cache task.to_str_params() to speedup Task initialization

gokart/task.py

@@ -1,3 +1,4 @@
+import functools
 import hashlib
 import inspect
 import os
@@ -118,6 +119,11 @@ def __init__(self, *args, **kwargs):
         self._rerun_state = self.rerun
         self._lock_at_dump = True
 
+        # Cache to_str_params to avoid slow task creation in a deep task tree.
+        # For example, gokart.build(RecursiveTask(dep=RecursiveTask(dep=RecursiveTask(dep=HelloWorldTask())))) results in O(n^2) calls to to_str_params.
+        # However, @lru_cache cannot be used as a decorator because luigi.Task employs metaclass tricks.
+        self.to_str_params = functools.lru_cache(maxsize=None)(self.to_str_params)  # type: ignore[method-assign]
+
         if self.complete_check_at_run:
             self.run = task_complete_check_wrapper(run_func=self.run, complete_check_func=self.complete)  # type: ignore
 

test/test_task_on_kart.py

@@ -584,6 +584,17 @@ def test_serialize_and_deserialize_default_values(self):
         deserialized: gokart.TaskOnKart = luigi.task_register.load_task(None, task.get_task_family(), task.to_str_params())
         self.assertDictEqual(task.to_str_params(), deserialized.to_str_params())
 
+    def test_to_str_params_changes_on_values_and_flags(self):
+        class _DummyTaskWithParams(gokart.TaskOnKart):
+            task_namespace = __name__
+            param: str = luigi.Parameter()
+
+        t1 = _DummyTaskWithParams(param='a')
+        self.assertEqual(t1.to_str_params(), t1.to_str_params())  # cache
+        self.assertEqual(t1.to_str_params(), _DummyTaskWithParams(param='a').to_str_params())  # same value
+        self.assertNotEqual(t1.to_str_params(), _DummyTaskWithParams(param='b').to_str_params())  # different value
+        self.assertNotEqual(t1.to_str_params(), t1.to_str_params(only_significant=True))
+
     def test_should_lock_run_when_set(self):
         class _DummyTaskWithLock(gokart.TaskOnKart):
             def run(self):