param.ListSelector#

class param.ListSelector(default=None, *, objects=[], instantiate=False, compute_default_fn=None, check_on_set=None, allow_None=None, empty_default=False, doc=None, label=None, precedence=None, constant=False, readonly=False, pickle_default_value=True, per_instance=True, allow_refs=False, nested_refs=False, default_factory=None, metadata=None)[source]#

Variant of Selector where the value can be multiple objects from a list of possible objects.

__init__(default=None, *, objects=[], instantiate=False, compute_default_fn=None, check_on_set=None, allow_None=None, empty_default=False, doc=None, label=None, precedence=None, constant=False, readonly=False, pickle_default_value=True, per_instance=True, allow_refs=False, nested_refs=False, default_factory=None, metadata=None)[source]#

Initialize a new Parameter object with the specified attributes.

Parameters:

  • default (Any, optional) – The owning class’s value for the attribute represented by this parameter, which can be overridden in an instance. Default is None.

  • default_factory (callable, optional) –

    A callable used to generate the attribute value. It can be provided directly (in which case it is invoked with no arguments) or wrapped in a DefaultFactory for more advanced behavior (in which case it receives cls, self, and parameter as arguments). When specified, default_factory takes precedence over default. The factory is invoked once the Parameterized instance is initialized.

    Added in version 2.3.0.

  • doc (str | None, optional) – A documentation string describing the purpose of the parameter. Default is None.

  • label (str | None, optional) – An optional text label used when displaying this parameter, such as in a listing. If not specified, the parameter’s attribute name in the owning Parameterized object is used.

  • precedence (float | None, optional) – A numeric value, usually in the range 0.0 to 1.0, that determines the order of the parameter in a listing or user interface. A negative precedence indicates a parameter that should be hidden in such listings. Default is None.

  • instantiate (bool, optional) – Whether the default value of this parameter will be deepcopied when a Parameterized object is instantiated (True), or if the single default value will be shared by all Parameterized instances (False, the default). For an immutable Parameter value, it is best to leave instantiate at the default of False, so that a user can choose to change the value at the Parameterized instance level (affecting only that instance) or at the Parameterized class or superclass level (affecting all existing and future instances of that class or superclass). For a mutable Parameter value, the default of False is also appropriate if you want all instances to share the same value state, e.g. if they are each simply referring to a single global object like a singleton. If instead each Parameterized should have its own independently mutable value, instantiate should be set to True, but note that there is then no simple way to change the value of this parameter at the class or superclass level, because each instance, once created, will then have an independently deepcopied value. Default is False.

  • constant (bool, optional) – If True, the parameter value can only be set at the class level or in a Parameterized constructor call. The value is otherwise constant on the Parameterized instance, once it has been constructed. Default is False.

  • readonly (bool, optional) – If True, the parameter value cannot be modified at the class or instance level. Default is False.

  • pickle_default_value (bool, optional) –

    Whether the default value should be pickled. Set to False in rare cases, such as system-specific file paths.

    Deprecated since version 2.3.0.

  • allow_None (bool, optional) – If True, allows None as a valid parameter value. If the default value is None, this is automatically set to True. Default is False.

  • per_instance (bool, optional) – Whether to create a separate Parameter object for each instance of the Parameterized class (True), or share the same Parameter object across all instances (False). See also instantiate, which is conceptually similar but affects the parameter value rather than the parameter object. Default is True.

  • allow_refs (bool, optional) – If True, allows linking parameter references to this parameter, meaning the value will automatically reflect the current value of the reference that is passed in. Default is False.

  • nested_refs (bool, optional) – If True and allow_refs=True, inspects nested objects (e.g., dictionaries, lists, slices, tuples) for references and resolves them automatically. Default is False.

  • metadata (Mapping, optional) –

    An arbitrary mapping, to be used to store additional metadata than cannot be declared with the other attributes. Useful for third-party libraries to extend Param. Default is None.

    Added in version 2.3.0.

Examples

Define a parameter with a default value:

>>> import param
>>> class MyClass(param.Parameterized):
...     my_param = param.Parameter(default=10, doc="An example parameter.")
>>> instance = MyClass()
>>> instance.my_param
10

Use a constant parameter:

>>> class ConstantExample(param.Parameterized):
...     my_param = param.Parameter(default=5, constant=True)
>>> instance = ConstantExample()
>>> instance.my_param = 10  # Raises an error
...
TypeError: Constant parameter 'my_param' cannot be modified.

Methods

__init__([default, objects, instantiate, ...])

Initialize a new Parameter object with the specified attributes.

compute_default()

If this parameter's compute_default_fn is callable, call it and store the result in self.default.

deserialize(value)

Given a serializable Python value, return a value that the parameter can be set to.

get_range()

Return the possible objects to which this parameter could be set.

schema([safe, subset, mode])

Generate a schema for the parameters of the Parameterized object.

serialize(value)

Given the parameter value, return a Python value suitable for serialization.

Attributes

allow_None

allow_refs

check_on_set

compute_default_fn

constant

default

default_factory

doc

instantiate

label

Get the label for this parameter.

metadata

name

names

nested_refs

objects

owner

per_instance

pickle_default_value

precedence

readonly

rx

The reactive operations namespace.

watchers