Improve the implementation of itertools.product()

* Fix-up issues pointed-out by Neal Norwitz.
* Add extensive comments.
* The lz->result variable is now a tuple instead of a list.
* Use fast macro getitem/setitem calls so most code is in-line.
* Re-use the result tuple if available (modify in-place instead of copy).

Lib/test/test_itertools.py

@@ -274,6 +274,9 @@ class TestBasicOps(unittest.TestCase):
             args = map(iter, args)
             self.assertEqual(len(list(product(*args))), n)
 
+        # Test implementation detail:  tuple re-use
+        self.assertEqual(len(set(map(id, product('abc', 'def')))), 1)
+        self.assertNotEqual(len(set(map(id, list(product('abc', 'def'))))), 1)
 
     def test_repeat(self):
         self.assertEqual(zip(xrange(3),repeat('a')),

Modules/itertoolsmodule.c

@@ -1796,7 +1796,7 @@ product_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 	lz = (productobject *)type->tp_alloc(type, 0);
 	if (lz == NULL) {
 		Py_DECREF(pools);
-		return NULL;
+		goto error;
 	}
 
 	lz->pools = pools;
@@ -1840,7 +1840,7 @@ product_next(productobject *lz)
 {
 	PyObject *pool;
 	PyObject *elem;
-	PyObject *tuple_result;
+	PyObject *oldelem;
 	PyObject *pools = lz->pools;
 	PyObject *result = lz->result;
 	Py_ssize_t npools = PyTuple_GET_SIZE(pools);
@@ -1848,10 +1848,14 @@ product_next(productobject *lz)
 
 	if (lz->stopped)
 		return NULL;
+
 	if (result == NULL) {
+                /* On the first pass, return an initial tuple filled with the 
+                   first element from each pool.  If any pool is empty, then 
+                   whole product is empty and we're already done */
 		if (npools == 0)
 			goto empty;
-		result = PyList_New(npools);
+		result = PyTuple_New(npools);
 		if (result == NULL)
             		goto empty;
 		lz->result = result;
@@ -1861,34 +1865,61 @@ product_next(productobject *lz)
 				goto empty;
     			elem = PyTuple_GET_ITEM(pool, 0);
     			Py_INCREF(elem);
-    			PyList_SET_ITEM(result, i, elem);
+    			PyTuple_SET_ITEM(result, i, elem);
 		}
 	} else {
 		Py_ssize_t *indices = lz->indices;
 		Py_ssize_t *maxvec = lz->maxvec;
+
+		/* Copy the previous result tuple or re-use it if available */
+		if (Py_REFCNT(result) > 1) {
+			PyObject *old_result = result;
+			result = PyTuple_New(npools);
+			if (result == NULL)
+				goto empty;
+			lz->result = result;
+			for (i=0; i < npools; i++) {
+				elem = PyTuple_GET_ITEM(old_result, i);
+    				Py_INCREF(elem);
+    				PyTuple_SET_ITEM(result, i, elem);
+			}
+			Py_DECREF(old_result);
+		}
+		/* Now, we've got the only copy so we can update it in-place */
+		assert (Py_REFCNT(result) == 1);
+
+                /* Update the pool indices right-to-left.  Only advance to the
+                   next pool when the previous one rolls-over */
 		for (i=npools-1 ; i >= 0 ; i--) {
 			pool = PyTuple_GET_ITEM(pools, i);
 			indices[i]++;
 			if (indices[i] == maxvec[i]) {
+				/* Roll-over and advance to next pool */
 				indices[i] = 0;
 				elem = PyTuple_GET_ITEM(pool, 0);
 				Py_INCREF(elem);
-				PyList_SetItem(result, i, elem);
+				oldelem = PyTuple_GET_ITEM(result, i);
+				PyTuple_SET_ITEM(result, i, elem);
+				Py_DECREF(oldelem);
 			} else {
+				/* No rollover. Just increment and stop here. */
 				elem = PyTuple_GET_ITEM(pool, indices[i]);
 				Py_INCREF(elem);
-				PyList_SetItem(result, i, elem);
+				oldelem = PyTuple_GET_ITEM(result, i);
+				PyTuple_SET_ITEM(result, i, elem);
+				Py_DECREF(oldelem);
 				break;
 			}
 		}
+
+		/* If i is negative, then the indices have all rolled-over
+                   and we're done. */
 		if (i < 0)
-			return NULL;
+			goto empty;
 	}
 
-	tuple_result = PySequence_Tuple(result);
-	if (tuple_result == NULL)
-		lz->stopped = 1;
-	return tuple_result;
+	Py_INCREF(result);
+	return result;
 
 empty:
 	lz->stopped = 1;
@@ -1898,7 +1929,7 @@ empty:
 PyDoc_STRVAR(product_doc,
 "product(*iterables) --> product object\n\
 \n\
-Cartesian product of input interables.  Equivalent to nested for-loops.\n\n\
+Cartesian product of input iterables.  Equivalent to nested for-loops.\n\n\
 For example, product(A, B) returns the same as:  ((x,y) for x in A for y in B).\n\
 The leftmost iterators are in the outermost for-loop, so the output tuples\n\
 cycle in a manner similar to an odometer (with the rightmost element changing\n\