cache task.to_str_params() to speedup Task initialization

gokart/task.py

@@ -117,6 +117,7 @@ def __init__(self, *args, **kwargs):
         super(TaskOnKart, self).__init__(*args, **kwargs)
         self._rerun_state = self.rerun
         self._lock_at_dump = True
+        self._str_params_cache = None
 
         if self.complete_check_at_run:
             self.run = task_complete_check_wrapper(run_func=self.run, complete_check_func=self.complete)  # type: ignore
@@ -373,6 +374,18 @@ def make_unique_id(self) -> str:
             self.task_unique_id = unique_id
         return unique_id
 
+    def to_str_params(self, only_significant=False, only_public=False):
+        if only_significant == True and only_public == False:
+            # cache to_str_params to avoid too slow task creation of deep task tree
+            # e.g. gokart.build(RecursiveTask(dep=RecursiveTask(dep=RecursiveTask(dep=HelloWorldTask())))) takes O(n*n) to_str_params calls
+            if self._str_params_cache is not None:
+                return self._str_params_cache
+            else:
+                self._str_params_cache = super().to_str_params(only_significant, only_public)
+                return self._str_params_cache
+        else:
+            return super().to_str_params(only_significant, only_public)
+
     def _make_hash_id(self) -> str:
         def _to_str_params(task):
             if isinstance(task, TaskOnKart):