io.wb`` modules are removed in favor of
  the `pandas-datareader package <https://github.com/pydata/pandas-datareader>`__ (:issue:`13724`).
- The ``pandas.tools.rplot`` module has been removed in favor of
  the `seaborn package <https://github.com/mwaskom/seaborn>`__ (:issue:`13855`)
- ``DataFrame.to_csv()`` has dropped the ``engine`` parameter, as was deprecated in 0.17.1 (:issue:`11274`, :issue:`13419`)
- ``DataFrame.to_dict()`` has dropped the ``outtype`` parameter in favor of ``orient`` (:issue:`13627`, :issue:`8486`)
- ``pd.Categorical`` has dropped setting of the ``ordered`` attribute directly in favor of the ``set_ordered`` method (:issue:`13671`)
- ``pd.Categorical`` has dropped the ``levels`` attribute in favour of ``categories`` (:issue:`8376`)
- ``DataFrame.to_sql()`` has dropped the ``mysql`` option for the ``flavor`` parameter (:issue:`13611`)
- ``pd.Index`` has dropped the ``diff`` method in favour of ``difference`` (:issue:`13669`)

- ``Series.to_csv`` has dropped the ``nanRep`` parameter in favor of ``na_rep`` (:issue:`13804`)
- ``Series.xs``, ``DataFrame.xs``, ``Panel.xs``, ``Panel.major_xs``, and ``Panel.minor_xs`` have dropped the ``copy`` parameter (:issue:`13781`)
- ``str.split`` has dropped the ``return_type`` parameter in favor of ``expand`` (:issue:`13701`)
- Removal of the legacy time rules (offset aliases), deprecated since 0.17.0 (this has been alias since 0.8.0) (:issue:`13590`, :issue:`13868`)

  Previous Behavior:

  .. code-block:: ipython

     In [2]: pd.date_range('2016-07-01', freq='W@MON', periods=3)
     pandas/tseries/frequencies.py:465: FutureWarning: Freq "W@MON" is deprecated, use "W-MON" as alternative.
     Out[2]: DatetimeIndex(['2016-07-04', '2016-07-11', '2016-07-18'], dtype='datetime64[ns]', freq='W-MON')

  Now legacy time rules raises ``ValueError``. For the list of currently supported offsets, see :ref:`here <timeseries.offset_aliases>`

- The ``tquery`` and ``uquery`` functions in the ``pandas.io.sql`` module are removed (:issue:`5950`).


.. _whatsnew_0190.performance:

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

- Improved performance of sparse ``IntIndex.intersect`` (:issue:`13082`)
- Improved performance of sparse arithmetic with ``BlockIndex`` when the number of blocks are large, though recommended to use ``IntIndex`` in such cases (:issue:`13082`)
- increased performance of ``DataFrame.quantile()`` as it now operates per-block (:issue:`11623`)

- Improved performance of float64 hash table operations, fixing some very slow indexing and groupby operations in python 3 (:issue:`13166`, :issue:`13334`)
- Improved performance of ``DataFrameGroupBy.transform`` (:issue:`12737`)
- Improved performance of ``Index`` and ``Series`` ``.duplicated`` (:issue:`10235`)
- Improved performance of ``Index.difference`` (:issue:`12044`)
- Improved performance of ``RangeIndex.is_monotonic_increasing`` and ``is_monotonic_decreasing`` (:issue:`13749`)
- Improved performance of datetime string parsing in ``DatetimeIndex`` (:issue:`13692`)
- Improved performance of hashing ``Period`` (:issue:`12817`)
- Improved performance of ``factorize`` of datetime with timezone (:issue:`13750`)



.. _whatsnew_0190.bug_fixes:

Bug Fixes
~~~~~~~~~

- Bug in ``groupby().shift()``, which could cause a segfault or corruption in rare circumstances when grouping by columns with missing values (:issue:`13813`)
- Bug in ``pd.read_csv()``, which may cause a segfault or corruption when iterating in large chunks over a stream/file under rare circumstances (:issue:`13703`)
- Bug in ``io.json.json_normalize()``, where non-ascii keys raised an exception (:issue:`13213`)
- Bug in ``SparseSeries`` with ``MultiIndex`` ``[]`` indexing may raise ``IndexError`` (:issue:`13144`)
- Bug in ``SparseSeries`` with ``MultiIndex`` ``[]`` indexing result may have normal ``Index`` (:issue:`13144`)
- Bug in ``SparseDataFrame`` in which ``axis=None`` did not default to ``axis=0`` (:issue:`13048`)
- Bug in ``SparseSeries`` and ``SparseDataFrame`` creation with ``object`` dtype may raise ``TypeError`` (:issue:`11633`)
- Bug when passing a not-default-indexed ``Series`` as ``xerr`` or ``yerr`` in ``.plot()`` (:issue:`11858`)
- Bug in matplotlib ``AutoDataFormatter``; this restores the second scaled formatting and re-adds micro-second scaled formatting (:issue:`13131`)
- Bug in selection from a ``HDFStore`` with a fixed format and ``start`` and/or ``stop`` specified will now return the selected range (:issue:`8287`)
- Bug in ``Series`` construction from a tuple of integers on windows not returning default dtype (int64) (:issue:`13646`)

- Bug in ``.groupby(..).resample(..)`` when the same object is called multiple times (:issue:`13174`)
- Bug in ``.to_records()`` when index name is a unicode string (:issue:`13172`)

- Bug in calling ``.memory_usage()`` on object which doesn't implement (:issue:`12924`)

- Regression in ``Series.quantile`` with nans (also shows up in ``.median()`` and ``.describe()`` ); furthermore now names the ``Series`` with the quantile (:issue:`13098`, :issue:`13146`)

- Bug in ``SeriesGroupBy.transform`` with datetime values and missing groups (:issue:`13191`)

- Bug in ``Series.str.extractall()`` with ``str`` index raises ``ValueError``  (:issue:`13156`)
- Bug in ``Series.str.extractall()`` with single group and quantifier  (:issue:`13382`)
- Bug in ``DatetimeIndex`` and ``Period`` subtraction raises ``ValueError`` or ``AttributeError`` rather than ``TypeError`` (:issue:`13078`)
- Bug in ``Index`` and ``Series`` created with ``NaN`` and ``NaT`` mixed data may not have ``datetime64`` dtype  (:issue:`13324`)
- Bug in ``Index`` and ``Series`` may ignore ``np.datetime64('nat')`` and ``np.timdelta64('nat')`` to infer dtype (:issue:`13324`)
- Bug in ``PeriodIndex`` and ``Period`` subtraction raises ``AttributeError`` (:issue:`13071`)
- Bug in ``PeriodIndex`` construction returning a ``float64`` index in some circumstances (:issue:`13067`)
- Bug in ``.resample(..)`` with a ``PeriodIndex`` not changing its ``freq`` appropriately when empty (:issue:`13067`)
- Bug in ``.resample(..)`` with a ``PeriodIndex`` not retaining its type or name with an empty ``DataFrame`` appropriately when empty (:issue:`13212`)
- Bug in ``groupby(..).apply(..)`` when the passed function returns scalar values per group (:issue:`13468`).
- Bug in ``groupby(..).resample(..)`` where passing some keywords would raise an exception (:issue:`13235`)
- Bug in ``.tz_convert`` on a tz-aware ``DateTimeIndex`` that relied on index being sorted for correct results (:issue:`13306`)
- Bug in ``.tz_localize`` with ``dateutil.tz.tzlocal`` may return incorrect result (:issue:`13583`)
- Bug in ``DatetimeTZDtype`` dtype with ``dateutil.tz.tzlocal`` cannot be regarded as valid dtype (:issue:`13583`)
- Bug in ``pd.read_hdf()`` where attempting to load an HDF file with a single dataset, that had one or more categorical columns, failed unless the key argument was set to the name of the dataset. (:issue:`13231`)
- Bug in ``.rolling()`` that allowed a negative integer window in contruction of the ``Rolling()`` object, but would later fail on aggregation (:issue:`13383`)

- Bug in printing ``pd.DataFrame`` where unusual elements with the ``object`` dtype were causing segfaults (:issue:`13717`)
- Bug in various index types, which did not propagate the name of passed index (:issue:`12309`)
- Bug in ``DatetimeIndex``, which did not honour the ``copy=True`` (:issue:`13205`)
- Bug in ``DatetimeIndex.is_normalized`` returns incorrectly for normalized date_range in case of local timezones (:issue:`13459`)

- Bug in ``DataFrame.to_csv()`` in which float values were being quoted even though quotations were specified for non-numeric values only (:issue:`12922`, :issue:`13259`)
- Bug in ``MultiIndex`` slicing where extra elements were returned when level is non-unique (:issue:`12896`)
- Bug in ``.str.replace`` does not raise ``TypeError`` for invalid replacement (:issue:`13438`)
- Bug in ``MultiIndex.from_arrays`` which didn't check for input array lengths matching (:issue:`13599`)


- Bug in ``pd.read_csv()`` with ``engine='python'`` in which ``NaN`` values weren't being detected after data was converted to numeric values (:issue:`13314`)
- Bug in ``pd.read_csv()`` in which the ``nrows`` argument was not properly validated for both engines (:issue:`10476`)
- Bug in ``pd.read_csv()`` with ``engine='python'`` in which infinities of mixed-case forms were not being interpreted properly (:issue:`13274`)
- Bug in ``pd.read_csv()`` with ``engine='python'`` in which trailing ``NaN`` values were not being parsed (:issue:`13320`)
- Bug in ``pd.read_csv()`` with ``engine='python'`` when reading from a ``tempfile.TemporaryFile`` on Windows with Python 3 (:issue:`13398`)
- Bug in ``pd.read_csv()`` that prevents ``usecols`` kwarg from accepting single-byte unicode strings (:issue:`13219`)
- Bug in ``pd.read_csv()`` that prevents ``usecols`` from being an empty set (:issue:`13402`)
- Bug in ``pd.read_csv()`` with ``engine='c'`` in which null ``quotechar`` was not accepted even though ``quoting`` was specified as ``None`` (:issue:`13411`)
- Bug in ``pd.read_csv()`` with ``engine='c'`` in which fields were not properly cast to float when quoting was specified as non-numeric (:issue:`13411`)
- Bug in ``pd.pivot_table()`` where ``margins_name`` is ignored when ``aggfunc`` is a list (:issue:`13354`)
- Bug in ``pd.Series.str.zfill``, ``center``, ``ljust``, ``rjust``, and ``pad`` when passing non-integers, did not raise ``TypeError`` (:issue:`13598`)
- Bug in checking for any null objects in a ``TimedeltaIndex``, which always returned ``True`` (:issue:`13603`)



- Bug in ``Series`` arithmetic raises ``TypeError`` if it contains datetime-like as ``object`` dtype (:issue:`13043`)

- Bug ``Series.isnull`` and ``Series.notnull`` ignore ``Period('NaT')``  (:issue:`13737`)
- Bug ``Series.fillna`` and ``Series.dropna`` don't affect to ``Period('NaT')``  (:issue:`13737`)

- Bug in ``pd.to_datetime()`` when passing invalid datatypes (e.g. bool); will now respect the ``errors`` keyword (:issue:`13176`)
- Bug in ``pd.to_datetime()`` which overflowed on ``int8``, and ``int16`` dtypes (:issue:`13451`)
- Bug in extension dtype creation where the created types were not is/identical (:issue:`13285`)
- Bug in ``.resample(..)`` where incorrect warnings were triggered by IPython introspection (:issue:`13618`)
- Bug in ``NaT`` - ``Period`` raises ``AttributeError`` (:issue:`13071`)
- Bug in ``Series`` comparison may output incorrect result if rhs contains ``NaT`` (:issue:`9005`)
- Bug in ``Series`` and ``Index`` comparison may output incorrect result if it contains ``NaT`` with ``object`` dtype (:issue:`13592`)
- Bug in ``Period`` addition raises ``TypeError`` if ``Period`` is on right hand side (:issue:`13069`)
- Bug in ``Peirod`` and ``Series`` or ``Index`` comparison raises ``TypeError`` (:issue:`13200`)
- Bug in ``pd.set_eng_float_format()`` that would prevent NaN's from formatting (:issue:`11981`)
- Bug in ``.unstack`` with ``Categorical`` dtype resets ``.ordered`` to ``True`` (:issue:`13249`)
- Clean some compile time warnings in datetime parsing (:issue:`13607`)
- Bug in ``factorize`` raises ``AmbiguousTimeError`` if data contains datetime near DST boundary (:issue:`13750`)
- Bug in ``.set_index`` raises ``AmbiguousTimeError`` if new index contains DST boundary and multi levels (:issue:`12920`)
- Bug in ``pd.read_hdf()`` returns incorrect result when a ``DataFrame`` with a ``categorical`` column and a query which doesn't match any values (:issue:`13792`)


- Bug in ``Series`` comparison operators when dealing with zero dim NumPy arrays (:issue:`13006`)
- Bug in ``groupby`` where ``apply`` returns different result depending on whether first result is ``None`` or not (:issue:`12824`)
- Bug in ``groupby(..).nth()`` where the group key is included inconsistently if called after ``.head()/.tail()`` (:issue:`12839`)
- Bug in ``.to_html``, ``.to_latex`` and ``.to_string`` silently ignore custom datetime formatter passed through the ``formatters`` key word (:issue:`10690`)

- Bug in ``pd.to_numeric`` when ``errors='coerce'`` and input contains non-hashable objects (:issue:`13324`)
- Bug in invalid ``Timedelta`` arithmetic and comparison may raise ``ValueError`` rather than ``TypeError`` (:issue:`13624`)
- Bug in invalid datetime parsing in ``to_datetime`` and ``DatetimeIndex`` may raise ``TypeError`` rather than ``ValueError`` (:issue:`11169`, :issue:`11287`)
- Bug in ``Index`` created with tz-aware ``Timestamp`` and mismatched ``tz`` option incorrectly coerces timezone (:issue:`13692`)
- Bug in ``DatetimeIndex`` with nanosecond frequency does not include timestamp specified with ``end`` (:issue:`13672`)

- Bug in ``Index`` raises ``OutOfBoundsDatetime`` if ``datetime`` exceeds ``datetime64[ns]`` bounds, rather than coercing to ``object`` dtype (:issue:`13663`)
- Bug in ``RangeIndex`` can be created without no arguments rather than raises ``TypeError`` (:issue:`13793`)
- Bug in ``.value_counts`` raises ``OutOfBoundsDatetime`` if data exceeds ``datetime64[ns]`` bounds (:issue:`13663`)
- Bug in ``DatetimeIndex`` may raise ``OutOfBoundsDatetime`` if input ``np.datetime64`` has other unit than ``ns`` (:issue:`9114`)
- Bug in ``Series`` creation with ``np.datetime64`` which has other unit than ``ns`` as ``object`` dtype results in incorrect values (:issue:`13876`)

- Bug in ``isnull`` ``notnull`` raise ``TypeError`` if input datetime-like has other unit than ``ns`` (:issue:`13389`)
- Bug in ``.merge`` may raise ``TypeError`` if input datetime-like has other unit than ``ns`` (:issue:`13389`)



- Bug in ``Categorical.remove_unused_categories()`` changes ``.codes`` dtype to platform int (:issue:`13261`)
- Bug in ``groupby`` with ``as_index=False`` returns all NaN's when grouping on multiple columns including a categorical one (:issue:`13204`)
- Bug in ``df.groupby(...)[...]`` where getitem with ``Int64Index`` raised an error (:issue:`13731`)

- Bug where ``pd.read_gbq()`` could throw ``ImportError: No module named discovery`` as a result of a naming conflict with another python package called apiclient  (:issue:`13454`)
- Bug in ``Index.union`` returns an incorrect result with a named empty index (:issue:`13432`)
- Bugs in ``Index.difference`` and ``DataFrame.join`` raise in Python3 when using mixed-integer indexes (:issue:`13432`, :issue:`12814`)

- Bug in ``.to_excel()`` when DataFrame contains a MultiIndex which contains a label with a NaN value (:issue:`13511`)
- Bug in ``pd.read_csv`` in Python 2.x with non-UTF8 encoded, multi-character separated data (:issue:`3404`)
- Bug in ``Index`` raises ``KeyError`` displaying incorrect column when column is not in the df and columns contains duplicate values (:issue:`13822`)