If you define a sum type like

expr = Unary(expr e) | Binary(expr left, expr right)

Then you will get three pieces of generated code:

• expr_e.Unary: An integer tag.
  • typical usage: if node.tag == expr_e.Unary
• expr.Unary(): A Python class / type / constructor.
  • typical usage: node = expr.Unary(child)
  • Note that this is an alias for expr__Unary(). MyPy seems to like this form better. It makes it easier to statically type the code.
• expr_t: A common superclass of all the nodes.
  • typical usage: isinstance(node, expr_t)

The implementation is here, with some more notes in the README.

https://github.com/oilshell/oil/tree/master/asdl

For conceptual background, see blog posts tagged #ASDL or the original Zephyr technical report.