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# Version 0.18.1 (May 3, 2016)

This is a minor bug-fix release from 0.18.0 and includes a large number of bug fixes along with several new features, enhancements, and performance improvements. We recommend that all users upgrade to this version.

Highlights include:

- `.groupby(...)` has been enhanced to provide convenient syntax when working with `.rolling(..)`, `.expanding(..)` and `.resample(..)` per group, see here

- `pd.to_datetime()` has gained the ability to assemble dates from a `DataFrame`, see here

- Method chaining improvements, see here.

- Custom business hour offset, see here.

- Many bug fixes in the handling of `sparse`, see here

- Expanded the Tutorials section with a feature on modern pandas, courtesy of @TomAugsburger. (GH 13045).

## New features

### Custom business hour

The `CustomBusinessHour` is a mixture of `BusinessHour` and `CustomBusinessDay` which allows you to specify arbitrary holidays. For details, see Custom Business Hour (GH 11514)

    In [1]: from pandas.tseries.offsets import CustomBusinessHour

    In [2]: from pandas.tseries.holiday import USFederalHolidayCalendar

    In [3]: bhour_us = CustomBusinessHour(calendar=USFederalHolidayCalendar())

Friday before MLK Day

    In [4]: import datetime

    In [5]: dt = datetime.datetime(2014, 1, 17, 15)

    In [6]: dt + bhour_us
    Out[6]: Timestamp('2014-01-17 16:00:00')

Tuesday after MLK Day (Monday is skipped because it’s a holiday)

    In [7]: dt + bhour_us * 2
    Out[7]: Timestamp('2014-01-20 09:00:00')

### Method `.groupby(..)` syntax with window and resample operations

`.groupby(...)` has been enhanced to provide convenient syntax when working with `.rolling(..)`, `.expanding(..)` and `.resample(..)` per group, see (GH 12486, GH 12738).

You can now use `.rolling(..)` and `.expanding(..)` as methods on groupbys. These return another deferred object (similar to what `.rolling()` and `.expanding()` do on ungrouped pandas objects). You can then operate on these `RollingGroupby` objects in a similar manner.

Previously you would have to do this to get a rolling window mean per-group:

    In [8]: df = pd.DataFrame({"A": [1] * 20 + [2] * 12 + [3] * 8, "B": np.arange(40)})

    In [9]: df
    Out[9]:
        A   B
    0   1   0
    1   1   1
    2   1   2
    3   1   3
    4   1   4
    .. ..  ..
    35  3  35
    36  3  36
    37  3  37
    38  3  38
    39  3  39

    [40 rows x 2 columns]

    In [1]: df.groupby("A").apply(lambda x: x.rolling(4).B.mean())
    Out[1]:
    A
    1  0      NaN
       1      NaN
       2      NaN
       3      1.5
       4      2.5
       5      3.5
       6      4.5
       7      5.5
       8      6.5
       9      7.5
       10     8.5
       11     9.5
       12    10.5
       13    11.5
       14    12.5
       15    13.5
       16    14.5
       17    15.5
       18    16.5
       19    17.5
    2  20     NaN
       21     NaN
       22     NaN
       23    21.5
       24    22.5
       25    23.5
       26    24.5
       27    25.5
       28    26.5
       29    27.5
       30    28.5
       31    29.5
    3  32     NaN
       33     NaN
       34     NaN
       35    33.5
       36    34.5
       37    35.5
       38    36.5
       39    37.5
    Name: B, dtype: float64

Now you can do:

    In [10]: df.groupby("A").rolling(4).B.mean()
    Out[10]:
    A
    1  0      NaN
       1      NaN
       2      NaN
       3      1.5
       4      2.5
             ...
    3  35    33.5
       36    34.5
       37    35.5
       38    36.5
       39    37.5
    Name: B, Length: 40, dtype: float64

For `.resample(..)` type of operations, previously you would have to:

    In [11]: df = pd.DataFrame(
       ....:     {
       ....:         "date": pd.date_range(start="2016-01-01", periods=4, freq="W"),
       ....:         "group": [1, 1, 2, 2],
       ....:         "val": [5, 6, 7, 8],
       ....:     }
       ....: ).set_index("date")
       ....:

    In [12]: df
    Out[12]:
                group  val
    date
    2016-01-03      1    5
    2016-01-10      1    6
    2016-01-17      2    7
    2016-01-24      2    8

    [4 rows x 2 columns]

    In[1]: df.groupby("group").apply(lambda x: x.resample("1D").ffill())
    Out[1]:
                      group  val
    group date
    1     2016-01-03      1    5
          2016-01-04      1    5
          2016-01-05      1    5
          2016-01-06      1    5
          2016-01-07      1    5
          2016-01-08      1    5
          2016-01-09      1    5
          2016-01-10      1    6
    2     2016-01-17      2    7
          2016-01-18      2    7
          2016-01-19      2    7
          2016-01-20      2    7
          2016-01-21      2    7
          2016-01-22      2    7
          2016-01-23      2    7
          2016-01-24      2    8

Now you can do:

    In[1]: df.groupby("group").resample("1D").ffill()
    Out[1]:
                      group  val
    group date
    1     2016-01-03      1    5
          2016-01-04      1    5
          2016-01-05      1    5
          2016-01-06      1    5
          2016-01-07      1    5
          2016-01-08      1    5
          2016-01-09      1    5
          2016-01-10      1    6
    2     2016-01-17      2    7
          2016-01-18      2    7
          2016-01-19      2    7
          2016-01-20      2    7
          2016-01-21      2    7
          2016-01-22      2    7
          2016-01-23      2    7
          2016-01-24      2    8

### Method chaining improvements

The following methods / indexers now accept a `callable`. It is intended to make these more useful in method chains, see the documentation. (GH 11485, GH 12533)

- `.where()` and `.mask()`

- `.loc[]`, `iloc[]` and `.ix[]`

- `[]` indexing

#### Methods `.where()` and `.mask()`

These can accept a callable for the condition and `other` arguments.

    In [13]: df = pd.DataFrame({"A": [1, 2, 3], "B": [4, 5, 6], "C": [7, 8, 9]})

    In [14]: df.where(lambda x: x > 4, lambda x: x + 10)
    Out[14]:
        A   B  C
    0  11  14  7
    1  12   5  8
    2  13   6  9

    [3 rows x 3 columns]

#### Methods `.loc[]`, `.iloc[]`, `.ix[]`

These can accept a callable, and a tuple of callable as a slicer. The callable can return a valid boolean indexer or anything which is valid for these indexer’s input.

    # callable returns bool indexer
    In [15]: df.loc[lambda x: x.A >= 2, lambda x: x.sum() > 10]
    Out[15]:
       B  C
    1  5  8
    2  6  9

    [2 rows x 2 columns]

    # callable returns list of labels
    In [16]: df.loc[lambda x: [1, 2], lambda x: ["A", "B"]]
    Out[16]:
       A  B
    1  2  5
    2  3  6

    [2 rows x 2 columns]

#### Indexing with `[]`

Finally, you can use a callable in `[]` indexing of Series, DataFrame and Panel. The callable must return a valid input for `[]` indexing depending on its class and index type.

    In [17]: df[lambda x: "A"]
    Out[17]:
    0    1
    1    2
    2    3
    Name: A, Length: 3, dtype: int64

Using these methods / indexers, you can chain data selection operations without using temporary variable.

    In [18]: bb = pd.read_csv("data/baseball.csv", index_col="id")

    In [19]: (bb.groupby(["year", "team"]).sum(numeric_only=True).loc[lambda df: df.r > 100])
    Out[19]:
               stint    g    ab    r    h  X2b  ...     so   ibb   hbp    sh    sf  gidp
    year team                                   ...
    2007 CIN       6  379   745  101  203   35  ...  127.0  14.0   1.0   1.0  15.0  18.0
         DET       5  301  1062  162  283   54  ...  176.0   3.0  10.0   4.0   8.0  28.0
         HOU       4  311   926  109  218   47  ...  212.0   3.0   9.0  16.0   6.0  17.0
         LAN      11  413  1021  153  293   61  ...  141.0   8.0   9.0   3.0   8.0  29.0
         NYN      13  622  1854  240  509  101  ...  310.0  24.0  23.0  18.0  15.0  48.0
         SFN       5  482  1305  198  337   67  ...  188.0  51.0   8.0  16.0   6.0  41.0
         TEX       2  198   729  115  200   40  ...  140.0   4.0   5.0   2.0   8.0  16.0
         TOR       4  459  1408  187  378   96  ...  265.0  16.0  12.0   4.0  16.0  38.0

    [8 rows x 18 columns]

### Partial string indexing on `DatetimeIndex` when part of a `MultiIndex`

Partial string indexing now matches on `DateTimeIndex` when part of a `MultiIndex` (GH 10331)

    In [20]: dft2 = pd.DataFrame(
       ....:     np.random.randn(20, 1),
       ....:     columns=["A"],
       ....:     index=pd.MultiIndex.from_product(
       ....:         [pd.date_range("20130101", periods=10, freq="12H"), ["a", "b"]]
       ....:     ),
       ....: )
       ....:

    In [21]: dft2
    Out[21]:
                                  A
    2013-01-01 00:00:00 a  0.469112
                        b -0.282863
    2013-01-01 12:00:00 a -1.509059
                        b -1.135632
    2013-01-02 00:00:00 a  1.212112
    ...                         ...
    2013-01-04 12:00:00 b  0.271860
    2013-01-05 00:00:00 a -0.424972
                        b  0.567020
    2013-01-05 12:00:00 a  0.276232
                        b -1.087401

    [20 rows x 1 columns]

    In [22]: dft2.loc["2013-01-05"]
    Out[22]:
                                  A
    2013-01-05 00:00:00 a -0.424972
                        b  0.567020
    2013-01-05 12:00:00 a  0.276232
                        b -1.087401

    [4 rows x 1 columns]

On other levels

    In [26]: idx = pd.IndexSlice

    In [27]: dft2 = dft2.swaplevel(0, 1).sort_index()

    In [28]: dft2
    Out[28]:
                                  A
    a 2013-01-01 00:00:00  0.469112
      2013-01-01 12:00:00 -1.509059
      2013-01-02 00:00:00  1.212112
      2013-01-02 12:00:00  0.119209
      2013-01-03 00:00:00 -0.861849
    ...                         ...
    b 2013-01-03 12:00:00  1.071804
      2013-01-04 00:00:00 -0.706771
      2013-01-04 12:00:00  0.271860
      2013-01-05 00:00:00  0.567020
      2013-01-05 12:00:00 -1.087401

    [20 rows x 1 columns]

    In [29]: dft2.loc[idx[:, "2013-01-05"], :]
    Out[29]:
                                  A
    a 2013-01-05 00:00:00 -0.424972
      2013-01-05 12:00:00  0.276232
    b 2013-01-05 00:00:00  0.567020
      2013-01-05 12:00:00 -1.087401

    [4 rows x 1 columns]

### Assembling datetimes

`pd.to_datetime()` has gained the ability to assemble datetimes from a passed in `DataFrame` or a dict. (GH 8158).

    In [20]: df = pd.DataFrame(
       ....:     {"year": [2015, 2016], "month": [2, 3], "day": [4, 5], "hour": [2, 3]}
       ....: )
       ....:

    In [21]: df
    Out[21]:
       year  month  day  hour
    0  2015      2    4     2
    1  2016      3    5     3

    [2 rows x 4 columns]

Assembling using the passed frame.

    In [22]: pd.to_datetime(df)
    Out[22]:
    0   2015-02-04 02:00:00
    1   2016-03-05 03:00:00
    Length: 2, dtype: datetime64[us]

You can pass only the columns that you need to assemble.

    In [23]: pd.to_datetime(df[["year", "month", "day"]])
    Out[23]:
    0   2015-02-04
    1   2016-03-05
    Length: 2, dtype: datetime64[us]

### Other enhancements

- `pd.read_csv()` now supports `delim_whitespace=True` for the Python engine (GH 12958)

- `pd.read_csv()` now supports opening ZIP files that contains a single CSV, via extension inference or explicit `compression='zip'` (GH 12175)

- `pd.read_csv()` now supports opening files using xz compression, via extension inference or explicit `compression='xz'` is specified; `xz` compressions is also supported by `DataFrame.to_csv` in the same way (GH 11852)

- `pd.read_msgpack()` now always gives writeable ndarrays even when compression is used (GH 12359).

- `pd.read_msgpack()` now supports serializing and de-serializing categoricals with msgpack (GH 12573)

- `.to_json()` now supports `NDFrames` that contain categorical and sparse data (GH 10778)

- `interpolate()` now supports `method='akima'` (GH 7588).

- `pd.read_excel()` now accepts path objects (e.g. `pathlib.Path`, `py.path.local`) for the file path, in line with other `read_*` functions (GH 12655)

- Added `.weekday_name` property as a component to `DatetimeIndex` and the `.dt` accessor. (GH 11128)

- `Index.take` now handles `allow_fill` and `fill_value` consistently (GH 12631)

      In [24]: idx = pd.Index([1.0, 2.0, 3.0, 4.0], dtype="float")

      # default, allow_fill=True, fill_value=None
      In [25]: idx.take([2, -1])
      Out[25]: Index([3.0, 4.0], dtype='float64')

      In [26]: idx.take([2, -1], fill_value=True)
      Out[26]: Index([3.0, nan], dtype='float64')

- `Index` now supports `.str.get_dummies()` which returns `MultiIndex`, see Creating Indicator Variables (GH 10008, GH 10103)

      In [27]: idx = pd.Index(["a|b", "a|c", "b|c"])

      In [28]: idx.str.get_dummies("|")
      Out[28]:
      MultiIndex([(1, 1, 0),
                  (1, 0, 1),
                  (0, 1, 1)],
                 names=['a', 'b', 'c'])

- `pd.crosstab()` has gained a `normalize` argument for normalizing frequency tables (GH 12569). Examples in the updated docs here.

- `.resample(..).interpolate()` is now supported (GH 12925)

- `.isin()` now accepts passed `sets` (GH 12988)

## Sparse changes

These changes conform sparse handling to return the correct types and work to make a smoother experience with indexing.

`SparseArray.take` now returns a scalar for scalar input, `SparseArray` for others. Furthermore, it handles a negative indexer with the same rule as `Index` (GH 10560, GH 12796)

    s = pd.SparseArray([np.nan, np.nan, 1, 2, 3, np.nan, 4, 5, np.nan, 6])
    s.take(0)
    s.take([1, 2, 3])

- Bug in `SparseSeries[]` indexing with `Ellipsis` raises `KeyError` (GH 9467)

- Bug in `SparseArray[]` indexing with tuples are not handled properly (GH 12966)

- Bug in `SparseSeries.loc[]` with list-like input raises `TypeError` (GH 10560)

- Bug in `SparseSeries.iloc[]` with scalar input may raise `IndexError` (GH 10560)

- Bug in `SparseSeries.loc[]`, `.iloc[]` with `slice` returns `SparseArray`, rather than `SparseSeries` (GH 10560)

- Bug in `SparseDataFrame.loc[]`, `.iloc[]` may results in dense `Series`, rather than `SparseSeries` (GH 12787)

- Bug in `SparseArray` addition ignores `fill_value` of right hand side (GH 12910)

- Bug in `SparseArray` mod raises `AttributeError` (GH 12910)

- Bug in `SparseArray` pow calculates `1 ** np.nan` as `np.nan` which must be 1 (GH 12910)

- Bug in `SparseArray` comparison output may incorrect result or raise `ValueError` (GH 12971)

- Bug in `SparseSeries.__repr__` raises `TypeError` when it is longer than `max_rows` (GH 10560)

- Bug in `SparseSeries.shape` ignores `fill_value` (GH 10452)

- Bug in `SparseSeries` and `SparseArray` may have different `dtype` from its dense values (GH 12908)

- Bug in `SparseSeries.reindex` incorrectly handle `fill_value` (GH 12797)

- Bug in `SparseArray.to_frame()` results in `DataFrame`, rather than `SparseDataFrame` (GH 9850)

- Bug in `SparseSeries.value_counts()` does not count `fill_value` (GH 6749)

- Bug in `SparseArray.to_dense()` does not preserve `dtype` (GH 10648)

- Bug in `SparseArray.to_dense()` incorrectly handle `fill_value` (GH 12797)

- Bug in `pd.concat()` of `SparseSeries` results in dense (GH 10536)

- Bug in `pd.concat()` of `SparseDataFrame` incorrectly handle `fill_value` (GH 9765)

- Bug in `pd.concat()` of `SparseDataFrame` may raise `AttributeError` (GH 12174)

- Bug in `SparseArray.shift()` may raise `NameError` or `TypeError` (GH 12908)

## API changes

### Method `.groupby(..).nth()` changes

The index in `.groupby(..).nth()` output is now more consistent when the `as_index` argument is passed (GH 11039):

    In [29]: df = pd.DataFrame({"A": ["a", "b", "a"], "B": [1, 2, 3]})

    In [30]: df
    Out[30]:
       A  B
    0  a  1
    1  b  2
    2  a  3

    [3 rows x 2 columns]

Previous behavior:

    In [3]: df.groupby('A', as_index=True)['B'].nth(0)
    Out[3]:
    0    1
    1    2
    Name: B, dtype: int64

    In [4]: df.groupby('A', as_index=False)['B'].nth(0)
    Out[4]:
    0    1
    1    2
    Name: B, dtype: int64

New behavior:

    In [31]: df.groupby("A", as_index=True)["B"].nth(0)
    Out[31]:
    0    1
    1    2
    Name: B, Length: 2, dtype: int64

    In [32]: df.groupby("A", as_index=False)["B"].nth(0)
    Out[32]:
    0    1
    1    2
    Name: B, Length: 2, dtype: int64

Furthermore, previously, a `.groupby` would always sort, regardless if `sort=False` was passed with `.nth()`.

    In [33]: np.random.seed(1234)

    In [34]: df = pd.DataFrame(np.random.randn(100, 2), columns=["a", "b"])

    In [35]: df["c"] = np.random.randint(0, 4, 100)

Previous behavior:

    In [4]: df.groupby('c', sort=True).nth(1)
    Out[4]:
              a         b
    c
    0 -0.334077  0.002118
    1  0.036142 -2.074978
    2 -0.720589  0.887163
    3  0.859588 -0.636524

    In [5]: df.groupby('c', sort=False).nth(1)
    Out[5]:
              a         b
    c
    0 -0.334077  0.002118
    1  0.036142 -2.074978
    2 -0.720589  0.887163
    3  0.859588 -0.636524

New behavior:

    In [36]: df.groupby("c", sort=True).nth(1)
    Out[36]:
               a         b  c
    2  -0.720589  0.887163  2
    3   0.859588 -0.636524  3
    7  -0.334077  0.002118  0
    21  0.036142 -2.074978  1

    [4 rows x 3 columns]

    In [37]: df.groupby("c", sort=False).nth(1)
    Out[37]:
               a         b  c
    2  -0.720589  0.887163  2
    3   0.859588 -0.636524  3
    7  -0.334077  0.002118  0
    21  0.036142 -2.074978  1

    [4 rows x 3 columns]

### NumPy function compatibility

Compatibility between pandas array-like methods (e.g. `sum` and `take`) and their `numpy` counterparts has been greatly increased by augmenting the signatures of the `pandas` methods so as to accept arguments that can be passed in from `numpy`, even if they are not necessarily used in the `pandas` implementation (GH 12644, GH 12638, GH 12687)

- `.searchsorted()` for `Index` and `TimedeltaIndex` now accept a `sorter` argument to maintain compatibility with numpy’s `searchsorted` function (GH 12238)

- Bug in numpy compatibility of `np.round()` on a `Series` (GH 12600)

An example of this signature augmentation is illustrated below:

    sp = pd.SparseDataFrame([1, 2, 3])
    sp

Previous behaviour:

    In [2]: np.cumsum(sp, axis=0)
    ...
    TypeError: cumsum() takes at most 2 arguments (4 given)

New behaviour:

    np.cumsum(sp, axis=0)

### Using `.apply` on GroupBy resampling

Using `apply` on resampling groupby operations (using a `pd.TimeGrouper`) now has the same output types as similar `apply` calls on other groupby operations. (GH 11742).

    In [38]: df = pd.DataFrame(
       ....:     {"date": pd.to_datetime(["10/10/2000", "11/10/2000"]), "value": [10, 13]}
       ....: )
       ....:

    In [39]: df
    Out[39]:
            date  value
    0 2000-10-10     10
    1 2000-11-10     13

    [2 rows x 2 columns]

Previous behavior:

    In [1]: df.groupby(pd.TimeGrouper(key='date',
       ...:                           freq='M')).apply(lambda x: x.value.sum())
    Out[1]:
    ...
    TypeError: cannot concatenate a non-NDFrame object

    # Output is a Series
    In [2]: df.groupby(pd.TimeGrouper(key='date',
       ...:                           freq='M')).apply(lambda x: x[['value']].sum())
    Out[2]:
    date
    2000-10-31  value    10
    2000-11-30  value    13
    dtype: int64

New behavior:

    # Output is a Series
    In [55]: df.groupby(pd.TimeGrouper(key='date',
        ...:                           freq='M')).apply(lambda x: x.value.sum())
    Out[55]:
    date
    2000-10-31    10
    2000-11-30    13
    Freq: M, dtype: int64

    # Output is a DataFrame
    In [56]: df.groupby(pd.TimeGrouper(key='date',
        ...:                           freq='M')).apply(lambda x: x[['value']].sum())
    Out[56]:
                value
    date
    2000-10-31     10
    2000-11-30     13

### Changes in `read_csv` exceptions

In order to standardize the `read_csv` API for both the `c` and `python` engines, both will now raise an `EmptyDataError`, a subclass of `ValueError`, in response to empty columns or header (GH 12493, GH 12506)

Previous behaviour:

    In [1]: import io

    In [2]: df = pd.read_csv(io.StringIO(''), engine='c')
    ...
    ValueError: No columns to parse from file

    In [3]: df = pd.read_csv(io.StringIO(''), engine='python')
    ...
    StopIteration

New behaviour:

    In [1]: df = pd.read_csv(io.StringIO(''), engine='c')
    ...
    pandas.io.common.EmptyDataError: No columns to parse from file

    In [2]: df = pd.read_csv(io.StringIO(''), engine='python')
    ...
    pandas.io.common.EmptyDataError: No columns to parse from file

In addition to this error change, several others have been made as well:

- `CParserError` now sub-classes `ValueError` instead of just a `Exception` (GH 12551)

- A `CParserError` is now raised instead of a generic `Exception` in `read_csv` when the `c` engine cannot parse a column (GH 12506)

- A `ValueError` is now raised instead of a generic `Exception` in `read_csv` when the `c` engine encounters a `NaN` value in an integer column (GH 12506)

- A `ValueError` is now raised instead of a generic `Exception` in `read_csv` when `true_values` is specified, and the `c` engine encounters an element in a column containing unencodable bytes (GH 12506)

- `pandas.parser.OverflowError` exception has been removed and has been replaced with Python’s built-in `OverflowError` exception (GH 12506)

- `pd.read_csv()` no longer allows a combination of strings and integers for the `usecols` parameter (GH 12678)

### Method `to_datetime` error changes

Bugs in `pd.to_datetime()` when passing a `unit` with convertible entries and `errors='coerce'` or non-convertible with `errors='ignore'`. Furthermore, an `OutOfBoundsDateime` exception will be raised when an out-of-range value is encountered for that unit when `errors='raise'`. (GH 11758, GH 13052, GH 13059)

Previous behaviour:

    In [27]: pd.to_datetime(1420043460, unit='s', errors='coerce')
    Out[27]: NaT

    In [28]: pd.to_datetime(11111111, unit='D', errors='ignore')
    OverflowError: Python int too large to convert to C long

    In [29]: pd.to_datetime(11111111, unit='D', errors='raise')
    OverflowError: Python int too large to convert to C long

New behaviour:

    In [2]: pd.to_datetime(1420043460, unit='s', errors='coerce')
    Out[2]: Timestamp('2014-12-31 16:31:00')

    In [3]: pd.to_datetime(11111111, unit='D', errors='ignore')
    Out[3]: 11111111

    In [4]: pd.to_datetime(11111111, unit='D', errors='raise')
    OutOfBoundsDatetime: cannot convert input with unit 'D'

### Other API changes

- `.swaplevel()` for `Series`, `DataFrame`, `Panel`, and `MultiIndex` now features defaults for its first two parameters `i` and `j` that swap the two innermost levels of the index. (GH 12934)

- `.searchsorted()` for `Index` and `TimedeltaIndex` now accept a `sorter` argument to maintain compatibility with numpy’s `searchsorted` function (GH 12238)

- `Period` and `PeriodIndex` now raises `IncompatibleFrequency` error which inherits `ValueError` rather than raw `ValueError` (GH 12615)

- `Series.apply` for category dtype now applies the passed function to each of the `.categories` (and not the `.codes`), and returns a `category` dtype if possible (GH 12473)

- `read_csv` will now raise a `TypeError` if `parse_dates` is neither a boolean, list, or dictionary (matches the doc-string) (GH 5636)

- The default for `.query()/.eval()` is now `engine=None`, which will use `numexpr` if it’s installed; otherwise it will fallback to the `python` engine. This mimics the pre-0.18.1 behavior if `numexpr` is installed (and which, previously, if numexpr was not installed, `.query()/.eval()` would raise). (GH 12749)

- `pd.show_versions()` now includes `pandas_datareader` version (GH 12740)

- Provide a proper `__name__` and `__qualname__` attributes for generic functions (GH 12021)

- `pd.concat(ignore_index=True)` now uses `RangeIndex` as default (GH 12695)

- `pd.merge()` and `DataFrame.join()` will show a `UserWarning` when merging/joining a single- with a multi-leveled dataframe (GH 9455, GH 12219)

- Compat with `scipy` > 0.17 for deprecated `piecewise_polynomial` interpolation method; support for the replacement `from_derivatives` method (GH 12887)

### Deprecations

- The method name `Index.sym_diff()` is deprecated and can be replaced by `Index.symmetric_difference()` (GH 12591)

- The method name `Categorical.sort()` is deprecated in favor of `Categorical.sort_values()` (GH 12882)

## Performance improvements

- Improved speed of SAS reader (GH 12656, GH 12961)

- Performance improvements in `.groupby(..).cumcount()` (GH 11039)

- Improved memory usage in `pd.read_csv()` when using `skiprows=an_integer` (GH 13005)

- Improved performance of `DataFrame.to_sql` when checking case sensitivity for tables. Now only checks if table has been created correctly when table name is not lower case. (GH 12876)

- Improved performance of `Period` construction and time series plotting (GH 12903, GH 11831).

- Improved performance of `.str.encode()` and `.str.decode()` methods (GH 13008)

- Improved performance of `to_numeric` if input is numeric dtype (GH 12777)

- Improved performance of sparse arithmetic with `IntIndex` (GH 13036)

## Bug fixes

- `usecols` parameter in `pd.read_csv` is now respected even when the lines of a CSV file are not even (GH 12203)

- Bug in `groupby.transform(..)` when `axis=1` is specified with a non-monotonic ordered index (GH 12713)

- Bug in `Period` and `PeriodIndex` creation raises `KeyError` if `freq="Minute"` is specified. Note that “Minute” freq is deprecated in v0.17.0, and recommended to use `freq="T"` instead (GH 11854)

- Bug in `.resample(...).count()` with a `PeriodIndex` always raising a `TypeError` (GH 12774)

- Bug in `.resample(...)` with a `PeriodIndex` casting to a `DatetimeIndex` when empty (GH 12868)

- Bug in `.resample(...)` with a `PeriodIndex` when resampling to an existing frequency (GH 12770)

- Bug in printing data which contains `Period` with different `freq` raises `ValueError` (GH 12615)

- Bug in `Series` construction with `Categorical` and `dtype='category'` is specified (GH 12574)

- Bugs in concatenation with a coercible dtype was too aggressive, resulting in different dtypes in output formatting when an object was longer than `display.max_rows` (GH 12411, GH 12045, GH 11594, GH 10571, GH 12211)

- Bug in `float_format` option with option not being validated as a callable. (GH 12706)

- Bug in `GroupBy.filter` when `dropna=False` and no groups fulfilled the criteria (GH 12768)

- Bug in `__name__` of `.cum*` functions (GH 12021)

- Bug in `.astype()` of a `Float64Inde/Int64Index` to an `Int64Index` (GH 12881)

- Bug in round tripping an integer based index in `.to_json()/.read_json()` when `orient='index'` (the default) (GH 12866)

- Bug in plotting `Categorical` dtypes cause error when attempting stacked bar plot (GH 13019)

- Compat with >= `numpy` 1.11 for `NaT` comparisons (GH 12969)

- Bug in `.drop()` with a non-unique `MultiIndex`. (GH 12701)

- Bug in `.concat` of datetime tz-aware and naive DataFrames (GH 12467)

- Bug in correctly raising a `ValueError` in `.resample(..).fillna(..)` when passing a non-string (GH 12952)

- Bug fixes in various encoding and header processing issues in `pd.read_sas()` (GH 12659, GH 12654, GH 12647, GH 12809)

- Bug in `pd.crosstab()` where would silently ignore `aggfunc` if `values=None` (GH 12569).

- Potential segfault in `DataFrame.to_json` when serialising `datetime.time` (GH 11473).

- Potential segfault in `DataFrame.to_json` when attempting to serialise 0d array (GH 11299).

- Segfault in `to_json` when attempting to serialise a `DataFrame` or `Series` with non-ndarray values; now supports serialization of `category`, `sparse`, and `datetime64[ns, tz]` dtypes (GH 10778).

- Bug in `DataFrame.to_json` with unsupported dtype not passed to default handler (GH 12554).

- Bug in `.align` not returning the sub-class (GH 12983)

- Bug in aligning a `Series` with a `DataFrame` (GH 13037)

- Bug in `ABCPanel` in which `Panel4D` was not being considered as a valid instance of this generic type (GH 12810)

- Bug in consistency of `.name` on `.groupby(..).apply(..)` cases (GH 12363)

- Bug in `Timestamp.__repr__` that caused `pprint` to fail in nested structures (GH 12622)

- Bug in `Timedelta.min` and `Timedelta.max`, the properties now report the true minimum/maximum `timedeltas` as recognized by pandas. See the documentation. (GH 12727)

- Bug in `.quantile()` with interpolation may coerce to `float` unexpectedly (GH 12772)

- Bug in `.quantile()` with empty `Series` may return scalar rather than empty `Series` (GH 12772)

- Bug in `.loc` with out-of-bounds in a large indexer would raise `IndexError` rather than `KeyError` (GH 12527)

- Bug in resampling when using a `TimedeltaIndex` and `.asfreq()`, would previously not include the final fencepost (GH 12926)

- Bug in equality testing with a `Categorical` in a `DataFrame` (GH 12564)

- Bug in `GroupBy.first()`, `.last()` returns incorrect row when `TimeGrouper` is used (GH 7453)

- Bug in `pd.read_csv()` with the `c` engine when specifying `skiprows` with newlines in quoted items (GH 10911, GH 12775)

- Bug in `DataFrame` timezone lost when assigning tz-aware datetime `Series` with alignment (GH 12981)

- Bug in `.value_counts()` when `normalize=True` and `dropna=True` where nulls still contributed to the normalized count (GH 12558)

- Bug in `Series.value_counts()` loses name if its dtype is `category` (GH 12835)

- Bug in `Series.value_counts()` loses timezone info (GH 12835)

- Bug in `Series.value_counts(normalize=True)` with `Categorical` raises `UnboundLocalError` (GH 12835)

- Bug in `Panel.fillna()` ignoring `inplace=True` (GH 12633)

- Bug in `pd.read_csv()` when specifying `names`, `usecols`, and `parse_dates` simultaneously with the `c` engine (GH 9755)

- Bug in `pd.read_csv()` when specifying `delim_whitespace=True` and `lineterminator` simultaneously with the `c` engine (GH 12912)

- Bug in `Series.rename`, `DataFrame.rename` and `DataFrame.rename_axis` not treating `Series` as mappings to relabel (GH 12623).

- Clean in `.rolling.min` and `.rolling.max` to enhance dtype handling (GH 12373)

- Bug in `groupby` where complex types are coerced to float (GH 12902)

- Bug in `Series.map` raises `TypeError` if its dtype is `category` or tz-aware `datetime` (GH 12473)

- Bugs on 32bit platforms for some test comparisons (GH 12972)

- Bug in index coercion when falling back from `RangeIndex` construction (GH 12893)

- Better error message in window functions when invalid argument (e.g. a float window) is passed (GH 12669)

- Bug in slicing subclassed `DataFrame` defined to return subclassed `Series` may return normal `Series` (GH 11559)

- Bug in `.str` accessor methods may raise `ValueError` if input has `name` and the result is `DataFrame` or `MultiIndex` (GH 12617)

- Bug in `DataFrame.last_valid_index()` and `DataFrame.first_valid_index()` on empty frames (GH 12800)

- Bug in `CategoricalIndex.get_loc` returns different result from regular `Index` (GH 12531)

- Bug in `PeriodIndex.resample` where name not propagated (GH 12769)

- Bug in `date_range` `closed` keyword and timezones (GH 12684).

- Bug in `pd.concat` raises `AttributeError` when input data contains tz-aware datetime and timedelta (GH 12620)

- Bug in `pd.concat` did not handle empty `Series` properly (GH 11082)

- Bug in `.plot.bar` alignment when `width` is specified with `int` (GH 12979)

- Bug in `fill_value` is ignored if the argument to a binary operator is a constant (GH 12723)

- Bug in `pd.read_html()` when using bs4 flavor and parsing table with a header and only one column (GH 9178)

- Bug in `.pivot_table` when `margins=True` and `dropna=True` where nulls still contributed to margin count (GH 12577)

- Bug in `.pivot_table` when `dropna=False` where table index/column names disappear (GH 12133)

- Bug in `pd.crosstab()` when `margins=True` and `dropna=False` which raised (GH 12642)

- Bug in `Series.name` when `name` attribute can be a hashable type (GH 12610)

- Bug in `.describe()` resets categorical columns information (GH 11558)

- Bug where `loffset` argument was not applied when calling `resample().count()` on a timeseries (GH 12725)

- `pd.read_excel()` now accepts column names associated with keyword argument `names` (GH 12870)

- Bug in `pd.to_numeric()` with `Index` returns `np.ndarray`, rather than `Index` (GH 12777)

- Bug in `pd.to_numeric()` with datetime-like may raise `TypeError` (GH 12777)

- Bug in `pd.to_numeric()` with scalar raises `ValueError` (GH 12777)

## Contributors

A total of 60 people contributed patches to this release. People with a “+” by their names contributed a patch for the first time.

- Andrew Fiore-Gartland +

- Bastiaan +

- Benoît Vinot +

- Brandon Rhodes +

- DaCoEx +

- Drew Fustin +

- Ernesto Freitas +

- Filip Ter +

- Gregory Livschitz +

- Gábor Lipták

- Hassan Kibirige +

- Iblis Lin

- Israel Saeta Pérez +

- Jason Wolosonovich +

- Jeff Reback

- Joe Jevnik

- Joris Van den Bossche

- Joshua Storck +

- Ka Wo Chen

- Kerby Shedden

- Kieran O’Mahony

- Leif Walsh +

- Mahmoud Lababidi +

- Maoyuan Liu +

- Mark Roth +

- Matt Wittmann

- MaxU +

- Maximilian Roos

- Michael Droettboom +

- Nick Eubank

- Nicolas Bonnotte

- OXPHOS +

- Pauli Virtanen +

- Peter Waller +

- Pietro Battiston

- Prabhjot Singh +

- Robin Wilson

- Roger Thomas +

- Sebastian Bank

- Stephen Hoover

- Tim Hopper +

- Tom Augspurger

- WANG Aiyong

- Wes Turner

- Winand +

- Xbar +

- Yan Facai +

- adneu +

- ajenkins-cargometrics +

- behzad nouri

- chinskiy +

- gfyoung

- jeps-journal +

- jonaslb +

- kotrfa +

- nileracecrew +

- onesandzeroes

- rs2 +

- sinhrks

- tsdlovell +