diff --git a/fastNLP/core/batch.py b/fastNLP/core/batch.py
index 88d9185d..d07df047 100644
--- a/fastNLP/core/batch.py
+++ b/fastNLP/core/batch.py
@@ -14,15 +14,17 @@ class Batch(object):
 
     :param DataSet dataset: a DataSet object
     :param int batch_size: the size of the batch
-    :param Sampler sampler: a Sampler object
+    :param Sampler sampler: a Sampler object. If None, use fastNLP.sampler.RandomSampler
     :param bool as_numpy: If True, return Numpy array. Otherwise, return torch tensors.
     :param bool prefetch: If True, use multiprocessing to fetch next batch when training.
     :param str or torch.device device: the batch's device, if as_numpy is True, device is ignored.
     """
 
-    def __init__(self, dataset, batch_size, sampler=RandomSampler(), as_numpy=False, prefetch=False):
+    def __init__(self, dataset, batch_size, sampler=None, as_numpy=False, prefetch=False):
         self.dataset = dataset
         self.batch_size = batch_size
+        if sampler is None:
+            sampler = RandomSampler()
         self.sampler = sampler
         self.as_numpy = as_numpy
         self.idx_list = None
diff --git a/fastNLP/core/callback.py b/fastNLP/core/callback.py
index e3b4f36e..57f94bc4 100644
--- a/fastNLP/core/callback.py
+++ b/fastNLP/core/callback.py
@@ -17,37 +17,37 @@ class Callback(object):
         super(Callback, self).__init__()
         self.trainer = None  # 在Trainer内部被重新赋值
 
-        # callback只读属性
-        self._n_epochs = None
-        self._n_steps = None
-        self._batch_size = None
-        self._model = None
-        self._pbar = None
-        self._optimizer = None
-
     @property
     def n_epochs(self):
-        return self._n_epochs
+        return self.trainer.n_epochs
+
+    @property
+    def epoch(self):
+        return self.trainer.epoch
 
     @property
     def n_steps(self):
-        return self._n_steps
+        return self.trainer.n_steps
+
+    @property
+    def step(self):
+        return self.trainer.step
 
     @property
     def batch_size(self):
-        return self._batch_size
+        return self.trainer.batch_size
 
     @property
     def model(self):
-        return self._model
+        return self.trainer.model
 
     @property
     def pbar(self):
-        return self._pbar
+        return self.trainer.pbar
 
     @property
     def optimizer(self):
-        return self._optimizer
+        return self.trainer.optimizer
 
     def on_train_begin(self):
         # before the main training loop
@@ -82,13 +82,14 @@ class Callback(object):
     def on_valid_begin(self):
         pass
 
-    def on_valid_end(self, eval_result, metric_key, optimizer):
+    def on_valid_end(self, eval_result, metric_key, optimizer, is_better_eval):
         """
         每次执行验证机的evaluation后会调用。传入eval_result
 
         :param eval_result: Dict[str: Dict[str: float]], evaluation的结果
         :param metric_key: str
-        :param optimizer:
+        :param optimizer: optimizer passed to trainer
+        :param is_better_eval: bool, 当前dev结果是否比之前的好
         :return:
         """
         pass
@@ -145,11 +146,10 @@ class CallbackManager(Callback):
 
     """
 
-    def __init__(self, env, attr, callbacks=None):
+    def __init__(self, env, callbacks=None):
         """
 
         :param dict env: The key is the name of the Trainer attribute(str). The value is the attribute itself.
-        :param dict attr: read-only attributes for all callbacks
         :param Callback callbacks:
         """
         super(CallbackManager, self).__init__()
@@ -170,19 +170,6 @@ class CallbackManager(Callback):
             for callback in self.callbacks:
                 setattr(callback, env_name, env_val)  # Callback.trainer
 
-        self.set_property(**attr)
-
-    def set_property(self, **kwargs):
-        """设置所有callback的只读属性
-
-        :param kwargs:
-        :return:
-        """
-        for callback in self.callbacks:
-            for k, v in kwargs.items():
-                setattr(callback, "_" + k, v)
-
-
     @transfer
     def on_train_begin(self):
         pass
@@ -220,7 +207,7 @@ class CallbackManager(Callback):
         pass
 
     @transfer
-    def on_valid_end(self, eval_result, metric_key, optimizer):
+    def on_valid_end(self, eval_result, metric_key, optimizer, is_better_eval):
         pass
 
     @transfer
diff --git a/fastNLP/core/dataset.py b/fastNLP/core/dataset.py
index 24376a72..068afb38 100644
--- a/fastNLP/core/dataset.py
+++ b/fastNLP/core/dataset.py
@@ -90,7 +90,7 @@ class DataSet(object):
             data_set = DataSet()
             for field in self.field_arrays.values():
                 data_set.add_field(name=field.name, fields=field.content[idx], padder=field.padder,
-                                   is_input=field.is_input, is_target=field.is_target)
+                                   is_input=field.is_input, is_target=field.is_target, ignore_type=field.ignore_type)
             return data_set
         elif isinstance(idx, str):
             if idx not in self:
@@ -313,16 +313,23 @@ class DataSet(object):
         else:
             return results
 
-    def drop(self, func):
+    def drop(self, func, inplace=True):
         """Drop instances if a condition holds.
 
         :param func: a function that takes an Instance object as input, and returns bool.
             The instance will be dropped if the function returns True.
+        :param inplace: bool, whether to drop inpalce. Otherwise a new dataset will be returned.
 
         """
-        results = [ins for ins in self._inner_iter() if not func(ins)]
-        for name, old_field in self.field_arrays.items():
-            self.field_arrays[name].content = [ins[name] for ins in results]
+        if inplace:
+            results = [ins for ins in self._inner_iter() if not func(ins)]
+            for name, old_field in self.field_arrays.items():
+                self.field_arrays[name].content = [ins[name] for ins in results]
+        else:
+            results = [ins for ins in self if not func(ins)]
+            data = DataSet(results)
+            for field_name, field in self.field_arrays.items():
+                data.field_arrays[field_name].to(field)
 
     def split(self, dev_ratio):
         """Split the dataset into training and development(validation) set.
@@ -346,19 +353,8 @@ class DataSet(object):
         for idx in train_indices:
             train_set.append(self[idx])
         for field_name in self.field_arrays:
-            train_set.field_arrays[field_name].is_input = self.field_arrays[field_name].is_input
-            train_set.field_arrays[field_name].is_target = self.field_arrays[field_name].is_target
-            train_set.field_arrays[field_name].padder = self.field_arrays[field_name].padder
-            train_set.field_arrays[field_name].dtype = self.field_arrays[field_name].dtype
-            train_set.field_arrays[field_name].pytype = self.field_arrays[field_name].pytype
-            train_set.field_arrays[field_name].content_dim = self.field_arrays[field_name].content_dim
-
-            dev_set.field_arrays[field_name].is_input = self.field_arrays[field_name].is_input
-            dev_set.field_arrays[field_name].is_target = self.field_arrays[field_name].is_target
-            dev_set.field_arrays[field_name].padder = self.field_arrays[field_name].padder
-            dev_set.field_arrays[field_name].dtype = self.field_arrays[field_name].dtype
-            dev_set.field_arrays[field_name].pytype = self.field_arrays[field_name].pytype
-            dev_set.field_arrays[field_name].content_dim = self.field_arrays[field_name].content_dim
+            train_set.field_arrays[field_name].to(self.field_arrays[field_name])
+            dev_set.field_arrays[field_name].to(self.field_arrays[field_name])
 
         return train_set, dev_set
 
diff --git a/fastNLP/core/fieldarray.py b/fastNLP/core/fieldarray.py
index 72bb30b5..10fbbebe 100644
--- a/fastNLP/core/fieldarray.py
+++ b/fastNLP/core/fieldarray.py
@@ -383,6 +383,23 @@ class FieldArray(object):
         """
         return len(self.content)
 
+    def to(self, other):
+        """
+        将other的属性复制给本fieldarray(必须通过fieldarray类型). 包含 is_input, is_target, padder, dtype, pytype, content_dim
+            ignore_type
+
+        :param other: FieldArray
+        :return:
+        """
+        assert isinstance(other, FieldArray), "Only support FieldArray type, not {}.".format(type(other))
+
+        self.is_input = other.is_input
+        self.is_target = other.is_target
+        self.padder = other.padder
+        self.dtype = other.dtype
+        self.pytype = other.pytype
+        self.content_dim = other.content_dim
+        self.ignore_type = other.ignore_type
 
 def is_iterable(content):
     try:
diff --git a/fastNLP/core/metrics.py b/fastNLP/core/metrics.py
index 64555e12..3d3647c4 100644
--- a/fastNLP/core/metrics.py
+++ b/fastNLP/core/metrics.py
@@ -91,7 +91,6 @@ class MetricBase(object):
     Besides, before passing params into self.evaluate, this function will filter out params from output_dict and
     target_dict which are not used in self.evaluate. (but if **kwargs presented in self.evaluate, no filtering
     will be conducted.)
-    However, in some cases where type check is not necessary, ``_fast_param_map`` will be used.
 
     """
     def __init__(self):
@@ -146,21 +145,6 @@ class MetricBase(object):
     def get_metric(self, reset=True):
         raise NotImplemented
 
-    def _fast_param_map(self, pred_dict, target_dict):
-        """Only used as inner function. When the pred_dict, target is unequivocal. Don't need users to pass key_map.
-            such as pred_dict has one element, target_dict has one element
-
-        :param pred_dict:
-        :param target_dict:
-        :return: dict, if dict is not {}, pass it to self.evaluate. Otherwise do mapping.
-        """
-        fast_param = {}
-        if len(self.param_map) == 2 and len(pred_dict) == 1 and len(target_dict) == 1:
-            fast_param['pred'] = list(pred_dict.values())[0]
-            fast_param['target'] = list(target_dict.values())[0]
-            return fast_param
-        return fast_param
-
     def __call__(self, pred_dict, target_dict):
         """
 
@@ -172,7 +156,6 @@ class MetricBase(object):
         Besides, before passing params into self.evaluate, this function will filter out params from output_dict and
             target_dict which are not used in self.evaluate. (but if **kwargs presented in self.evaluate, no filtering
             will be conducted.)
-        This function also support _fast_param_map.
         :param pred_dict: usually the output of forward or prediction function
         :param target_dict: usually features set as target..
         :return:
@@ -180,11 +163,6 @@ class MetricBase(object):
         if not callable(self.evaluate):
             raise TypeError(f"{self.__class__.__name__}.evaluate has to be callable, not {type(self.evaluate)}.")
 
-        fast_param = self._fast_param_map(pred_dict=pred_dict, target_dict=target_dict)
-        if fast_param:
-            self.evaluate(**fast_param)
-            return
-
         if not self._checked:
             # 1. check consistence between signature and param_map
             func_spect = inspect.getfullargspec(self.evaluate)
@@ -262,41 +240,6 @@ class AccuracyMetric(MetricBase):
         self.total = 0
         self.acc_count = 0
 
-    def _fast_param_map(self, pred_dict, target_dict):
-        """Only used as inner function. When the pred_dict, target is unequivocal. Don't need users to pass key_map.
-            such as pred_dict has one element, target_dict has one element
-
-        :param pred_dict:
-        :param target_dict:
-        :return: dict, if dict is not None, pass it to self.evaluate. Otherwise do mapping.
-        """
-        fast_param = {}
-        targets = list(target_dict.values())
-        if len(targets) == 1 and isinstance(targets[0], torch.Tensor):
-            if len(pred_dict) == 1:
-                pred = list(pred_dict.values())[0]
-                fast_param['pred'] = pred
-            elif len(pred_dict) == 2:
-                pred1 = list(pred_dict.values())[0]
-                pred2 = list(pred_dict.values())[1]
-                if not (isinstance(pred1, torch.Tensor) and isinstance(pred2, torch.Tensor)):
-                    return fast_param
-                if len(pred1.size()) < len(pred2.size()) and len(pred1.size()) == 1:
-                    seq_lens = pred1
-                    pred = pred2
-                elif len(pred1.size()) > len(pred2.size()) and len(pred2.size()) == 1:
-                    seq_lens = pred2
-                    pred = pred1
-                else:
-                    return fast_param
-                fast_param['pred'] = pred
-                fast_param['seq_lens'] = seq_lens
-            else:
-                return fast_param
-            fast_param['target'] = targets[0]
-        # TODO need to make sure they all have same batch_size
-        return fast_param
-
     def evaluate(self, pred, target, seq_lens=None):
         """
 
@@ -321,7 +264,7 @@ class AccuracyMetric(MetricBase):
                             f"got {type(seq_lens)}.")
 
         if seq_lens is not None:
-            masks = seq_lens_to_masks(seq_lens=seq_lens, float=True)
+            masks = seq_lens_to_masks(seq_lens=seq_lens).long()
         else:
             masks = None
 
@@ -334,14 +277,12 @@ class AccuracyMetric(MetricBase):
                                f"size:{pred.size()}, target should have size: {pred.size()} or "
                                f"{pred.size()[:-1]}, got {target.size()}.")
 
-        pred = pred.float()
-        target = target.float()
 
         if masks is not None:
-            self.acc_count += torch.sum(torch.eq(pred, target).float() * masks.float()).item()
-            self.total += torch.sum(masks.float()).item()
+            self.acc_count += torch.sum(torch.eq(pred, target) * masks).item()
+            self.total += torch.sum(masks).item()
         else:
-            self.acc_count += torch.sum(torch.eq(pred, target).float()).item()
+            self.acc_count += torch.sum(torch.eq(pred, target)).item()
             self.total += np.prod(list(pred.size()))
 
     def get_metric(self, reset=True):
@@ -350,7 +291,7 @@ class AccuracyMetric(MetricBase):
         :param bool reset: whether to recount next time.
         :return evaluate_result: {"acc": float}
         """
-        evaluate_result = {'acc': round(self.acc_count / self.total, 6)}
+        evaluate_result = {'acc': round(float(self.acc_count) / (self.total + 1e-12), 6)}
         if reset:
             self.acc_count = 0
             self.total = 0
@@ -441,8 +382,7 @@ def bio_tag_to_spans(tags, ignore_labels=None):
         prev_bio_tag = bio_tag
     return [(span[0], (span[1][0], span[1][1]+1))
                     for span in spans
-                        if span[0] not in ignore_labels
-            ]
+                        if span[0] not in ignore_labels]
 
 
 class SpanFPreRecMetric(MetricBase):
diff --git a/fastNLP/core/trainer.py b/fastNLP/core/trainer.py
index ca2ff93b..e678ea3d 100644
--- a/fastNLP/core/trainer.py
+++ b/fastNLP/core/trainer.py
@@ -34,7 +34,7 @@ class Trainer(object):
     def __init__(self, train_data, model, loss=None, metrics=None, n_epochs=3, batch_size=32, print_every=50,
                  validate_every=-1, dev_data=None, save_path=None, optimizer=None,
                  check_code_level=0, metric_key=None, sampler=None, prefetch=False, use_tqdm=True,
-                 use_cuda=False, callbacks=None):
+                 use_cuda=False, callbacks=None, update_every=1):
         """
         :param DataSet train_data: the training data
         :param torch.nn.modules.module model: a PyTorch model
@@ -62,6 +62,8 @@ class Trainer(object):
         :param bool use_tqdm: whether to use tqdm to show train progress.
         :param callbacks: List[Callback]. 用于在train过程中起调节作用的回调函数。比如early stop，negative sampling等可以
             通过callback机制实现。
+        :param update_every: int, 多少步更新一次梯度。用于希望累计梯度的场景，比如需要128的batch_size, 但是直接设为128会导致内存
+            不足，通过设置batch_size=32, update_every=4达到目的
         """
         super(Trainer, self).__init__()
 
@@ -76,6 +78,10 @@ class Trainer(object):
         if metrics and (dev_data is None):
             raise ValueError("No dev_data for evaluations, pass dev_data or set metrics to None. ")
 
+        # check update every
+        assert update_every>=1, "update_every must be no less than 1."
+        self.update_every = int(update_every)
+
         # check save_path
         if not (save_path is None or isinstance(save_path, str)):
             raise ValueError("save_path can only be None or `str`.")
@@ -144,11 +150,9 @@ class Trainer(object):
         self.start_time = None  # start timestamp
 
         self.callback_manager = CallbackManager(env={"trainer": self},
-                                                attr={"n_epochs": self.n_epochs, "n_steps": self.step,
-                                                      "batch_size": self.batch_size, "model": self.model,
-                                                      "optimizer": self.optimizer},
                                                 callbacks=callbacks)
 
+
     def train(self, load_best_model=True):
         """
 
@@ -241,7 +245,6 @@ class Trainer(object):
             avg_loss = 0
             data_iterator = Batch(self.train_data, batch_size=self.batch_size, sampler=self.sampler, as_numpy=False,
                                   prefetch=self.prefetch)
-            self.callback_manager.set_property(pbar=pbar)
             for epoch in range(1, self.n_epochs+1):
                 pbar.set_description_str(desc="Epoch {}/{}".format(epoch, self.n_epochs))
                 # early stopping
@@ -257,6 +260,7 @@ class Trainer(object):
                     self.callback_manager.on_loss_begin(batch_y, prediction)
                     loss = self._compute_loss(prediction, batch_y)
                     avg_loss += loss.item()
+                    loss = loss/self.update_every
 
                     # Is loss NaN or inf? requires_grad = False
                     self.callback_manager.on_backward_begin(loss, self.model)
@@ -267,8 +271,9 @@ class Trainer(object):
                     self.callback_manager.on_step_end(self.optimizer)
 
                     if (self.step+1) % self.print_every == 0:
+                        avg_loss = avg_loss / self.print_every
                         if self.use_tqdm:
-                            print_output = "loss:{0:<6.5f}".format(avg_loss / self.print_every)
+                            print_output = "loss:{0:<6.5f}".format(avg_loss)
                             pbar.update(self.print_every)
                         else:
                             end = time.time()
@@ -286,8 +291,8 @@ class Trainer(object):
                         eval_res = self._do_validation(epoch=epoch, step=self.step)
                         eval_str = "Evaluation at Epoch {}/{}. Step:{}/{}. ".format(epoch, self.n_epochs, self.step,
                                                                                     total_steps) + \
-                                   self.tester._format_eval_results(eval_res)
-                        pbar.write(eval_str)
+                                            self.tester._format_eval_results(eval_res)
+                        pbar.write(eval_str + '\n')
 
                 # ================= mini-batch end ==================== #
 
@@ -301,6 +306,7 @@ class Trainer(object):
         self.callback_manager.on_valid_begin()
         res = self.tester.test()
 
+        is_better_eval = False
         if self._better_eval_result(res):
             if self.save_path is not None:
                 self._save_model(self.model,
@@ -310,8 +316,9 @@ class Trainer(object):
             self.best_dev_perf = res
             self.best_dev_epoch = epoch
             self.best_dev_step = step
+            is_better_eval = True
         # get validation results; adjust optimizer
-        self.callback_manager.on_valid_end(res, self.metric_key, self.optimizer)
+        self.callback_manager.on_valid_end(res, self.metric_key, self.optimizer, is_better_eval)
         return res
 
     def _mode(self, model, is_test=False):
@@ -330,7 +337,8 @@ class Trainer(object):
         """Perform weight update on a model.
 
         """
-        self.optimizer.step()
+        if (self.step+1)%self.update_every==0:
+            self.optimizer.step()
 
     def _data_forward(self, network, x):
         x = _build_args(network.forward, **x)
@@ -346,7 +354,8 @@ class Trainer(object):
 
         For PyTorch, just do "loss.backward()"
         """
-        self.model.zero_grad()
+        if self.step%self.update_every==0:
+            self.model.zero_grad()
         loss.backward()
 
     def _compute_loss(self, predict, truth):
diff --git a/setup.py b/setup.py
index a8b4834e..b7834d8d 100644
--- a/setup.py
+++ b/setup.py
@@ -13,12 +13,12 @@ with open('requirements.txt', encoding='utf-8') as f:
 
 setup(
     name='FastNLP',
-    version='0.1.1',
+    version='0.4.0',
     description='fastNLP: Deep Learning Toolkit for NLP, developed by Fudan FastNLP Team',
     long_description=readme,
     license=license,
     author='FudanNLP',
-    python_requires='>=3.5',
+    python_requires='>=3.6',
     packages=find_packages(),
     install_requires=reqs.strip().split('\n'),
 )
diff --git a/test/automl/test_enas.py b/test/automl/test_enas.py
index d2d3af05..4fea1063 100644
--- a/test/automl/test_enas.py
+++ b/test/automl/test_enas.py
@@ -35,7 +35,7 @@ class TestENAS(unittest.TestCase):
         print(dataset[0])
 
         # DataSet.drop(func)筛除数据
-        dataset.drop(lambda x: x['seq_len'] <= 3)
+        dataset.drop(lambda x: x['seq_len'] <= 3, inplace=True)
         print(len(dataset))
 
         # 设置DataSet中，哪些field要转为tensor
diff --git a/test/core/test_dataset.py b/test/core/test_dataset.py
index 607f9a13..7f4a7184 100644
--- a/test/core/test_dataset.py
+++ b/test/core/test_dataset.py
@@ -125,7 +125,7 @@ class TestDataSetMethods(unittest.TestCase):
 
     def test_drop(self):
         ds = DataSet({"x": [[1, 2, 3, 4]] * 40, "y": [[5, 6], [7, 8, 9, 0]] * 20})
-        ds.drop(lambda ins: len(ins["y"]) < 3)
+        ds.drop(lambda ins: len(ins["y"]) < 3, inplace=True)
         self.assertEqual(len(ds), 20)
 
     def test_contains(self):
@@ -169,7 +169,7 @@ class TestDataSetMethods(unittest.TestCase):
 
         dataset = DataSet.read_csv('test/data_for_tests/tutorial_sample_dataset.csv', headers=('raw_sentence', 'label'),
                                    sep='\t')
-        dataset.drop(lambda x: len(x['raw_sentence'].split()) == 0)
+        dataset.drop(lambda x: len(x['raw_sentence'].split()) == 0, inplace=True)
         dataset.apply(split_sent, new_field_name='words', is_input=True)
         # print(dataset)
 
diff --git a/test/test_tutorials.py b/test/test_tutorials.py
index 68c874fa..0056dff7 100644
--- a/test/test_tutorials.py
+++ b/test/test_tutorials.py
@@ -35,7 +35,7 @@ class TestTutorial(unittest.TestCase):
         print(dataset[0])
 
         # DataSet.drop(func)筛除数据
-        dataset.drop(lambda x: x['seq_len'] <= 3)
+        dataset.drop(lambda x: x['seq_len'] <= 3, inplace=True)
         print(len(dataset))
 
         # 设置DataSet中，哪些field要转为tensor
@@ -296,7 +296,7 @@ class TestTutorial(unittest.TestCase):
 
         # 筛选数据
         origin_data_set_len = len(data_set)
-        data_set.drop(lambda x: len(x['premise']) <= 6)
+        data_set.drop(lambda x: len(x['premise']) <= 6, inplace=True)
         origin_data_set_len, len(data_set)
 
         # In[17]: