Source code for lena.context.context

""":class:`Context` provides a better representation for context."""
import functools
import json

import lena.core
import lena.flow


class Context(dict):
    """Dictionary with easy-to-read formatting.

    :class:`Context` provides a better representation for context. Example:

    >>> from lena.context import Context
    >>> c = Context({"1": 1, "2": {"3": 4}})
    >>> print(c) # doctest: +NORMALIZE_WHITESPACE
    {
        "1": 1,
        "2": {
            "3": 4
        }
    }
    """

    def __init__(self, d=None, formatter=None):
        """Initialize from a dictionary *d* (empty by default).

        Representation is defined by the *formatter*.
        That must be a callable accepting a dictionary
        and returning a string. The default is ``json.dumps``.

        All public attributes of a :class:`Context`
        can be got or set using dot notation
        (for example, *context["data_path"]*
        is equal to *context.data_path*).
        Only one level of nesting is accessible using dot notation.

        Tip
        ---
            `JSON <https://docs.python.org/3/library/json.html>`_
            and Python representations are different.
            In particular, JSON *True* is written as lowercase *true*.
            To convert JSON back to Python, use ``json.loads(string)``.

        If *formatter* is given but is not callable,
        :exc:`.LenaTypeError` is raised.
        If the attribute to be got is missing,
        :exc:`.LenaAttributeError` is raised.
        An attempt to get a private attribute raises
        :exc:`AttributeError`.
        """
        if d is None:
            d = {}
        super(Context, self).__init__(d)
        if formatter is not None:
            if not callable(formatter):
                raise lena.core.LenaTypeError(
                    "formatter must be callable, "
                    "{} given".format(formatter)
                )
            self._formatter = formatter
        else:
            self._formatter = functools.partial(json.dumps,
                                                sort_keys=True, indent=4)
            # same, but doesn't allow pickling
            # lambda s: json.dumps(s, sort_keys=True, indent=4)
        # formatter should be private,
        # otherwise it'll mess with other attributes
        # self._formatter = pprint.PrettyPrinter(indent=1)

    def __call__(self, value):
        """Convert *value*'s context to :class:`Context` on the fly.

        If the *value* is a *(data, context)* pair,
        convert its context part to :class:`Context`.
        If the *value* doesn't contain a context,
        it is created as an empty :class:`Context`.

        When a :class:`Context` is used as a sequence element,
        its initialization argument *d*
        has no effect on the produced values.
        """
        data, context = lena.flow.get_data_context(value)
        return (data, Context(context))

    def __getattr__(self, name):
        # we don't implement getting nested attributes,
        # because that would require creating proxy objects
        # -- maybe in the future if needed.
        if name.startswith('_'):
            # this is not LenaAttributeError,
            # as it wouldn't be so for other Lena classes
            # that don't implement __getattr__ .
            # See comment for Variable.
            raise AttributeError(name)
        try:
            return self[name]
        except KeyError:
            raise lena.core.LenaAttributeError(
                "{} missing".format(name)
            )

    def __repr__(self):
        return self._formatter(self)

    def __setattr__(self, attr, value):
        if attr in ["_formatter"]:
            # from https://stackoverflow.com/a/17020163/952234
            super(Context, self).__setattr__(attr, value)
        elif attr.startswith('_'):
            raise AttributeError(attr)
        else:
            self[attr] = value