Performance of list([]) in 2.3 came up in a thread on comp.lang.python,

which can be reviewed via
http://coding.derkeiler.com/Archive/Python/comp.lang.python/2003-12/1011.html

Duncan Booth investigated, and discovered that an "optimisation" was
in fact a pessimisation for small numbers of elements in a source list,
compared to not having the optimisation, although with large numbers
of elements in the source list the optimisation was quite beneficial.

He posted his change to comp.lang.python (but not to SF).

Further research has confirmed his assessment that the optimisation only
becomes a net win when the source list has more than 100 elements.

I also found that the optimisation could apply to tuples as well,
but the gains only arrive with source tuples larger than about 320
elements and are nowhere near as significant as the gains with lists,
(~95% gain @ 10000 elements for lists, ~20% gain @ 10000 elements for
tuples) so I haven't proceeded with this.

The code as it was applied the optimisation to list subclasses as
well, and this also appears to be a net loss for all reasonable sized
sources (~80-100% for up to 100 elements, ~20% for more than 500
elements; I tested up to 10000 elements).

Duncan also suggested special casing empty lists, which I've extended
to all empty sequences.

On the basis that list_fill() is only ever called with a list for the
result argument, testing for the source being the destination has
now happens before testing source types.

1 files changed, 28 insertions, 6 deletions

diff --git a/Objects/listobject.c b/Objects/listobject.c
index 47673be..66e4460 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -2234,6 +2234,13 @@ list_richcompare(PyObject *v, PyObject *w, int op)
 	return PyObject_RichCompare(vl->ob_item[i], wl->ob_item[i], op);
 }
 
+/* empirically determined threshold for activating an optimisation
+ * in list_fill() - 100 seems close to optimum for current CPUs and
+ * compilers, as of December '03.
+ * see also comment in list_fill().
+ */
+#define	LISTFILL_OPT_THRESHOLD	100
+
 /* Adapted from newer code by Tim */
 static int
 list_fill(PyListObject *result, PyObject *v)
@@ -2242,14 +2249,29 @@ list_fill(PyListObject *result, PyObject *v)
 	int n;		   /* guess for result list size */
 	int i;
 
+	/* if source is destination, we're done. */
+	if (v == (PyObject *)result)
+		return 0;
+
 	n = result->ob_size;
 
-	/* Special-case list(a_list), for speed. */
-	if (PyList_Check(v)) {
-		if (v == (PyObject *)result)
-			return 0; /* source is destination, we're done */
-		return list_ass_slice(result, 0, n, v);
-	}
+	/* Special-case list(a_list), for speed:
+	 * - if the source has 0 elements, there's nothing to copy.
+	 * - if the source has more than a threshold number of elements
+	 *   slice assignment is a faster way of filling the target
+	 *   (the exact threshold is subject to empirical determination).
+	 * Also special case any other zero length sequence, including
+	 * subclasses of list, there being nothing to copy.
+	 */
+	if (PyList_CheckExact(v)) {
+		i = ((PyListObject*)v)->ob_size;
+		if (i == 0)
+			return 0;
+		if (i > LISTFILL_OPT_THRESHOLD)
+			return list_ass_slice(result, 0, n, v);
+	} else
+		if (PySequence_Check(v) && PySequence_Size(v) == 0)
+			return 0;
 
 	/* Empty previous contents */
 	if (n != 0) {