diff --git a/mmdet/datasets/__init__.py b/mmdet/datasets/__init__.py
index b478acffebb30d475042259a9b97c8307627b5ec..dda1a1eb4a514b93254623693753ecf8f3839c12 100644
--- a/mmdet/datasets/__init__.py
+++ b/mmdet/datasets/__init__.py
@@ -5,6 +5,7 @@ from .utils import to_tensor, random_scale, show_ann, get_dataset
 from .concat_dataset import ConcatDataset
 
 __all__ = [
-    'CustomDataset', 'CocoDataset', 'GroupSampler', 'DistributedGroupSampler', 'ConcatDataset',
-    'build_dataloader', 'to_tensor', 'random_scale', 'show_ann', 'get_dataset'
+    'CustomDataset', 'CocoDataset', 'GroupSampler', 'DistributedGroupSampler',
+    'ConcatDataset', 'build_dataloader', 'to_tensor', 'random_scale',
+    'show_ann', 'get_dataset'
 ]
diff --git a/mmdet/datasets/concat_dataset.py b/mmdet/datasets/concat_dataset.py
index 47e073a2bf3e83413c1eb0446c6d009445987b78..605d112f628ba0fc2f411574324b552b59cd9862 100644
--- a/mmdet/datasets/concat_dataset.py
+++ b/mmdet/datasets/concat_dataset.py
@@ -5,7 +5,7 @@ from torch.utils.data.dataset import ConcatDataset as _ConcatDataset
 
 class ConcatDataset(_ConcatDataset):
     """
-    Same as torch.utils.data.dataset.ConcatDataset, but 
+    Same as torch.utils.data.dataset.ConcatDataset, but
     concat the group flag for image aspect ratio.
     """
     def __init__(self, datasets):
@@ -13,7 +13,7 @@ class ConcatDataset(_ConcatDataset):
         flag: Images with aspect ratio greater than 1 will be set as group 1,
               otherwise group 0.
         """
-        super(ConcatDataset, self).__init__(datasets)        
+        super(ConcatDataset, self).__init__(datasets)
         if hasattr(datasets[0], 'flag'):
             flags = []
             for i in range(0, len(datasets)):
@@ -27,4 +27,3 @@ class ConcatDataset(_ConcatDataset):
         else:
             sample_idx = idx - self.cumulative_sizes[dataset_idx - 1]
         return dataset_idx, sample_idx
-
diff --git a/mmdet/datasets/utils.py b/mmdet/datasets/utils.py
index d13ae892d485946f122cc33516cfbf78367f7582..92b4e5d57b2ce28664298a51cfc85f450b5c63b3 100644
--- a/mmdet/datasets/utils.py
+++ b/mmdet/datasets/utils.py
@@ -9,6 +9,7 @@ import numpy as np
 from .concat_dataset import ConcatDataset
 from .. import datasets
 
+
 def to_tensor(data):
     """Convert objects of various python types to :obj:`torch.Tensor`.
 
@@ -72,7 +73,8 @@ def show_ann(coco, img, ann_info):
 
 
 def get_dataset(data_cfg):
-    if isinstance(data_cfg['ann_file'], list) or isinstance(data_cfg['ann_file'], tuple):
+    if isinstance(data_cfg['ann_file'], list) or \
+            isinstance(data_cfg['ann_file'], tuple):
         ann_files = data_cfg['ann_file']
         dsets = []
         for ann_file in ann_files:
@@ -81,9 +83,9 @@ def get_dataset(data_cfg):
             dset = obj_from_dict(data_info, datasets)
             dsets.append(dset)
         if len(dsets) > 1:
-            dset = ConcatDataset(dsets)            
+            dset = ConcatDataset(dsets)
         else:
             dset = dsets[0]
     else:
         dset = obj_from_dict(data_cfg, datasets)
-    return dset
\ No newline at end of file
+    return dset
diff --git a/tools/train.py b/tools/train.py
index 006fc1197f5ea4e1df866c05d8323070b08d51df..49c46f05f88393f24f14a5f3a2f2fc4f6b58bbaf 100644
--- a/tools/train.py
+++ b/tools/train.py
@@ -2,7 +2,6 @@ from __future__ import division
 
 import argparse
 from mmcv import Config
-from mmcv.runner import obj_from_dict
 
 from mmdet import datasets, __version__
 from mmdet.apis import (train_detector, init_dist, get_root_logger,