Have dict.__getitem__ check __missing__

This is needed for defaultdict

src/runtime/capi.cpp

@@ -280,7 +280,13 @@ extern "C" PyObject* PyObject_GetItem(PyObject* o, PyObject* key) noexcept {
 }
 
 extern "C" int PyObject_SetItem(PyObject* o, PyObject* key, PyObject* v) noexcept {
-    Py_FatalError("unimplemented");
+    try {
+        setitem(o, key, v);
+        return 0;
+    } catch (ExcInfo e) {
+        setCAPIException(e);
+        return -1;
+    }
 }
 
 extern "C" int PyObject_DelItem(PyObject* o, PyObject* key) noexcept {

src/runtime/dict.cpp

@@ -174,6 +174,15 @@ Box* dictGetitem(BoxedDict* self, Box* k) {
 
     auto it = self->d.find(k);
     if (it == self->d.end()) {
+        // Try calling __missing__ if this is a subclass
+        if (self->cls != dict_cls) {
+            static const std::string missing("__missing__");
+            Box* r = callattr(self, &missing, CallattrFlags({.cls_only = true, .null_on_nonexistent = true }),
+                              ArgPassSpec(1), k, NULL, NULL, NULL, NULL);
+            if (r)
+                return r;
+        }
+
         raiseExcHelper(KeyError, k);
     }
 

test/tests/collections_test.py

@@ -7,3 +7,8 @@ for i in xrange(30):
 print o
 
 print collections.deque().maxlen
+
+d = collections.defaultdict(lambda: [])
+print d[1]
+d[2].append(3)
+print sorted(d.items())

test/tests/dict_subclassing.py

@@ -9,3 +9,11 @@ print d.items()
 print list(d.iterkeys())
 print list(d.itervalues())
 print list(d.iteritems())
+
+class DictWithMissing(dict):
+    def __missing__(self, key):
+        print key
+        return key
+
+d = DictWithMissing()
+print d[5]