Parameter nodes are often used in scripted modules to store data such that it can be easily saved to the MRML scene. They are simply MRML nodes that exist in a scene and store data. One of their most common uses is to save GUI state in module widgets.

The parameter node concept is implemented in C++ in the vtkMRMLScriptedModuleNode which has GetParameter and SetParameter methods for saving arbitrary string data to the scene. While the base vtkMRMLScriptedModuleNode is great for scene saving, treating all data as strings is not ideal, so a parameter node wrapper was implemented.

Parameter Node Wrapper

The parameter node wrapper allows wrapping around a vtkMRMLScriptedModuleNode parameter node with typed member access. The wrapper will serialize values into an underlying vtkMRMLScriptedModuleNode, and has a caching mechanism so multiple reads of a parameter without a write won’t convert from string every time.

A simple example is as follows.

import slicer
from slicer import vtkMRMLModelNode
from slicer.parameterNodeWrapper import *

class CustomParameterNode:
  numIterations: int
  inputs: list[vtkMRMLModelNode]
  output: vtkMRMLModelNode

This will create a new class called CustomParameterNode that has 3 members properties, an int named numIterations, a list of vtkMRMLModelNodes named inputs, and a vtkMRMLModelNode named output.

The @parameterNodeWrapper decorator will generate a constructor for this class that takes one argument, a vtkMRMLScriptedModuleNode parameter node.

An example usage is as follows:

parameterNode = slicer.mrmlScene.AddNewNodeByClass('vtkMRMLScriptedModuleNode')
param = CustomParameterNode(parameterNode)

# can set the property directly with an appropriate type
param.numIterations = 500
param.inputs = [slicer.mrmlScene.AddNewNodeByClass('vtkMRMLModelNode') for _ in range(5)]
param.output = slicer.mrmlScene.AddNewNodeByClass('vtkMRMLModelNode')

# pythonic list usage
for inputModel in param.inputs:
  mesh = inputModel.GetMesh()
  # ...

for iteration in range(param.numIterations):
  # run iteration


Much more complex wrappers can be written:

For the full list of supported types, including how to make custom types with invariants, see Supported types

For restricting valid values for a parameter, see Validators.

GUI binding and creation

Qt widgets can automatically be created for and bound to parameterNodeWrappers.

See GUI Connection for how to bind a parameterNodeWrapper to existing widgets (including directly in a .ui file).

See GUI Creation for how to generate widgets for parameterNodeWrappers.