Creating and Working with Nodes in LionWeb

LionWeb provides a flexible and language-agnostic model for working with models (or trees, or ASTs: let's consider these as synonyms in this context).

The main component is the Node.

When working with LionWeb nodes in Python, there are two complementary approaches depending on your needs:

1. Homogeneous nodes, using generic, universal APIs which work with all form of nodes. When choosing this approach, we may want to consider DynamicNode.
2. Heterogeneous nodes, using language-specific, statically-typed Python classes, defined for a certain LionWeb language and just that one.

The Core Abstraction: Node

At the heart of LionWeb is the Node interface. Implementing it guarantees:

• Serialization and deserialization
• Compatibility with the LionWeb Repository
• Introspection through classifiers and features
• Tool support (e.g., editors, model processors)

By relying on this interface, LionWeb tooling can process, manipulate, and analyze any conforming node in a uniform manner.

Option 1: Homogeneous Nodes

This approach is ideal for generic tools and runtime interoperability. The key class here is DynamicNode.

When to Use

• You receive nodes from external systems or clients
• You want to handle unknown or dynamic languages
• You’re building generic tools (e.g., validators, browsers)

How it Works

DynamicNode implements Node and stores features dynamically. You can query and manipulate the node’s structure by name.

Example

from lionweb.language import Language, Concept, Property, Containment
from lionweb.model import DynamicNode
from lionweb.language import LionCoreBuiltins
from lionweb.utils.node_tree_validator import NodeTreeValidator

# === Define the Language ===

# Define the 'TaskList' concept
task_list_concept = Concept(
    name="TaskList", key="TaskList", id="TaskList-id", abstract=False, partition=True
)

# Define the 'Task' concept
task_concept = Concept(
    name="Task", key="Task", id="Task-id", abstract=False, partition=False
)

# Add a 'tasks' containment
tasks_containment = Containment(
    name="tasks",
    key="TasksList-tasks",
    id="TasksList-tasks-id",
    type=task_concept,
    multiple=True,
    optional=False,
)
task_list_concept.add_feature(tasks_containment)

# Add a 'name' property
name_property = Property(
    name="name", key="task-name", id="task-name-id", type=LionCoreBuiltins.get_string()
)
task_concept.add_feature(name_property)

# Define the language container
task_language = Language(
    name="Task Language",
    key="task",
    id="task-id",
    version="1.0"
)
task_language.add_element(task_list_concept)
task_language.add_element(task_concept)

# === Use DynamicNode to Build a Model ===

# Create root node (a TaskList)
errands = DynamicNode(id="errands", concept=task_list_concept)

# Create first Task node
task1 = DynamicNode(id="task1-id", concept=task_concept)
task1.set_property_value(property=name_property, value="My Task #1")
errands.add_child(tasks_containment, task1)

# Create second Task node
task2 = DynamicNode(id="task2-id", concept=task_concept)
task2.set_property_value(name_property, "My Task #2")
errands.add_child(tasks_containment, task2)

# Validate the model tree
result = NodeTreeValidator().validate(errands)
if result.has_errors():
    raise ValueError(f"The tree is invalid: {result}")

# Access the model using direct containment
tasks = errands.get_children(tasks_containment)
print(f"Tasks found: {len(tasks)}")
for task in tasks:
    print(f" - {task.get_property_value(name_property)}")

# Access the model using name-based utilities
tasks_again = errands.get_children("tasks")
print(f"Tasks found again: {len(tasks_again)}")
for task in tasks_again:
    print(f" - {task.get_property_value('name')}")

Evaluation

• No static typing
• No compile-time safety
• No code completion or type checking
• Work out of the box, without the need to write any code for each language

If you misspell "name" or access a non-existent feature, you’ll get a runtime exception.

Option 2: Heterogeneous Nodes

This approach is recommended when building interpreters, compilers, or other tools for a specific language.

You define a Python class for each concept, typically:

• Implementing the Node interface
• Optionally extending DynamicNode for convenience

But how can you define these classes?

Of course, you can do that in the good old way: writing the code yourself.

Or you can use the built-in generator which, given a language, produce the corresponding classes. See more under Generating Code.

In the rest of this section we will show how to define the classes manually.

When to Use

• You are building tooling for a specific DSL or language
• You want type-safe code with IDE support
• You require structured, validated access to features

Example

import uuid
from typing import List, cast

from lionweb.language import Language, Concept, Property, Containment
from lionweb.model import DynamicNode
from lionweb.language import LionCoreBuiltins
from lionweb.utils.node_tree_validator import NodeTreeValidator


# === Define the Language ===

# Global elements
task_list_concept: Concept
task_concept: Concept
name_property: Property
tasks_containment: Containment
task_language: Language


def define_language():
    global task_list_concept, task_concept, name_property, tasks_containment, task_language

    # Define the 'TaskList' concept
    task_list_concept = Concept(
        name="TaskList", key="TaskList", id="TaskList-id", abstract=False, partition=True
    )

    # Define the 'Task' concept
    task_concept = Concept(
        name="Task", key="Task", id="Task-id", abstract=False, partition=False
    )

    # Add a 'tasks' containment
    tasks_containment = Containment(
        name="tasks",
        key="TasksList-tasks",
        id="TasksList-tasks-id",
        type=task_concept,
        multiple=True,
        optional=False,
    )
    task_list_concept.add_feature(tasks_containment)

    # Add a 'name' property
    name_property = Property(
        name="name", key="task-name", id="task-name-id", type=LionCoreBuiltins.get_string()
    )
    task_concept.add_feature(name_property)

    # Define the language container
    task_language = Language(
        name="Task Language",
        key="task",
        id="task-id",
        version="1.0"
    )
    task_language.add_element(task_list_concept)
    task_language.add_element(task_concept)


# === Define specific DynamicNode subclasses ===

class Task(DynamicNode):
    def __init__(self, name: str):
        super().__init__(str(uuid.uuid4()), task_concept)
        self.set_name(name)

    def set_name(self, name: str):
        self.set_property_value(name_property, name)

    def get_name(self) -> str:
        return self.get_property_value(name_property)


class TaskList(DynamicNode):
    def __init__(self):
        super().__init__(str(uuid.uuid4()), task_list_concept)

    def add_task(self, task: Task):
        self.add_child(tasks_containment, task)

    def get_tasks(self) -> List[Task]:
        return self.get_children(tasks_containment)


# === Main usage function ===

def use_specific_classes():
    define_language()

    errands = TaskList()

    task1 = Task("My Task #1")
    errands.add_task(task1)

    task2 = Task("My Task #2")
    errands.add_task(task2)

    # Validate
    result = NodeTreeValidator().validate(errands)
    if result.has_errors():
        raise ValueError(f"The tree is invalid: {result}")

    # Access
    tasks = errands.get_tasks()
    print(f"Tasks found: {len(tasks)}")
    for task in tasks:
        print(f" - {task.get_name()}")


# === Entry Point ===

if __name__ == "__main__":
    use_specific_classes()

Evaluation

• Full IDE support (auto-completion, navigation)
• Catch errors at compile time
• Clear API for collaborators
• Require extra work for defining the classes

Suggested approach

• Use DynamicNode in model editors, importers, migrators
• Use custom classes (like PersonNode) in interpreters, generators, type checkers

Interoperability

Both approaches can co-exist. For example, you might parse a file into DynamicNode objects and then convert them into typed classes using a factory or builder.