Brett: can you clarify---in what sense is the argument optional?  I see that there's a doc issue at http://docs.python.org/dev/library/ast.html, where it's marked as optional but shouldn't but.  Is there something more that's wrong?

If you mean that ast.YieldFrom can be instantiated without an argument, isn't that the same for all the ast classes?

>>> ast.YieldFrom()
<_ast.YieldFrom object at 0x100760f50>
>>> ast.BoolOp()
<_ast.BoolOp object at 0x100760f90>