update python annotation

Author: seiriosPlus

python/paddle/fluid/io.py

@@ -463,8 +463,11 @@ def save_checkpoint(executor,
                     save_interval_secs=600,
                     main_program=None):
     """
-    Save Variables to Checkpoint Directory
-
+    Save Checkpoint will save persistable LodTensor variables from main_program in checkpoint directory,
+    directory named by serial number from 0 to (n -1), save_checkpoint use LRU strategy
+    to keep numbers of checkpoint directory,  the numbers of checkpoint directory are max_num_checkpoints at most,
+    The interval time between two save_checkpoint must great than or equal to save_interval_secs.
+    
     :param dirname
     :param max_num_checkpoints
     :param save_secs
@@ -489,18 +492,19 @@ def save_checkpoint(executor,
         dirname=cur_dir,
         main_program=main_program,
         vars=None,
-        predicate=is_checkpoint_var,
+        predicate=_is_checkpoint_var,
         filename=None)
     _write_success(cur_dir)
     _lru_delete(dirname, max_num_checkpoints)
 
 
 def load_checkpoint(executor, dirname=None, main_program=None):
     """
-    Load Variables from Checkpint Dir
+    Load checkpoint from directory by executor,
+    it will find lastest checkpoint file and load it auto.
 
-    :param dirname
     :param executor
+    :param dirname
     :param main_program
     """
 
@@ -517,14 +521,16 @@ def load_checkpoint(executor, dirname=None, main_program=None):
         executor,
         dirname=cur_dir,
         main_program=main_program,
-        predicate=is_checkpoint_var,
+        predicate=_is_checkpoint_var,
         filename=None)
 
 
-def is_checkpoint_var(var):
+def _is_checkpoint_var(var):
     """
-    VarType will fliter out FEED_MINIBATCH FETCH_LIST RAW
-    VarName will fliter out Gradient
+    checkpoint will not save or load all the variables.
+    var type is FEED_MINIBATCH/FETCH_LIST/RAW and var name is end with @GRAD are discarded.
+    
+    :param var
     """
     if var.desc.type() == core.VarDesc.VarType.FEED_MINIBATCH or \
             var.desc.type() == core.VarDesc.VarType.FETCH_LIST or \
@@ -545,9 +551,6 @@ def _interval_secs_exceed(dirname, save_interval_secs):
 
 
 def _lru_delete(dirname, max_num_checkpoints=3):
-    """
-    retain checkpoint nums with max_num_checkpoints
-    """
     dirs = os.listdir(dirname)
     serials = []
     for serial in dirs:
@@ -568,7 +571,9 @@ def _lru_delete(dirname, max_num_checkpoints=3):
 
 def _write_success(dirname):
     """
-    write _SUCCESS to checkpoint dir
+    write an empty _SUCCESS file to checkpoint dir, indicate this checkpoint is correct.
+
+    :param dirname
     """
     success_file = os.path.join(dirname, SUCCESS_MARK_FILENAME)
     with open(success_file, 'a'):
@@ -577,7 +582,9 @@ def _write_success(dirname):
 
 def _get_lastest_checkpoint_dir(checkpoint_dir):
     """
-    get the biggest number in checkpoint_dir, which has _SUCCESS
+    get the latest file in checkpoint directory, the _SUCCESS file must exist in the directory
+
+    :param checkpoint_dir
     """
     if not checkpoint_dir.strip():
         return -1