From 0e371f2cb6178de7344c97e4a546703c4bba140e Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <python@rcn.com>
Date: Wed, 12 May 2004 20:55:56 +0000
Subject: [PATCH] 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  | 19 ++++++++---------
 Lib/test/test_deque.py      | 17 ++++++++++++---
 Modules/collectionsmodule.c | 41 +++++++++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 14 deletions(-)

diff --git a/Doc/lib/libcollections.tex b/Doc/lib/libcollections.tex
index 148ddea84c2..c7d5c508d3e 100644
--- a/Doc/lib/libcollections.tex
+++ b/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},
diff --git a/Lib/test/test_deque.py b/Lib/test/test_deque.py
index e8d9ce49c75..9b857c5d479 100644
--- a/Lib/test/test_deque.py
+++ b/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']]]]
 
-
 """
 
 
diff --git a/Modules/collectionsmodule.c b/Modules/collectionsmodule.c
index fc30c996e92..e49224d83d4 100644
--- a/Modules/collectionsmodule.c
+++ b/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;