demo.py

from contextlib import contextmanager

class Node:
    def __init__(self, name, args, children):
        self.name = name
        self.args = args
        self.children = children

    def __str__(self):
        out = []
        if self.name:
            out.append(str(self.name))
        else:
            out.append("node")
        if self.args:
            out.append("(")
            out.append(",".join(f"{k}:{v}" for k,v in self.args.items()))
            out.append(")")
        if self.children:
            out.append("{")
            out.append(",".join(f"{c}" for c in self.children))
            out.append("}")
        return "".join(out)

class Builder:
    def __init__(self):
        self.nodes = []
        self.inside = []

    def Node(self, name=None, **args):
        self.nodes.append(Node(name, args, []))

    @contextmanager
    def Group(self, name=None, **args):
        self.inside.append(name)
        old_nodes = self.nodes
        self.nodes = []
        yield
        old_nodes.append(Node(name, args, self.nodes))
        self.nodes = old_nodes

def build():
    def _decorate(fn):
        b = Builder()
        fn(b)
        if len(b.nodes) == 1:
            return b.nodes[0]
        return Node(None, {}, b.nodes)
    return _decorate

def main(argv):
    @build()
    def node(b):
        b.Node("one")
        with b.Group("two"):
            b.Node(1)
            b.Node(2)
            b.Node(3)
        b.Node("three")

    print(node)

if __name__ == "__main__":
    import sys
    sys.exit(main(sys.argv[1:]))