converters

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 `attr.Factory` is supported, however the ``takes_self`` option
       is *not*.
    :param callable factory: A callable that takes no 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 `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

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

    Type annotations will be inferred from the wrapped converter's, if it
    has any.

    :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)

    if not PY2:
        sig = None
        try:
            sig = inspect.signature(converter)
        except (ValueError, TypeError):  # inspect failed
            pass
        if sig:
            params = list(sig.parameters.values())
            if params and params[0].annotation is not inspect.Parameter.empty:
                optional_converter.__annotations__["val"] = typing.Optional[
                    params[0].annotation
                ]
            if sig.return_annotation is not inspect.Signature.empty:
                optional_converter.__annotations__["return"] = typing.Optional[
                    sig.return_annotation
                ]

    return optional_converter

def pipe(*converters):
    """
    A converter that composes multiple converters into one.

    When called on a value, it runs all wrapped converters, returning the
    *last* value.

    Type annotations will be inferred from the wrapped converters', if
    they have any.

    :param callables converters: Arbitrary number of converters.

    .. versionadded:: 20.1.0
    """

    def pipe_converter(val):
        for converter in converters:
            val = converter(val)

        return val

    if not PY2:
        if not converters:
            # If the converter list is empty, pipe_converter is the identity.
            A = typing.TypeVar("A")
            pipe_converter.__annotations__ = {"val": A, "return": A}
        else:
            # Get parameter type.
            sig = None
            try:
                sig = inspect.signature(converters[0])
            except (ValueError, TypeError):  # inspect failed
                pass
            if sig:
                params = list(sig.parameters.values())
                if (
                    params
                    and params[0].annotation is not inspect.Parameter.empty
                ):
                    pipe_converter.__annotations__["val"] = params[
                        0
                    ].annotation
            # Get return type.
            sig = None
            try:
                sig = inspect.signature(converters[-1])
            except (ValueError, TypeError):  # inspect failed
                pass
            if sig and sig.return_annotation is not inspect.Signature().empty:
                pipe_converter.__annotations__[
                    "return"
                ] = sig.return_annotation

    return pipe_converter