bpo-29737: Optimize concatenating with empty tuple. (python#524)

Author: serhiy-storchaka

Objects/tupleobject.c

@@ -446,13 +446,21 @@ tupleconcat(PyTupleObject *a, PyObject *bb)
     Py_ssize_t i;
     PyObject **src, **dest;
     PyTupleObject *np;
+    if (Py_SIZE(a) == 0 && PyTuple_CheckExact(bb)) {
+        Py_INCREF(bb);
+        return bb;
+    }
     if (!PyTuple_Check(bb)) {
         PyErr_Format(PyExc_TypeError,
              "can only concatenate tuple (not \"%.200s\") to tuple",
                  Py_TYPE(bb)->tp_name);
         return NULL;
     }
 #define b ((PyTupleObject *)bb)
+    if (Py_SIZE(b) == 0 && PyTuple_CheckExact(a)) {
+        Py_INCREF(a);
+        return (PyObject *)a;
+    }
     if (Py_SIZE(a) > PY_SSIZE_T_MAX - Py_SIZE(b))
         return PyErr_NoMemory();
     size = Py_SIZE(a) + Py_SIZE(b);