src/attr/converters.py

"""
Commonly useful converters.
"""

from __future__ import absolute_import, division, print_function

from ._make import NOTHING, Factory


def optional(converter):
    """
    A converter that allows an attribute to be optional. An optional attribute
    is one which can be set to ``None``.

    :param callable converter: the converter that is used for non-``None``
        values.

    .. versionadded:: 17.1.0
    """

    def optional_converter(val):
        if val is None:
            return None
        return converter(val)

    return optional_converter


def default_if_none(default=NOTHING, factory=None):
    """
    A converter that allows to replace ``None`` values by *default* or the
    result of *factory*.

    :param default: Value to be used if ``None`` is passed. Passing an instance
       of :class:`attr.Factory` is supported, however the ``takes_self`` option
       is *not*.
    :param callable factory: A callable that takes not parameters whose result
       is used if ``None`` is passed.

    :raises TypeError: If **neither** *default* or *factory* is passed.
    :raises TypeError: If **both** *default* and *factory* are passed.
    :raises ValueError: If an instance of :class:`attr.Factory` is passed with
       ``takes_self=True``.

    .. versionadded:: 18.2.0
    """
    if default is NOTHING and factory is None:
        raise TypeError("Must pass either `default` or `factory`.")

    if default is not NOTHING and factory is not None:
        raise TypeError(
            "Must pass either `default` or `factory` but not both."
        )

    if factory is not None:
        default = Factory(factory)

    if isinstance(default, Factory):
        if default.takes_self:
            raise ValueError(
                "`takes_self` is not supported by default_if_none."
            )

        def default_if_none_converter(val):
            if val is not None:
                return val

            return default.factory()

    else:

        def default_if_none_converter(val):
            if val is not None:
                return val

            return default

    return default_if_none_converter