- The I/O API is now much more consistent with a set of top level ``reader`` functions
    accessed like ``pd.read_csv()`` that generally return a ``pandas`` object.

    * ``read_csv``
    * ``read_excel``
    * ``read_hdf``
    * ``read_sql``
    * ``read_json``
    * ``read_html``
    * ``read_stata``
    * ``read_clipboard``

    The corresponding ``writer`` functions are object methods that are accessed like ``df.to_csv()``

    * ``to_csv``
    * ``to_excel``
    * ``to_hdf``
    * ``to_sql``
    * ``to_json``
    * ``to_html``
    * ``to_stata``
    * ``to_clipboard``


  - Fix modulo and integer division on Series,DataFrames to act similarly to ``float`` dtypes to return
    ``np.nan`` or ``np.inf`` as appropriate (:issue:`3590`). This correct a numpy bug that treats ``integer``
    and ``float`` dtypes differently.

    .. ipython:: python

        p = pd.DataFrame({"first": [4, 5, 8], "second": [0, 0, 3]})
        p % 0
        p % p
        p / p
        p / 0

  - Add ``squeeze`` keyword to ``groupby`` to allow reduction from
    DataFrame -> Series if groups are unique. This is a Regression from 0.10.1.
    We are reverting back to the prior behavior. This means groupby will return the
    same shaped objects whether the groups are unique or not. Revert this issue (:issue:`2893`)
    with (:issue:`3596`).

    .. code-block:: ipython

        In [2]: df2 = pd.DataFrame([{"val1": 1, "val2": 20},
           ...:                     {"val1": 1, "val2": 19},
           ...:                     {"val1": 1, "val2": 27},
           ...:                     {"val1": 1, "val2": 12}])

        In [3]: def func(dataf):
           ...:     return dataf["val2"] - dataf["val2"].mean()
           ...:

        In [4]: # squeezing the result frame to a series (because we have unique groups)
           ...: df2.groupby("val1", squeeze=True).apply(func)
        Out[4]:
        0    0.5
        1   -0.5
        2    7.5
        3   -7.5
        Name: 1, dtype: float64

        In [5]: # no squeezing (the default, and behavior in 0.10.1)
           ...: df2.groupby("val1").apply(func)
        Out[5]:
        val2    0    1    2    3
        val1
        1     0.5 -0.5  7.5 -7.5

  - Raise on ``iloc`` when boolean indexing with a label based indexer mask
    e.g. a boolean Series, even with integer labels, will raise. Since ``iloc``
    is purely positional based, the labels on the Series are not alignable (:issue:`3631`)

    This case is rarely used, and there are plenty of alternatives. This preserves the
    ``iloc`` API to be *purely* positional based.

    .. ipython:: python

       df = pd.DataFrame(range(5), index=list("ABCDE"), columns=["a"])
       mask = df.a % 2 == 0
       mask

       # this is what you should use
       df.loc[mask]

       # this will work as well
       df.iloc[mask.values]

    ``df.iloc[mask]`` will raise a ``ValueError``

  - The ``raise_on_error`` argument to plotting functions is removed. Instead,
    plotting functions raise a ``TypeError`` when the ``dtype`` of the object
    is ``object`` to remind you to avoid ``object`` arrays whenever possible
    and thus you should cast to an appropriate numeric dtype if you need to
    plot something.

  - Add ``colormap`` keyword to DataFrame plotting methods. Accepts either a
    matplotlib colormap object (ie, matplotlib.cm.jet) or a string name of such
    an object (ie, 'jet'). The colormap is sampled to select the color for each
    column. Please see :ref:`visualization.colormaps` for more information.
    (:issue:`3860`)

  - ``DataFrame.interpolate()`` is now deprecated. Please use
    ``DataFrame.fillna()`` and ``DataFrame.replace()`` instead. (:issue:`3582`,
    :issue:`3675`, :issue:`3676`)

  - the ``method`` and ``axis`` arguments of ``DataFrame.replace()`` are
    deprecated

  - ``DataFrame.replace`` 's ``infer_types`` parameter is removed and now
    performs conversion by default. (:issue:`3907`)

  - Add the keyword ``allow_duplicates`` to ``DataFrame.insert`` to allow a duplicate column
    to be inserted if ``True``, default is ``False`` (same as prior to 0.12) (:issue:`3679`)
  - Implement ``__nonzero__`` for ``NDFrame`` objects (:issue:`3691`, :issue:`3696`)

  - IO API

    - Added top-level function ``read_excel`` to replace the following,
      The original API is deprecated and will be removed in a future version

      .. code-block:: python

         from pandas.io.parsers import ExcelFile

         xls = ExcelFile("path_to_file.xls")
         xls.parse("Sheet1", index_col=None, na_values=["NA"])

      With

      .. code-block:: python

         import pandas as pd

         pd.read_excel("path_to_file.xls", "Sheet1", index_col=None, na_values=["NA"])

    - Added top-level function ``read_sql`` that is equivalent to the following

      .. code-block:: python

         from pandas.io.sql import read_frame

         read_frame(...)

  - ``DataFrame.to_html`` and ``DataFrame.to_latex`` now accept a path for
    their first argument (:issue:`3702`)

  - Do not allow astypes on ``datetime64[ns]`` except to ``object``, and
    ``timedelta64[ns]`` to ``object/int`` (:issue:`3425`)

  - The behavior of ``datetime64`` dtypes has changed with respect to certain
    so-called reduction operations (:issue:`3726`). The following operations now
    raise a ``TypeError`` when performed on a ``Series`` and return an *empty*
    ``Series`` when performed on a ``DataFrame`` similar to performing these
    operations on, for example, a ``DataFrame`` of ``slice`` objects:

    - sum, prod, mean, std, var, skew, kurt, corr, and cov

  - ``read_html`` now defaults to ``None`` when reading, and falls back on
    ``bs4`` + ``html5lib`` when lxml fails to parse. a list of parsers to try
    until success is also valid

  - The internal ``pandas`` class hierarchy has changed (slightly). The
    previous ``PandasObject`` now is called ``PandasContainer`` and a new
    ``PandasObject`` has become the base class for ``PandasContainer`` as well
    as ``Index``, ``Categorical``, ``GroupBy``, ``SparseList``, and
    ``SparseArray`` (+ their base classes). Currently, ``PandasObject``
    provides string methods (from ``StringMixin``). (:issue:`4090`, :issue:`4092`)

  - New ``StringMixin`` that, given a ``__unicode__`` method, gets python 2 and
    python 3 compatible string methods (``__str__``, ``__bytes__``, and
    ``__repr__``). Plus string safety throughout. Now employed in many places
    throughout the pandas library. (:issue:`4090`, :issue:`4092`)

IO enhancements

  - ``DataFrame.replace()`` now allows regular expressions on contained
    ``Series`` with object dtype. See the examples section in the regular docs
    :ref:`Replacing via String Expression <missing_data.replace_expression>`

    For example you can do

    .. ipython:: python

        df = pd.DataFrame({"a": list("ab.."), "b": [1, 2, 3, 4]})
        df.replace(regex=r"\s*\.\s*", value=np.nan)

    to replace all occurrences of the string ``'.'`` with zero or more
    instances of surrounding white space with ``NaN``.

    Regular string replacement still works as expected. For example, you can do

    .. ipython:: python

        df.replace(".", np.nan)

    to replace all occurrences of the string ``'.'`` with ``NaN``.

  - ``pd.melt()`` now accepts the optional parameters ``var_name`` and ``value_name``
    to specify custom column names of the returned DataFrame.

  - ``pd.set_option()`` now allows N option, value pairs (:issue:`3667`).

    Let's say that we had an option ``'a.b'`` and another option ``'b.c'``.
    We can set them at the same time:

    .. code-block:: ipython

        In [31]: pd.get_option('a.b')
        Out[31]: 2

        In [32]: pd.get_option('b.c')
        Out[32]: 3

        In [33]: pd.set_option('a.b', 1, 'b.c', 4)

        In [34]: pd.get_option('a.b')
        Out[34]: 1

        In [35]: pd.get_option('b.c')
        Out[35]: 4

  - The ``filter`` method for group objects returns a subset of the original
    object. Suppose we want to take only elements that belong to groups with a
    group sum greater than 2.

    .. ipython:: python

       sf = pd.Series([1, 1, 2, 3, 3, 3])
       sf.groupby(sf).filter(lambda x: x.sum() > 2)

    The argument of ``filter`` must a function that, applied to the group as a
    whole, returns ``True`` or ``False``.

    Another useful operation is filtering out elements that belong to groups
    with only a couple members.

    .. ipython:: python

       dff = pd.DataFrame({"A": np.arange(8), "B": list("aabbbbcc")})
       dff.groupby("B").filter(lambda x: len(x) > 2)

    Alternatively, instead of dropping the offending groups, we can return a
    like-indexed objects where the groups that do not pass the filter are
    filled with NaNs.

    .. ipython:: python

       dff.groupby("B").filter(lambda x: len(x) > 2, dropna=False)

  - Series and DataFrame hist methods now take a ``figsize`` argument (:issue:`3834`)

  - DatetimeIndexes no longer try to convert mixed-integer indexes during join
    operations (:issue:`3877`)

  - Timestamp.min and Timestamp.max now represent valid Timestamp instances instead
    of the default datetime.min and datetime.max (respectively), thanks @SleepingPills

  - ``read_html`` now raises when no tables are found and BeautifulSoup==4.2.0
    is detected (:issue:`4214`)


Experimental features

  - Plotting functions now raise a ``TypeError`` before trying to plot anything
    if the associated objects have a dtype of ``object`` (:issue:`1818`,
    :issue:`3572`, :issue:`3911`, :issue:`3912`), but they will try to convert object arrays to
    numeric arrays if possible so that you can still plot, for example, an
    object array with floats. This happens before any drawing takes place which
    eliminates any spurious plots from showing up.

  - ``fillna`` methods now raise a ``TypeError`` if the ``value`` parameter is
    a list or tuple.

  - ``Series.str`` now supports iteration (:issue:`3638`). You can iterate over the
    individual elements of each string in the ``Series``. Each iteration yields
    a ``Series`` with either a single character at each index of the original
    ``Series`` or ``NaN``. For example,

    .. code-block:: ipython

        In [38]: strs = "go", "bow", "joe", "slow"

        In [32]: ds = pd.Series(strs)

        In [33]: for s in ds.str:
            ...:     print(s)

        0    g
        1    b
        2    j
        3    s
        dtype: object
        0    o
        1    o
        2    o
        3    l
        dtype: object
        0    NaN
        1      w
        2      e
        3      o
        dtype: object
        0    NaN
        1    NaN
        2    NaN
        3      w
        dtype: object

        In [41]: s
        Out[41]:
        0    NaN
        1    NaN
        2    NaN
        3      w
        dtype: object

        In [42]: s.dropna().values.item() == "w"
        Out[42]: True

    The last element yielded by the iterator will be a ``Series`` containing
    the last element of the longest string in the ``Series`` with all other
    elements being ``NaN``. Here since ``'slow'`` is the longest string
    and there are no other strings with the same length ``'w'`` is the only
    non-null string in the yielded ``Series``.

  - ``HDFStore``

    - Will retain index attributes (freq,tz,name) on recreation (:issue:`3499`)
    - Will warn with a ``AttributeConflictWarning`` if you are attempting to append
      an index with a different frequency than the existing, or attempting
      to append an index with a different name than the existing
    - Support datelike columns with a timezone as data_columns (:issue:`2852`)

  - Non-unique index support clarified (:issue:`3468`).

    - Fix assigning a new index to a duplicate index in a DataFrame would fail (:issue:`3468`)
    - Fix construction of a DataFrame with a duplicate index
    - ref_locs support to allow duplicative indices across dtypes,
      allows iget support to always find the index (even across dtypes) (:issue:`2194`)
    - applymap on a DataFrame with a non-unique index now works
      (removed warning) (:issue:`2786`), and fix (:issue:`3230`)
    - Fix to_csv to handle non-unique columns (:issue:`3495`)
    - Duplicate indexes with getitem will return items in the correct order (:issue:`3455`, :issue:`3457`)
      and handle missing elements like unique indices (:issue:`3561`)
    - Duplicate indexes with and empty DataFrame.from_records will return a correct frame (:issue:`3562`)
    - Concat to produce a non-unique columns when duplicates are across dtypes is fixed (:issue:`3602`)
    - Allow insert/delete to non-unique columns (:issue:`3679`)
    - Non-unique indexing with a slice via ``loc`` and friends fixed (:issue:`3659`)
    - Allow insert/delete to non-unique columns (:issue:`3679`)
    - Extend ``reindex`` to correctly deal with non-unique indices (:issue:`3679`)
    - ``DataFrame.itertuples()`` now works with frames with duplicate column
      names (:issue:`3873`)
    - Bug in non-unique indexing via ``iloc`` (:issue:`4017`); added ``takeable`` argument to
      ``reindex`` for location-based taking
    - Allow non-unique indexing in series via ``.ix/.loc`` and ``__getitem__`` (:issue:`4246`)
    - Fixed non-unique indexing memory allocation issue with ``.ix/.loc`` (:issue:`4280`)

  - ``DataFrame.from_records`` did not accept empty recarrays (:issue:`3682`)
  - ``read_html`` now correctly skips tests (:issue:`3741`)
  - Fixed a bug where ``DataFrame.replace`` with a compiled regular expression
    in the ``to_replace`` argument wasn't working (:issue:`3907`)
  - Improved ``network`` test decorator to catch ``IOError`` (and therefore
    ``URLError`` as well). Added ``with_connectivity_check`` decorator to allow
    explicitly checking a website as a proxy for seeing if there is network
    connectivity. Plus, new ``optional_args`` decorator factory for decorators.
    (:issue:`3910`, :issue:`3914`)
  - Fixed testing issue where too many sockets where open thus leading to a
    connection reset issue (:issue:`3982`, :issue:`3985`, :issue:`4028`,
    :issue:`4054`)
  - Fixed failing tests in test_yahoo, test_google where symbols were not
    retrieved but were being accessed (:issue:`3982`, :issue:`3985`,
    :issue:`4028`, :issue:`4054`)
  - ``Series.hist`` will now take the figure from the current environment if
    one is not passed
  - Fixed bug where a 1xN DataFrame would barf on a 1xN mask (:issue:`4071`)
  - Fixed running of ``tox`` under python3 where the pickle import was getting
    rewritten in an incompatible way (:issue:`4062`, :issue:`4063`)
  - Fixed bug where sharex and sharey were not being passed to grouped_hist
    (:issue:`4089`)
  - Fixed bug in ``DataFrame.replace`` where a nested dict wasn't being
    iterated over when regex=False (:issue:`4115`)
  - Fixed bug in the parsing of microseconds when using the ``format``
    argument in ``to_datetime`` (:issue:`4152`)
  - Fixed bug in ``PandasAutoDateLocator`` where ``invert_xaxis`` triggered
    incorrectly ``MilliSecondLocator``  (:issue:`3990`)
  - Fixed bug in plotting that wasn't raising on invalid colormap for
    matplotlib 1.1.1 (:issue:`4215`)
  - Fixed the legend displaying in ``DataFrame.plot(kind='kde')`` (:issue:`4216`)
  - Fixed bug where Index slices weren't carrying the name attribute
    (:issue:`4226`)
  - Fixed bug in initializing ``DatetimeIndex`` with an array of strings
    in a certain time zone (:issue:`4229`)
  - Fixed bug where html5lib wasn't being properly skipped (:issue:`4265`)
  - Fixed bug where get_data_famafrench wasn't using the correct file edges
    (:issue:`4281`)

See the :ref:`full release notes
<release>` or issue tracker
on GitHub for a complete list.


.. _whatsnew_0.12.0.contributors:

Contributors