Class List

traitlets/traitlets.py:3588–3650

An instance of a Python list.

class List(Container[t.List[T]]):
    """An instance of a Python list."""

    klass = list  # type:ignore[assignment]
    _cast_types: t.Any = (tuple,)

    def __init__(
        self,
        trait: t.List[T] | t.Tuple[T] | t.Set[T] | Sentinel | TraitType[T, t.Any] | None = None,
        default_value: t.List[T] | t.Tuple[T] | t.Set[T] | Sentinel | None = Undefined,
        minlen: int = 0,
        maxlen: int = sys.maxsize,
        **kwargs: t.Any,
    ) -> None:
        """Create a List trait type from a list, set, or tuple.

        The default value is created by doing ``list(default_value)``,
        which creates a copy of the ``default_value``.

        ``trait`` can be specified, which restricts the type of elements
        in the container to that TraitType.

        If only one arg is given and it is not a Trait, it is taken as
        ``default_value``:

        ``c = List([1, 2, 3])``

        Parameters
        ----------
        trait : TraitType [ optional ]
            the type for restricting the contents of the Container.
            If unspecified, types are not checked.
        default_value : SequenceType [ optional ]
            The default value for the Trait.  Must be list/tuple/set, and
            will be cast to the container type.
        minlen : Int [ default 0 ]
            The minimum length of the input list
        maxlen : Int [ default sys.maxsize ]
            The maximum length of the input list
        """
        self._maxlen = maxlen
        self._minlen = minlen
        super().__init__(trait=trait, default_value=default_value, **kwargs)

    def length_error(self, obj: t.Any, value: t.Any) -> None:
        e = (
            "The '%s' trait of %s instance must be of length %i <= L <= %i, but a value of %s was specified."
            % (self.name, class_of(obj), self._minlen, self._maxlen, value)
        )
        raise TraitError(e)

    def validate_elements(self, obj: t.Any, value: t.Any) -> t.Any:
        length = len(value)
        if length < self._minlen or length > self._maxlen:
            self.length_error(obj, value)

        return super().validate_elements(obj, value)