diff --git a/Lib/test/test_bytes.py b/Lib/test/test_bytes.py
index b9c5b62..145411e 100644
--- a/Lib/test/test_bytes.py
+++ b/Lib/test/test_bytes.py
@@ -113,6 +113,23 @@
         b = self.type2test([1, 2, 3])
         self.assertEqual(b, b"\x01\x02\x03")
 
+    def test_from_mutating_list(self):
+        # Issue #34973: Crash in bytes constructor with mutating list.
+        class X:
+            def __index__(self):
+                a.clear()
+                return 42
+        a = [X(), X()]
+        self.assertEqual(bytes(a), b'*')
+
+        class Y:
+            def __index__(self):
+                if len(a) < 1000:
+                    a.append(self)
+                return 42
+        a = [Y()]
+        self.assertEqual(bytes(a), b'*' * 1000)  # should not crash
+
     def test_from_index(self):
         b = self.type2test([Indexable(), Indexable(1), Indexable(254),
                             Indexable(255)])
diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-10-13-16-42-03.bpo-34973.B5M-3g.rst b/Misc/NEWS.d/next/Core and Builtins/2018-10-13-16-42-03.bpo-34973.B5M-3g.rst
new file mode 100644
index 0000000..6e403cd
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-10-13-16-42-03.bpo-34973.B5M-3g.rst
@@ -0,0 +1,2 @@
+Fixed crash in :func:`bytes` when the :class:`list` argument is mutated
+while it is iterated.
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index 22d4687..d1057b9 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -2639,49 +2639,83 @@
     return NULL;
 }
 
-#define _PyBytes_FROM_LIST_BODY(x, GET_ITEM)                                \
-    do {                                                                    \
-        PyObject *bytes;                                                    \
-        Py_ssize_t i;                                                       \
-        Py_ssize_t value;                                                   \
-        char *str;                                                          \
-        PyObject *item;                                                     \
-                                                                            \
-        bytes = PyBytes_FromStringAndSize(NULL, Py_SIZE(x));                \
-        if (bytes == NULL)                                                  \
-            return NULL;                                                    \
-        str = ((PyBytesObject *)bytes)->ob_sval;                            \
-                                                                            \
-        for (i = 0; i < Py_SIZE(x); i++) {                                  \
-            item = GET_ITEM((x), i);                                        \
-            value = PyNumber_AsSsize_t(item, NULL);                         \
-            if (value == -1 && PyErr_Occurred())                            \
-                goto error;                                                 \
-                                                                            \
-            if (value < 0 || value >= 256) {                                \
-                PyErr_SetString(PyExc_ValueError,                           \
-                                "bytes must be in range(0, 256)");          \
-                goto error;                                                 \
-            }                                                               \
-            *str++ = (char) value;                                          \
-        }                                                                   \
-        return bytes;                                                       \
-                                                                            \
-    error:                                                                  \
-        Py_DECREF(bytes);                                                   \
-        return NULL;                                                        \
-    } while (0)
-
 static PyObject*
 _PyBytes_FromList(PyObject *x)
 {
-    _PyBytes_FROM_LIST_BODY(x, PyList_GET_ITEM);
+    Py_ssize_t i, size = PyList_GET_SIZE(x);
+    Py_ssize_t value;
+    char *str;
+    PyObject *item;
+    _PyBytesWriter writer;
+
+    _PyBytesWriter_Init(&writer);
+    str = _PyBytesWriter_Alloc(&writer, size);
+    if (str == NULL)
+        return NULL;
+    writer.overallocate = 1;
+    size = writer.allocated;
+
+    for (i = 0; i < PyList_GET_SIZE(x); i++) {
+        item = PyList_GET_ITEM(x, i);
+        Py_INCREF(item);
+        value = PyNumber_AsSsize_t(item, NULL);
+        Py_DECREF(item);
+        if (value == -1 && PyErr_Occurred())
+            goto error;
+
+        if (value < 0 || value >= 256) {
+            PyErr_SetString(PyExc_ValueError,
+                            "bytes must be in range(0, 256)");
+            goto error;
+        }
+
+        if (i >= size) {
+            str = _PyBytesWriter_Resize(&writer, str, size+1);
+            if (str == NULL)
+                return NULL;
+            size = writer.allocated;
+        }
+        *str++ = (char) value;
+    }
+    return _PyBytesWriter_Finish(&writer, str);
+
+  error:
+    _PyBytesWriter_Dealloc(&writer);
+    return NULL;
 }
 
 static PyObject*
 _PyBytes_FromTuple(PyObject *x)
 {
-    _PyBytes_FROM_LIST_BODY(x, PyTuple_GET_ITEM);
+    PyObject *bytes;
+    Py_ssize_t i, size = PyTuple_GET_SIZE(x);
+    Py_ssize_t value;
+    char *str;
+    PyObject *item;
+
+    bytes = PyBytes_FromStringAndSize(NULL, size);
+    if (bytes == NULL)
+        return NULL;
+    str = ((PyBytesObject *)bytes)->ob_sval;
+
+    for (i = 0; i < size; i++) {
+        item = PyTuple_GET_ITEM(x, i);
+        value = PyNumber_AsSsize_t(item, NULL);
+        if (value == -1 && PyErr_Occurred())
+            goto error;
+
+        if (value < 0 || value >= 256) {
+            PyErr_SetString(PyExc_ValueError,
+                            "bytes must be in range(0, 256)");
+            goto error;
+        }
+        *str++ = (char) value;
+    }
+    return bytes;
+
+  error:
+    Py_DECREF(bytes);
+    return NULL;
 }
 
 static PyObject *