diff --git a/fastNLP/core/dataset.py b/fastNLP/core/dataset.py
index 702d37a1..2075515e 100644
--- a/fastNLP/core/dataset.py
+++ b/fastNLP/core/dataset.py
@@ -34,21 +34,29 @@ class DataSet(object):
 
         def __next__(self):
             self.idx += 1
-            try:
-                self.fields = {name: field[self.idx] for name, field in self.dataset.get_fields().items()}
-            except IndexError:
+            if self.idx >= len(self.dataset):
                 raise StopIteration
             return self
 
         def __getitem__(self, name):
-            return self.fields[name]
+            return self.dataset[name][self.idx]
 
         def __setitem__(self, name, val):
             if name not in self.dataset:
                 new_fields = [None] * len(self.dataset)
                 self.dataset.add_field(name, new_fields)
             self.dataset[name][self.idx] = val
-            self.fields[name] = val
+
+        def __getattr__(self, item):
+            if item == 'fields':
+                self.fields = {name: field[self.idx] for name, field in self.dataset.get_fields().items()}
+                return self.fields
+            else:
+                raise AttributeError('{} does not exist.'.format(item))
+
+        def __setattr__(self, key, value):
+            self.__setitem__(key, value)
+
 
         def __repr__(self):
             return "\n".join(['{}: {}'.format(name, repr(self.dataset[name][self.idx])) for name
@@ -201,23 +209,19 @@ class DataSet(object):
                 raise KeyError
         return self
 
-    def __getattribute__(self, name):
-        if name in _READERS:
+    def __getattr__(self, item):
+        if item in self.field_arrays:
+            return self.field_arrays[item]
+        elif item in _READERS:
             # add read_*data() support
             def _read(*args, **kwargs):
-                data = _READERS[name]().load(*args, **kwargs)
+                data = _READERS[item]().load(*args, **kwargs)
                 self.extend(data)
                 return self
 
             return _read
         else:
-            return object.__getattribute__(self, name)
-
-    def __getattr__(self, item):
-        if item in self.field_arrays:
-            return self.field_arrays[item]
-        else:
-            self.__getattribute__(item)
+            raise AttributeError('{} does not exist.'.format(item))
 
     @classmethod
     def set_reader(cls, method_name):
diff --git a/fastNLP/core/instance.py b/fastNLP/core/instance.py
index 12de4efa..89cf1221 100644
--- a/fastNLP/core/instance.py
+++ b/fastNLP/core/instance.py
@@ -12,19 +12,6 @@ class Instance(object):
         self.fields[field_name] = field
         return self
 
-    def rename_field(self, old_name, new_name):
-        if old_name in self.fields:
-            self.fields[new_name] = self.fields.pop(old_name)
-        else:
-            raise KeyError("error, no such field: {}".format(old_name))
-        return self
-
-    def set_target(self, **fields):
-        for name, val in fields.items():
-            if name in self.fields:
-                self.fields[name].is_target = val
-        return self
-
     def __getitem__(self, name):
         if name in self.fields:
             return self.fields[name]
@@ -34,5 +21,14 @@ class Instance(object):
     def __setitem__(self, name, field):
         return self.add_field(name, field)
 
+    def __getattr__(self, item):
+        if item in self.fields:
+            return self.fields[item]
+        else:
+            raise AttributeError('{} does not exist.'.format(item))
+
+    def __setattr__(self, key, value):
+        self.__setitem__(key, value)
+
     def __repr__(self):
         return self.fields.__repr__()