Python supports keyword arguments in its superclasses: (argument_list …), which somehow modify the superclass. The stanza capturing superclasses was erroneously capturing these keyword arguments as superclasses themselves as well, which would then cause an error upon emitting the edge:

0: Error executing statement edge @class.super_scope -> @superclass.output at (938, 3)
    src/stack-graphs.tsg:938:3:
    938 |   edge @class.super_scope -> @superclass.output
        |   ^
    in stanza
    src/stack-graphs.tsg:934:1:
    934 | (class_definition
        | ^
    matching (class_definition) node
    test/superclasses.py:11:1:
    11 | class B(A, foo="Bar"):
        | ^
1: Evaluating edge sink
2: Undefined scoped variable [syntax node keyword_argument (11, 12)].output

The fragment of the parsed Tree-sitter tree for such a superclass argument list with keyword arguments is:

superclasses: (argument_list
  (identifier)
  (keyword_argument …))

As of v0.23.5 of the Python grammar, the superclasses field is indeed an argument_list syntax node, which in turn can contain either an expression or a keyword_argument (among other things), and expression in turn can be a whole host of types of syntax nodes.

Explicitly capturing only identifier and attribute (which are dotted identifier chains like foo.bar) syntax nodes fixes the issue. This is possibly too strict, since technically any expression can be used here, but it likely captures the vast majority of cases out there.