Make __parameters__ lazy (python#15)

Now we can also remove `__setstate__`.

Author: emmatyping

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;
 }