Make sure "del d[n]" is properly supported. Was necessary because the

same method that implements __setitem__ also implements __delitem__.
Also, there were several good use cases (removing items from a queue
and implementing Forth style stack ops).

Doc/lib/libcollections.tex

@@ -137,24 +137,21 @@ This section shows various approaches to working with deques.
 The \method{rotate()} method provides a way to implement \class{deque}
 slicing and deletion:
 
+This pure python implementation of \code{del d[n]} shows how to use the
+\method{rotate()} method as a building block for implementing a variety
+of class{deque} operations:
+
 \begin{verbatim}
 def delete_nth(d, n):
-    "del d[n]"
     d.rotate(-n)
     d.popleft()
     d.rotate(n)
-
->>> d = deque('abcdef')
->>> delete_nth(d, 2)   # remove the entry at d[2]
->>> d
-deque(['a', 'b', 'd', 'e', 'f'])
-
 \end{verbatim}
 
-For slicing, the idea is the same.  Use \method{rotate()} to bring a target
-element to the left side of the deque.  Remove old entries with
-\method{popleft()}, add new entries with \method{extend()}, and then
-reverse the rotation.
+To implement \class{deque} slicing, use a similar approach applying
+\method{rotate()} to bring a target element to the left side of the deque.
+Remove old entries with \method{popleft()}, add new entries with
+\method{extend()}, and then reverse the rotation.
 
 With minor variations on that approach, it is easy to implement Forth style
 stack manipulations such as \code{dup}, \code{drop}, \code{swap}, \code{over},

Lib/test/test_deque.py

@@ -90,6 +90,20 @@ class TestBasic(unittest.TestCase):
             l[i] = 7*i
         self.assertEqual(list(d), l)
 
+    def test_delitem(self):
+        n = 500         # O(n**2) test, don't make this too big
+        d = deque(xrange(n))
+        self.assertRaises(IndexError, d.__delitem__, -n-1)
+        self.assertRaises(IndexError, d.__delitem__, n)
+        for i in xrange(n):
+            self.assertEqual(len(d), n-i)
+            j = random.randrange(-len(d), len(d))
+            val = d[j]
+            self.assert_(val in d)
+            del d[j]
+            self.assert_(val not in d)
+        self.assertEqual(len(d), 0)
+
     def test_rotate(self):
         s = tuple('abcde')
         n = len(s)
@@ -476,9 +490,7 @@ deque(['c', 'b', 'a'])
 
 
 
-
 >>> def delete_nth(d, n):
-...     "del d[n]"
 ...     d.rotate(-n)
 ...     d.popleft()
 ...     d.rotate(n)
@@ -524,7 +536,6 @@ h
 >>> print maketree('abcdefgh')
 [[[['a', 'b'], ['c', 'd']], [['e', 'f'], ['g', 'h']]]]
 
-
 """
 
 

Modules/collectionsmodule.c

@@ -352,6 +352,44 @@ deque_item(dequeobject *deque, int i)
 	return item;
 }
 
+static int
+deque_del_item(dequeobject *deque, int i)
+{
+	PyObject *item=NULL, *minus_i=NULL, *plus_i=NULL;
+	int rv = -1;
+
+	assert (i >= 0 && i < deque->len); 
+
+	minus_i = Py_BuildValue("(i)", -i);
+	if (minus_i == NULL)
+		goto fail;
+
+	plus_i = Py_BuildValue("(i)", i);
+	if (plus_i == NULL)
+		goto fail;
+
+	item = deque_rotate(deque, minus_i);
+	if (item == NULL) 
+		goto fail;
+	Py_DECREF(item);
+
+	item = deque_popleft(deque, NULL);
+	if (item == NULL) 
+		goto fail;
+	Py_DECREF(item);
+
+	item = deque_rotate(deque, plus_i);
+	if (item == NULL) 
+		goto fail;
+
+	rv = 0;
+fail:
+	Py_XDECREF(item);
+	Py_XDECREF(minus_i);
+	Py_XDECREF(plus_i);
+	return rv;
+}
+
 static int
 deque_ass_item(dequeobject *deque, int i, PyObject *v)
 {
@@ -364,6 +402,9 @@ deque_ass_item(dequeobject *deque, int i, PyObject *v)
 				"deque index out of range");
 		return -1;
 	}
+	if (v == NULL)
+		return deque_del_item(deque, i);
+
 	i += deque->leftindex;
 	n = i / BLOCKLEN;
 	i %= BLOCKLEN;