@@ -220,6 +220,12 @@ _options = [
             "'plotting.sample_ratio' should be 1.0 >= value >= 0.0.",
         ),
     ),
+    Option(
+        key="plotting.backend",
+        doc=("Backend to use for plotting. Default is matplotlib."),
+        default="matplotlib",
+        types=str,
+    ),
 ]  # type: List[Option]
 
 _options_dict = dict(zip((option.key for option in _options), _options))  # type: Dict[str, Option]
#
# Copyright (C) 2019 Databricks, Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

"""
Infrastructure of options for Koalas.
"""
from contextlib import contextmanager
import json
from typing import Union, Any, Tuple, Callable, List, Dict

from pyspark._globals import _NoValue, _NoValueType

from databricks.koalas.utils import default_session


__all__ = ["get_option", "set_option", "reset_option", "options", "option_context"]


class Option:
    """
    Option class that defines an option with related properties.

    This class holds all information relevant to the one option. Also,
    Its instance can validate if the given value is acceptable or not.

    It is currently for internal usage only.

    Parameters
    ----------
    key: str, keyword-only argument
        the option name to use.
    doc: str, keyword-only argument
        the documentation for the current option.
    default: Any, keyword-only argument
        default value for this option.
    types: Union[Tuple[type, ...], type], keyword-only argument
        default is str. It defines the expected types for this option. It is
        used with `isinstance` to validate the given value to this option.
    check_func: Tuple[Callable[[Any], bool], str], keyword-only argument
        default is a function that always returns `True` with a empty string.
        It defines:
          - a function to check the given value to this option
          - the error message to show when this check is failed
        When new value is set to this option, this function is called to check
        if the given value is valid.

    Examples
    --------
    >>> option = Option(
    ...     key='option.name',
    ...     doc="this is a test option",
    ...     default="default",
    ...     types=(float, int),
    ...     check_func=(lambda v: v > 0, "should be a positive float"))

    >>> option.validate('abc')  # doctest: +NORMALIZE_WHITESPACE
    Traceback (most recent call last):
      ...
    ValueError: The value for option 'option.name' was <class 'str'>;
    however, expected types are [(<class 'float'>, <class 'int'>)].

    >>> option.validate(-1.1)
    Traceback (most recent call last):
      ...
    ValueError: should be a positive float

    >>> option.validate(1.1)
    """

    def __init__(
        self,
        *,
        key: str,
        doc: str,
        default: Any,
        types: Union[Tuple[type, ...], type] = str,
        check_func: Tuple[Callable[[Any], bool], str] = (lambda v: True, "")
    ):
        self.key = key
        self.doc = doc
        self.default = default
        self.types = types
        self.check_func = check_func

    def validate(self, v: Any) -> None:
        """
        Validate the given value and throw an exception with related information such as key.
        """
        if not isinstance(v, self.types):
            raise ValueError(
                "The value for option '%s' was %s; however, expected types are "
                "[%s]." % (self.key, type(v), str(self.types))
            )
        if not self.check_func[0](v):
            raise ValueError(self.check_func[1])


# Available options.
#
# NOTE: if you are fixing or adding an option here, make sure you execute `show_options()` and
#     copy & paste the results into show_options 'docs/source/user_guide/options.rst' as well.
#     See the examples below:
#     >>> from databricks.koalas.config import show_options
#     >>> show_options()
_options = [
    Option(
        key="display.max_rows",
        doc=(
            "This sets the maximum number of rows Koalas should output when printing out "
            "various output. For example, this value determines the number of rows to be "
            "shown at the repr() in a dataframe. Set `None` to unlimit the input length. "
            "Default is 1000."
        ),
        default=1000,
        types=(int, type(None)),
        check_func=(
            lambda v: v is None or v >= 0,
            "'display.max_rows' should be greater than or equal to 0.",
        ),
    ),
    Option(
        key="compute.max_rows",
        doc=(
            "'compute.max_rows' sets the limit of the current DataFrame. Set `None` to unlimit "
            "the input length. When the limit is set, it is executed by the shortcut by "
            "collecting the data into driver side, and then using pandas API. If the limit is "
            "unset, the operation is executed by PySpark. Default is 1000."
        ),
        default=1000,
        types=(int, type(None)),
        check_func=(
            lambda v: v is None or v >= 0,
            "'compute.max_rows' should be greater than or equal to 0.",
        ),
    ),
    Option(
        key="compute.shortcut_limit",
        doc=(
            "'compute.shortcut_limit' sets the limit for a shortcut. "
            "It computes specified number of rows and use its schema. When the dataframe "
            "length is larger than this limit, Koalas uses PySpark to compute."
        ),
        default=1000,
        types=int,
        check_func=(
            lambda v: v >= 0,
            "'compute.shortcut_limit' should be greater than or equal to 0.",
        ),
    ),
    Option(
        key="compute.ops_on_diff_frames",
        doc=(
            "This determines whether or not to operate between two different dataframes. "
            "For example, 'combine_frames' function internally performs a join operation which "
            "can be expensive in general. So, if `compute.ops_on_diff_frames` variable is not "
            "True, that method throws an exception."
        ),
        default=False,
        types=bool,
    ),
    Option(
        key="compute.default_index_type",
        doc=("This sets the default index type: sequence, distributed and distributed-sequence."),
        default="sequence",
        types=str,
        check_func=(
            lambda v: v in ("sequence", "distributed", "distributed-sequence"),
            "Index type should be one of 'sequence', 'distributed', 'distributed-sequence'.",
        ),
    ),
    Option(
        key="compute.ordered_head",
        doc=(
            "'compute.ordered_head' sets whether or not to operate head with natural ordering. "
            "Koalas does not guarantee the row ordering so `head` could return some rows from "
            "distributed partitions. If 'compute.ordered_head' is set to True, Koalas performs "
            "natural ordering beforehand, but it will cause a performance overhead."
        ),
        default=False,
        types=bool,
    ),
    Option(
        key="plotting.max_rows",
        doc=(
            "'plotting.max_rows' sets the visual limit on top-n-based plots such as `plot.bar` "
            "and `plot.pie`. If it is set to 1000, the first 1000 data points will be used "
            "for plotting. Default is 1000."
        ),
        default=1000,
        types=int,
        check_func=(
            lambda v: v is v >= 0,
            "'plotting.max_rows' should be greater than or equal to 0.",
        ),
    ),
    Option(
        key="plotting.sample_ratio",
        doc=(
            "'plotting.sample_ratio' sets the proportion of data that will be plotted for sample-"
            "based plots such as `plot.line` and `plot.area`. "
            "This option defaults to 'plotting.max_rows' option."
        ),
        default=None,
        types=(float, type(None)),
        check_func=(
            lambda v: v is None or 1 >= v >= 0,
            "'plotting.sample_ratio' should be 1.0 >= value >= 0.0.",
        ),
    ),
]  # type: List[Option]

_options_dict = dict(zip((option.key for option in _options), _options))  # type: Dict[str, Option]

_key_format = "koalas.{}".format


class OptionError(AttributeError, KeyError):
    pass


def show_options():
    """
    Make a pretty table that can be copied and pasted into public documentation.
    This is currently for an internal purpose.

    Examples
    --------
    >>> show_options()  # doctest: +ELLIPSIS, +NORMALIZE_WHITESPACE
    ================... =======... =====================...
    Option              Default    Description
    ================... =======... =====================...
    display.max_rows    1000       This sets the maximum...
    ...
    ================... =======... =====================...
    """

    import textwrap

    header = ["Option", "Default", "Description"]
    row_format = "{:<31} {:<14} {:<53}"

    print(row_format.format("=" * 31, "=" * 14, "=" * 53))
    print(row_format.format(*header))
    print(row_format.format("=" * 31, "=" * 14, "=" * 53))

    for option in _options:
        doc = textwrap.fill(option.doc, 53)
        formatted = "".join([line + "\n" + (" " * 47) for line in doc.split("\n")]).rstrip()
        print(row_format.format(option.key, repr(option.default), formatted))

    print(row_format.format("=" * 31, "=" * 14, "=" * 53))


def get_option(key: str, default: Union[Any, _NoValueType] = _NoValue) -> Any:
    """
    Retrieves the value of the specified option.

    Parameters
    ----------
    key : str
        The key which should match a single option.
    default : object
        The default value if the option is not set yet. The value should be JSON serializable.

    Returns
    -------
    result : the value of the option

    Raises
    ------
    OptionError : if no such option exists and the default is not provided
    """
    _check_option(key)
    if default is _NoValue:
        default = _options_dict[key].default
    _options_dict[key].validate(default)

    return json.loads(default_session().conf.get(_key_format(key), default=json.dumps(default)))


def set_option(key: str, value: Any) -> None:
    """
    Sets the value of the specified option.

    Parameters
    ----------
    key : str
        The key which should match a single option.
    value : object
        New value of option. The value should be JSON serializable.

    Returns
    -------
    None
    """
    _check_option(key)
    _options_dict[key].validate(value)

    default_session().conf.set(_key_format(key), json.dumps(value))


def reset_option(key: str) -> None:
    """
    Reset one option to their default value.

    Pass "all" as argument to reset all options.

    Parameters
    ----------
    key : str
        If specified only option will be reset.

    Returns
    -------
    None
    """
    _check_option(key)
    default_session().conf.unset(_key_format(key))


@contextmanager
def option_context(*args):
    """
    Context manager to temporarily set options in the `with` statement context.

    You need to invoke as ``option_context(pat, val, [(pat, val), ...])``.

    Examples
    --------
    >>> with option_context('display.max_rows', 10, 'compute.max_rows', 5):
    ...     print(get_option('display.max_rows'), get_option('compute.max_rows'))
    10 5
    >>> print(get_option('display.max_rows'), get_option('compute.max_rows'))
    1000 1000
    """
    if len(args) == 0 or len(args) % 2 != 0:
        raise ValueError("Need to invoke as option_context(pat, val, [(pat, val), ...]).")
    opts = dict(zip(args[::2], args[1::2]))
    orig_opts = {key: get_option(key) for key in opts}
    try:
        for key, value in opts.items():
            set_option(key, value)
        yield
    finally:
        for key, value in orig_opts.items():
            set_option(key, value)


def _check_option(key: str) -> None:
    if key not in _options_dict:
        raise OptionError(
            "No such option: '{}'. Available options are [{}]".format(
                key, ", ".join(list(_options_dict.keys()))
            )
        )


class DictWrapper:
    """ provide attribute-style access to a nested dict"""

    def __init__(self, d, prefix=""):
        object.__setattr__(self, "d", d)
        object.__setattr__(self, "prefix", prefix)

    def __setattr__(self, key, val):
        prefix = object.__getattribute__(self, "prefix")
        d = object.__getattribute__(self, "d")
        if prefix:
            prefix += "."
        canonical_key = prefix + key

        candidates = [
            k for k in d.keys() if all(x in k.split(".") for x in canonical_key.split("."))
        ]
        if len(candidates) == 1 and candidates[0] == canonical_key:
            return set_option(canonical_key, val)
        else:
            raise OptionError(
                "No such option: '{}'. Available options are [{}]".format(
                    key, ", ".join(list(_options_dict.keys()))
                )
            )

    def __getattr__(self, key):
        prefix = object.__getattribute__(self, "prefix")
        d = object.__getattribute__(self, "d")
        if prefix:
            prefix += "."
        canonical_key = prefix + key

        candidates = [
            k for k in d.keys() if all(x in k.split(".") for x in canonical_key.split("."))
        ]
        if len(candidates) == 1 and candidates[0] == canonical_key:
            return get_option(canonical_key)
        elif len(candidates) == 0:
            raise OptionError(
                "No such option: '{}'. Available options are [{}]".format(
                    key, ", ".join(list(_options_dict.keys()))
                )
            )
        else:
            return DictWrapper(d, canonical_key)

    def __dir__(self):
        prefix = object.__getattribute__(self, "prefix")
        d = object.__getattribute__(self, "d")

        if prefix == "":
            candidates = d.keys()
            offset = 0
        else:
            candidates = [k for k in d.keys() if all(x in k.split(".") for x in prefix.split("."))]
            offset = len(prefix) + 1  # prefix (e.g. "compute.") to trim.
        return [c[offset:] for c in candidates]


options = DictWrapper(_options_dict)

Could you add a list of currently supported backends?