Fix Issue 1045.

Factor-out common calling code by simplifying the length_hint API.
Speed-up the function by caching the PyObject_String for the attribute lookup.

Include/abstract.h

@@ -433,25 +433,12 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx*/
      PyAPI_FUNC(Py_ssize_t) PyObject_Length(PyObject *o);
 #define PyObject_Length PyObject_Size
 
-     PyAPI_FUNC(Py_ssize_t) _PyObject_LengthHint(PyObject *o);
+     PyAPI_FUNC(Py_ssize_t) _PyObject_LengthHint(PyObject *o, Py_ssize_t);
 
        /*
-         Return the size of object o.  If the object, o, provides
-	 both sequence and mapping protocols, the sequence size is
-	 returned. On error, -1 is returned.  If the object provides
-	 a __length_hint__() method, its value is returned.  This is an
-	 internal undocumented API provided for performance reasons;
-	 for compatibility, don't use it outside the core.  This is the
-	 equivalent to the Python expression: 
-		try:
-			return len(o)
-		except (AttributeError, TypeError):
-			exc_type, exc_value, exc_tb = sys.exc_info()
-			try:
-				return o.__length_hint__()
-			except:
-				pass
-			raise exc_type, exc_value, exc_tb
+         Guess the size of object o using len(o) or o.__length_hint__().
+         If neither of those return a non-negative value, then return the
+         default value.  This function never fails. All exceptions are cleared.
        */
 
      PyAPI_FUNC(PyObject *) PyObject_GetItem(PyObject *o, PyObject *key);

Lib/test/list_tests.py

@@ -523,7 +523,5 @@ class CommonTest(seq_tests.CommonTest):
         # Bug #1242657
         class F(object):
             def __iter__(self):
-                yield 23
-            def __len__(self):
                 raise KeyboardInterrupt
         self.assertRaises(KeyboardInterrupt, list, F())

Objects/abstract.c

@@ -82,29 +82,47 @@ PyObject_Length(PyObject *o)
 }
 #define PyObject_Length PyObject_Size
 
-Py_ssize_t
-_PyObject_LengthHint(PyObject *o)
-{
-	Py_ssize_t rv = PyObject_Size(o);
-	if (rv != -1)
-		return rv;
-	if (PyErr_ExceptionMatches(PyExc_TypeError) ||
-	    PyErr_ExceptionMatches(PyExc_AttributeError)) {
-		PyObject *err_type, *err_value, *err_tb, *ro;
 
-		PyErr_Fetch(&err_type, &err_value, &err_tb);
-		ro = PyObject_CallMethod(o, "__length_hint__", NULL);
-		if (ro != NULL) {
-			rv = PyInt_AsLong(ro);
-			Py_DECREF(ro);
-			Py_XDECREF(err_type);
-			Py_XDECREF(err_value);
-			Py_XDECREF(err_tb);
-			return rv;
-		}
-		PyErr_Restore(err_type, err_value, err_tb);
+/* The length hint function returns a non-negative value from o.__len__()
+   or o.__length_hint__().  If those methods aren't found or return a negative
+   value, then the defaultvalue is returned.  This function never fails. 
+   Accordingly, it will mask exceptions raised in either method.
+*/
+
+Py_ssize_t
+_PyObject_LengthHint(PyObject *o, Py_ssize_t defaultvalue)
+{
+	static PyObject *hintstrobj = NULL;
+	PyObject *ro;
+	Py_ssize_t rv;
+
+	/* try o.__len__() */
+	rv = PyObject_Size(o);
+	if (rv >= 0)
+		return rv;
+	if (PyErr_Occurred())
+		PyErr_Clear();
+
+	/* cache a hashed version of the attribute string */
+	if (hintstrobj == NULL) {
+		hintstrobj = PyString_InternFromString("__length_hint__");
+		if (hintstrobj == NULL)
+			goto defaultcase;
 	}
-	return -1;
+
+	/* try o.__length_hint__() */
+	ro = PyObject_CallMethodObjArgs(o, hintstrobj, NULL);
+	if (ro == NULL)
+		goto defaultcase;
+	rv = PyInt_AsLong(ro);
+	Py_DECREF(ro);
+	if (rv >= 0)
+		return rv;
+
+defaultcase:
+	if (PyErr_Occurred())
+		PyErr_Clear();
+	return defaultvalue;
 }
 
 PyObject *
@@ -1505,17 +1523,7 @@ PySequence_Tuple(PyObject *v)
 		return NULL;
 
 	/* Guess result size and allocate space. */
-	n = _PyObject_LengthHint(v);
-	if (n < 0) {
-		if (PyErr_Occurred()
-		    && !PyErr_ExceptionMatches(PyExc_TypeError)
-		    && !PyErr_ExceptionMatches(PyExc_AttributeError)) {
-			Py_DECREF(it);
-			return NULL;
-		}
-		PyErr_Clear();
-		n = 10;  /* arbitrary */
-	}
+	n = _PyObject_LengthHint(v, 10);
 	result = PyTuple_New(n);
 	if (result == NULL)
 		goto Fail;

Objects/listobject.c

@@ -794,17 +794,7 @@ listextend(PyListObject *self, PyObject *b)
 	iternext = *it->ob_type->tp_iternext;
 
 	/* Guess a result list size. */
-	n = _PyObject_LengthHint(b);
-	if (n < 0) {
-		if (PyErr_Occurred()
-		    && !PyErr_ExceptionMatches(PyExc_TypeError)
-		    && !PyErr_ExceptionMatches(PyExc_AttributeError)) {
-			Py_DECREF(it);
-			return NULL;
-		}
-		PyErr_Clear();
-		n = 8;	/* arbitrary */
-	}
+	n = _PyObject_LengthHint(b, 8);
 	m = Py_Size(self);
 	mn = m + n;
 	if (mn >= m) {

Python/bltinmodule.c

@@ -236,15 +236,7 @@ builtin_filter(PyObject *self, PyObject *args)
 		goto Fail_arg;
 
 	/* Guess a result list size. */
-	len = _PyObject_LengthHint(seq);
-	if (len < 0) {
-		if (!PyErr_ExceptionMatches(PyExc_TypeError)  &&
-		    !PyErr_ExceptionMatches(PyExc_AttributeError)) {
-			goto Fail_it;
-		}
-		PyErr_Clear();
-		len = 8;	/* arbitrary */
-	}
+	len = _PyObject_LengthHint(seq, 8);
 
 	/* Get a result list. */
 	if (PyList_Check(seq) && seq->ob_refcnt == 1) {
@@ -905,15 +897,7 @@ builtin_map(PyObject *self, PyObject *args)
 		}
 
 		/* Update len. */
-		curlen = _PyObject_LengthHint(curseq);
-		if (curlen < 0) {
-			if (!PyErr_ExceptionMatches(PyExc_TypeError)  &&
-			    !PyErr_ExceptionMatches(PyExc_AttributeError)) {
-				goto Fail_2;
-			}
-			PyErr_Clear();
-			curlen = 8;  /* arbitrary */
-		}
+		curlen = _PyObject_LengthHint(curseq, 8);
 		if (curlen > len)
 			len = curlen;
 	}
@@ -2243,13 +2227,8 @@ builtin_zip(PyObject *self, PyObject *args)
 	len = -1;	/* unknown */
 	for (i = 0; i < itemsize; ++i) {
 		PyObject *item = PyTuple_GET_ITEM(args, i);
-		Py_ssize_t thislen = _PyObject_LengthHint(item);
+		Py_ssize_t thislen = _PyObject_LengthHint(item, -1);
 		if (thislen < 0) {
-			if (!PyErr_ExceptionMatches(PyExc_TypeError)  &&
-			    !PyErr_ExceptionMatches(PyExc_AttributeError)) {
-				return NULL;
-			}
-			PyErr_Clear();
 			len = -1;
 			break;
 		}