diff --git a/paddle/py_paddle/dataprovider_converter.py b/paddle/py_paddle/dataprovider_converter.py
index f1ed57f13fcea65ad578880d65a1809c1f514da9..6d6a406cf61d467cb2701ca5e85e99648eea36eb 100644
--- a/paddle/py_paddle/dataprovider_converter.py
+++ b/paddle/py_paddle/dataprovider_converter.py
@@ -22,6 +22,19 @@ __all__ = ['DataProviderConverter']
 
 
 class IScanner(object):
+    """
+    The scanner will scan Python object two passes, then convert it to Paddle's
+    argument.
+
+    In the first pass, `pre_scan` will be invoked by every data instance, and
+    then invoke `finish_pre_scan` to arguments. And the second pass do the same
+    thing except the functions changed to `scan`, `finish_scan`.
+
+    During the first pass, a scanner may count the shape of input matrix and
+    allocate memory for this argument. Then fill the data into this  argument
+    in second pass.
+    """
+
     def __init__(self, input_type, pos):
         self.input_type = input_type
         if not isinstance(self.input_type, dp2.InputType):
@@ -38,15 +51,39 @@ class IScanner(object):
         ) and swig_paddle.getTrainerCount() == 1
 
     def pre_scan(self, dat):
+        """
+        First pass scan method. During this method, the scanner could count the
+        data number, and get the total memory size this batch would use.
+
+        :param dat: The python object.
+        """
         pass
 
     def finish_pre_scan(self, argument):
+        """
+        Finish first scan pass. Allocate the memory.
+
+        :param argument: Output arguments object.
+        :type argument: swig_paddle.Arguments
+        :return:
+        """
         pass
 
     def scan(self, dat):
+        """
+        Second pass scan method. Copy the data to arguments.
+
+        :param dat: The python object.
+        """
         pass
 
     def finish_scan(self, argument):
+        """
+        Finish second pass. Finalize the resources, etc.
+
+        :param argument: Output arguments object.
+        :type argument: swig_paddle.Arguments
+        """
         pass