simplify the code and add docs

qlib/workflow/exp.py

@@ -1,6 +1,7 @@
 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT License.
 
+from typing import Union
 import mlflow, logging
 from mlflow.entities import ViewType
 from mlflow.exceptions import MlflowException
@@ -324,14 +325,21 @@ def delete_recorder(self, recorder_id=None, recorder_name=None):
 
     UNLIMITED = 50000  # FIXME: Mlflow can only list 50000 records at most!!!!!!!
 
-    def list_recorders(self, max_results=UNLIMITED, status=None):
+    def list_recorders(self, max_results: int=UNLIMITED, status: Union[str, None]=None):
+        """
+        Parameters
+        ----------
+        max_results : int
+            the number limitation of the results
+        status : str
+            the criteria based on status to filter results.
+            `None` indicates no filtering.
+        """
         runs = self._client.search_runs(self.id, run_view_type=ViewType.ACTIVE_ONLY, max_results=max_results)
         recorders = dict()
         for i in range(len(runs)):
             recorder = MLflowRecorder(self.id, self._uri, mlflow_run=runs[i])
-            if status is not None:
-                if recorder.status != status:
-                    continue
-            recorders[runs[i].info.run_id] = recorder
+            if status is None or recorder.status == status:
+                recorders[runs[i].info.run_id] = recorder
 
         return recorders