Python 3: super() raises TypeError unexpectedly

Coming from Java, I'm struggling a bit getting down inheritance, abstract classes, static methods and similar concepts of OO programming in Python.

I have an implementation of an expression tree class, given (simplified) by

# Generic node class
class Node(ABC):
    @abstractmethod
    def to_expr(self):
        pass

    @staticmethod
    def bracket_complex(child):
        s = child.to_expr()
        return s if isinstance(child, Leaf) or isinstance(child, UnaryOpNode) else "(" + s + ")"


# Leaf class - used for values and variables
class Leaf(Node):
    def __init__(self, val):
        self.val = val

    def to_expr(self):
        return str(self.val)


# Unary operator node
class UnaryOpNode(Node):
    def __init__(self, op, child):
        self.op = op
        self.child = child

    def to_expr(self):
        return str(self.op) + super().bracket_complex(self.child)


# Binary operator node
class BinaryOpNode(Node):
    def __init__(self, op, lchild, rchild):
        self.op = op
        self.lchild = lchild
        self.rchild = rchild

    def to_expr(self):
        return super().bracket_complex(self.lchild) + " " + str(self.op) + " " + super().bracket_complex(self.rchild)


# Variadic operator node (arbitrary number of arguments)
# Assumes commutative operator
class VariadicOpNode(Node):
    def __init__(self, op, list_):
        self.op = op
        self.children = list_

    def to_expr(self):
        return (" " + str(self.op) + " ").join(super().bracket_complex(child) for child in self.children)

The method to_expr() works fine when called on instances of Leaf, UnaryOpNode and BinaryOpNode, but raises a TypeError when called on an instance of VariadicOpNode:

TypeError: super(type, obj): obj must be an instance or subtype of type

What am I doing wrong in that specific class that super() is suddenly not working?

In Java the static method would get inherited so I wouldn't even need the super call, but in Python this does not seem to be the case.