Make __parameters__ lazy (python#15)

Now we can also remove `__setstate__`.

Objects/descrobject.c

@@ -1969,6 +1969,15 @@ static PyObject *
 ga_getitem(PyObject *self, PyObject *item)
 {
  gaobject *alias = (gaobject *)self;
+ // do a lookup for __parameters__ so it gets populated (if not already)
+ if (alias->parameters == NULL) {
+ _Py_IDENTIFIER(__parameters__);
+ PyObject *params = _PyObject_GetAttrId(self, &PyId___parameters__);
+ if (params == NULL) {
+ return NULL;
+ }
+ Py_DECREF(params);
+ }
  Py_ssize_t nparams = PyTuple_GET_SIZE(alias->parameters);
  if (nparams == 0) {
  return PyErr_Format(PyExc_TypeError,
@@ -2031,7 +2040,6 @@ static const char* const attr_exceptions[] = {
  "__mro_entries__",
  "__reduce_ex__", // needed so we don't look up object.__reduce_ex__
  "__reduce__",
- "__setstate__",
  NULL,
 };
 
@@ -2118,29 +2126,37 @@ ga_reduce(PyObject *self, PyObject *Py_UNUSED(ignored))
  alias->origin, alias->args);
 }
 
-static PyObject *
-ga_setstate(PyObject *self, PyObject *state)
-{
- gaobject *alias = (gaobject *)self;
- PyObject *parameters = make_parameters(alias->args);
- Py_INCREF(parameters);
- alias->parameters = parameters;
- Py_RETURN_NONE;
-}
-
 static PyMethodDef ga_methods[] = {
  {"__mro_entries__", ga_mro_entries, METH_O},
  {"__instancecheck__", ga_instancecheck, METH_O},
  {"__subclasscheck__", ga_subclasscheck, METH_O},
  {"__reduce__", ga_reduce, METH_NOARGS},
- {"__setstate__", ga_setstate, METH_O},
  {0}
 };
 
 static PyMemberDef ga_members[] = {
  {"__origin__", T_OBJECT, offsetof(gaobject, origin), READONLY},
  {"__args__", T_OBJECT, offsetof(gaobject, args), READONLY},
- {"__parameters__", T_OBJECT, offsetof(gaobject, parameters), READONLY},
+ {0}
+};
+
+static PyObject *
+ga_parameters(PyObject *self, void *unused)
+{
+ gaobject *alias = (gaobject *)self;
+ if (alias->parameters == NULL) {
+ alias->parameters = make_parameters(alias->args);
+ if (alias->parameters == NULL) {
+ Py_DECREF(alias->parameters);
+ return NULL;
+ }
+ }
+ Py_INCREF(alias->parameters);
+ return alias->parameters;
+}
+
+static PyGetSetDef ga_properties[] = {
+ {"__parameters__", ga_parameters, (setter)NULL, "Type variables in the GenericAlias.", NULL},
  {0}
 };
 
@@ -2179,6 +2195,7 @@ PyTypeObject Py_GenericAliasType = {
  .tp_alloc = PyType_GenericAlloc,
  .tp_new = ga_new,
  .tp_free = PyObject_GC_Del,
+ .tp_getset = ga_properties,
 };
 
 PyObject *
@@ -2203,12 +2220,7 @@ Py_GenericAlias(PyObject *origin, PyObject *args)
  Py_INCREF(origin);
  alias->origin = origin;
  alias->args = args;
- // TODO: Make __parameters__ a lazy attribute
- alias->parameters = make_parameters(args);
- if (alias->parameters == NULL) {
- Py_DECREF(alias);
- return NULL;
- }
+ alias->parameters = NULL;
  _PyObject_GC_TRACK(alias);
  return (PyObject *)alias;
 }