SF bug 555042: zip() may trigger MemoryError.

NOT a bugfix candidate:  this is a fix to an optimization introduced
in 2.3.

Lib/test/test_b2.py

@@ -344,7 +344,17 @@ except:
 if not exc:
     raise TestFailed, 'zip(a, b) - missing expected TypeError'
 
+# Make sure zip doesn't try to allocate a billion elements for the
+# result list when one of its arguments doesn't say how long it is.
+# A MemoryError is the most likely failure mode.
+class SequenceWithoutALength:
+    def __getitem__(self, i):
+        if i == 5:
+            raise IndexError
+        else:
+            return i
+vereq(zip(SequenceWithoutALength(), xrange(2**30)),
+      list(enumerate(range(5))))
 
 # Epilogue -- unlink the temp file
-
 unlink(TESTFN)

Python/bltinmodule.c

@@ -1717,13 +1717,18 @@ builtin_zip(PyObject *self, PyObject *args)
 	/* args must be a tuple */
 	assert(PyTuple_Check(args));
 
-	/* Guess at result length:  the shortest of the input lengths. */
+	/* Guess at result length:  the shortest of the input lengths.
+	   If some argument refuses to say, we refuse to guess too, lest
+	   an argument like xrange(sys.maxint) lead us astray.*/
 	len = -1;	/* unknown */
 	for (i = 0; i < itemsize; ++i) {
 		PyObject *item = PyTuple_GET_ITEM(args, i);
 		int thislen = PySequence_Length(item);
-		if (thislen < 0)
+		if (thislen < 0) {
 			PyErr_Clear();
+			len = -1;
+			break;
+		}
 		else if (len < 0 || thislen < len)
 			len = thislen;
 	}