Python supports “nested” assignments like x = y = 42. In that particular context the nested y = 42 assignment sort of acts like an expression, and the result is bound to the LHS of the first assignment: x = <result>. However, assignments are not generally considered expressions, and as such the TSSG definition does not create an output node for it, causing an error in the the stanza that deals with assignments:

0: Error executing statement edge @pattern.input -> @value.output at (1299, 3)
     src/stack-graphs.tsg:1299:3:
     1299 |   edge @pattern.input -> @value.output
          |   ^
     in stanza
     src/stack-graphs.tsg:1288:1:
     1288 | [
          | ^
     matching (assignment) node
     test/assignments.py:6:1:
     6 | y = z = 1337
       | ^
1: Evaluating edge sink
2: Undefined scoped variable [syntax node assignment (6, 5)].output

The fragment of the parsed Tree-sitter tree for such a nested assignment is:

(assignment
  left: (…)
  right: (assignment
    left: (…)
    right: (…)))

As of v0.23.5 of the Python grammar, the right field of an assignment indeed accepts another assignment (via _right_hand_side).

Simply creating an output node for an assignment fixes the issue. Copious testing reveals no negative side effects.