1"""
2Commonly useful converters.
3"""
4
5from __future__ import absolute_import, division, print_function
6
7from ._make import NOTHING, Factory
8
9
10def optional(converter):
11    """
12    A converter that allows an attribute to be optional. An optional attribute
13    is one which can be set to ``None``.
14
15    :param callable converter: the converter that is used for non-``None``
16        values.
17
18    .. versionadded:: 17.1.0
19    """
20
21    def optional_converter(val):
22        if val is None:
23            return None
24        return converter(val)
25
26    return optional_converter
27
28
29def default_if_none(default=NOTHING, factory=None):
30    """
31    A converter that allows to replace ``None`` values by *default* or the
32    result of *factory*.
33
34    :param default: Value to be used if ``None`` is passed. Passing an instance
35       of :class:`attr.Factory` is supported, however the ``takes_self`` option
36       is *not*.
37    :param callable factory: A callable that takes not parameters whose result
38       is used if ``None`` is passed.
39
40    :raises TypeError: If **neither** *default* or *factory* is passed.
41    :raises TypeError: If **both** *default* and *factory* are passed.
42    :raises ValueError: If an instance of :class:`attr.Factory` is passed with
43       ``takes_self=True``.
44
45    .. versionadded:: 18.2.0
46    """
47    if default is NOTHING and factory is None:
48        raise TypeError("Must pass either `default` or `factory`.")
49
50    if default is not NOTHING and factory is not None:
51        raise TypeError(
52            "Must pass either `default` or `factory` but not both."
53        )
54
55    if factory is not None:
56        default = Factory(factory)
57
58    if isinstance(default, Factory):
59        if default.takes_self:
60            raise ValueError(
61                "`takes_self` is not supported by default_if_none."
62            )
63
64        def default_if_none_converter(val):
65            if val is not None:
66                return val
67
68            return default.factory()
69
70    else:
71
72        def default_if_none_converter(val):
73            if val is not None:
74                return val
75
76            return default
77
78    return default_if_none_converter
79