Introduce constant for ID_SIZE in python code. (#2517)

python/ray/plasma/test/test.py

@@ -18,6 +18,7 @@ import unittest
 import ray
 from ray.plasma.utils import (random_object_id, create_object_with_id,
                               create_object)
+import ray.ray_constants as ray_constants
 from ray import services
 import pyarrow as pa
 import pyarrow.plasma as plasma
@@ -377,7 +378,7 @@ class TestPlasmaManager(unittest.TestCase):
         # Make sure that wait returns when the requested number of object IDs
         # are available and does not wait for all object IDs to be available.
         object_ids = [random_object_id() for _ in range(9)] + \
-                     [plasma.ObjectID(20 * b'\x00')]
+                     [plasma.ObjectID(ray_constants.ID_SIZE * b'\x00')]
         object_ids_perm = object_ids[:]
         random.shuffle(object_ids_perm)
         for i in range(10):