Default values

Default values can easily be used when creating parameterNodeWrappers and parameterPacks by simply assigning a value to the parameter.


from typing import Annotated
from slicer.parameterNodeWrapper import parameterNodeWrapper

class ParameterNodeWrapper:
    iterations: int = 50
    text: str = "abc"
    tup: tuple[int, bool] = (7, True)


When constructing a parameterNodeWrapper using a parameter node that already has values set (e.g. when loading a .mrb or a .mrml scene file), those values will be maintained. The default will only come into play if parameter node does not have a value for the parameter.

Unspecified defaults

When a parameter in a parameterPack or parameterNodeWrapper is not given an explicit default value, the following values will be used:

Type Implicit default
int 0
float 0.0
str ""
bool False
vtkMRMLNode (including subclasses) None
list (hinted as list[int], list[str], etc) [] (empty list)
tuple (hinted as tuple[int, bool], etc) A tuple of the defaults of all the elements (e.g. tuple[int, bool] -> (0, False))
dict (hinted as dict[keyType, valueType]) {} (empty dictionary)
enum.Enum The first value in the enum
pathlib.Path pathlib.Path() (which is the current directory)
pathlib.PosixPath pathlib.PosixPath() (which is the current directory)
pathlib.WindowsPath pathlib.WindowsPath() (which is the current directory)
pathlib.PurePath pathlib.PurePath() (which is the current directory)
pathlib.PurePosixPath pathlib.PurePosixPath() (which is the current directory)
pathlib.PureWindowsPath pathlib.PureWindowsPath() (which is the current directory)
typing.Union (hinted as typing.Union[int, str], etc) The default value of the first item in the union.
typing.Optional (hinted as typing.Optional[int], typing.Optional[float], etc) None


If typing.Union[SomeType, None] is used, the default will be None. This will only happen if there are exactly 2 options in the union and the last one is None. In Python (not just the parameter node wrappers), writing typing.Union[SomeType, None] is equivalent to writing typing.Optional[SomeType].

Default generators

Occasionally, it may be useful to use a generator function to create the default value, rather than a fixed value. One possible use of this is to choose a default for a Path after Slicer has finished loading and the appropriate resource directories are known.


import os
from typing import Annotated
import slicer
from slicer.parameterNodeWrapper import parameterNodeWrapper, Default

def defaultIcon():
    # this will not change, but it can't be queried until this module is loaded
    return pathlib.Path(os.path.join(

class PipelineCreatorMk2ParameterNode:
    icon: Annotated[pathlib.Path, Default(generator=defaultIcon)]