Fix bug 1604. deque.__init__() did not clear existing contents like list.__init__. Not a backport candidate.

Lib/test/test_deque.py

@@ -29,8 +29,8 @@ class MutateCmp:
 class TestBasic(unittest.TestCase):
 
     def test_basics(self):
-        d = deque(xrange(100))
-        d.__init__(xrange(100, 200))
+        d = deque(xrange(-5125, -5000))
+        d.__init__(xrange(200))
         for i in xrange(200, 400):
             d.append(i)
         for i in reversed(xrange(-200, 0)):
@@ -451,8 +451,8 @@ class DequeWithBadIter(deque):
 class TestSubclass(unittest.TestCase):
 
     def test_basics(self):
-        d = Deque(xrange(100))
-        d.__init__(xrange(100, 200))
+        d = Deque(xrange(25))
+        d.__init__(xrange(200))
         for i in xrange(200, 400):
             d.append(i)
         for i in reversed(xrange(-200, 0)):

Misc/NEWS

@@ -881,6 +881,10 @@ Library
 Extension Modules
 -----------------
 
+- Bug #1604: collections.deque.__init__(iterable) now clears any prior contents
+  before adding elements from the iterable.  This fix brings the behavior into
+  line with that for list.__init__().
+
 - Added wide char functions to msvcrt module: getwch, getwche, putwch and
   ungetwch. The functions accept or return unicode.
 

Modules/_collectionsmodule.c

@@ -843,6 +843,7 @@ deque_init(dequeobject *deque, PyObject *args, PyObject *kwdargs)
 		}
 	}
 	deque->maxlen = maxlen;
+	deque_clear(deque);
 	if (iterable != NULL) {
 		PyObject *rv = deque_extend(deque, iterable);
 		if (rv == NULL)