bpo-29839: Raise ValueError rather than OverflowError in len() for negative values.

Lib/test/test_builtin.py

@@ -770,10 +770,18 @@ class FloatLen:
             def __len__(self):
                 return 4.5
         self.assertRaises(TypeError, len, FloatLen())
+        class NegativeLen:
+            def __len__(self):
+                return -10
+        self.assertRaises(ValueError, len, NegativeLen())
         class HugeLen:
             def __len__(self):
                 return sys.maxsize + 1
         self.assertRaises(OverflowError, len, HugeLen())
+        class HugeNegativeLen:
+            def __len__(self):
+                return -sys.maxsize-10
+        self.assertRaises(ValueError, len, HugeNegativeLen())
         class NoLenMethod(object): pass
         self.assertRaises(TypeError, len, NoLenMethod())
 

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
+- bpo-29839: len() now raises ValueError rather than OverflowError if
+  __len__() returned a large negative integer.
+
 - bpo-11913: README.rst is now included in the list of distutils standard
   READMEs and therefore included in source distributions.
 

Objects/typeobject.c

@@ -5924,14 +5924,21 @@ slot_sq_length(PyObject *self)
 
     if (res == NULL)
         return -1;
-    len = PyNumber_AsSsize_t(res, PyExc_OverflowError);
-    Py_DECREF(res);
-    if (len < 0) {
-        if (!PyErr_Occurred())
-            PyErr_SetString(PyExc_ValueError,
-                            "__len__() should return >= 0");
+
+    Py_SETREF(res, PyNumber_Index(res));
+    if (res == NULL)
+        return -1;
+
+    assert(PyLong_Check(res));
+    if (Py_SIZE(res) < 0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "__len__() should return >= 0");
         return -1;
     }
+
+    len = PyNumber_AsSsize_t(res, PyExc_OverflowError);
+    assert(len >= 0 || PyErr_ExceptionMatches(PyExc_OverflowError));
+    Py_DECREF(res);
     return len;
 }