Hey @cleoold! Technically the second AST you gave is invalid (fun2), considering that the FunctionType is not an expression;

    mod = Module(stmt* body, type_ignore* type_ignores)
        | FunctionType(expr* argtypes, expr returns)

    expr = BoolOp(boolop op, expr* values)
         | NamedExpr(expr target, expr value)
         | BinOp(expr left, operator op, expr right)

Though I am curious about use case. How did you come with the fun2's AST?