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AST type doesn't have any information about optional fields. And I don't know it's worth enough to add it.
When AST is instantiated by keyword arguments, there are no check about absence of some fields.
I suppose we can just loosen positional arguments too.
current:
 if (PyTuple_GET_SIZE(args) > 0) {
 if (numfields != PyTuple_GET_SIZE(args)) {
 PyErr_Format(PyExc_TypeError, "%.400s constructor takes %s"
 "%zd positional argument%s",
 Py_TYPE(self)->tp_name,
 numfields == 0 ? "" : "either 0 or ",
 numfields, numfields == 1 ? "" : "s");
will be:
 if (PyTuple_GET_SIZE(args) > 0) {
 if (numfields > PyTuple_GET_SIZE(args)) {
 PyErr_Format(PyExc_TypeError, "%.400s constructor takes at most "
 "%zd positional argument%s",
 Py_TYPE(self)->tp_name,
 numfields, numfields == 1 ? "" : "s");