アノテーションで囲まれた部分を画像ファイルで出力したい

VIAというアノテーションソフトを使って囲んだ部分のjsonファイルを使って、画像から囲んだ部分だけをとりだそうとしていますが。ファイルの書き出しが上手くいかないので教えていただきたいです。
まず、コードは以下のような感じです。

python
1def make_dataset_for_VIA(json_filename,base_dir,has_size_fitting=False):
2    import json
3    import os
4    import sys
5    sys.stdout.flush()
6    print 'the paramater of "has_size_fitting" is %s' % (str(has_size_fitting),)
7
8    with open(os.path.join(base_dir,json_filename), 'r') as fp:
9        tree = json.load(fp)
10    result = []
11    for v in tree['_via_img_metadata'].itervalues():
12        filename = v['filename']
13        full_filename = os.path.join(base_dir, filename)
14        if not os.path.isfile(full_filename):
15            print('there is no file named %s!'%(full_filename,))
16            continue
17            # raise IOError(full_filename)
18        sys.stdout.write('.')
19        sys.stdout.flush()
20        basename, ext = os.path.splitext(filename)
21        regions = v['regions']
22        arr = cv2.imread(os.path.join(base_dir, filename))
23        bboxes = []
24        wh = []
25        img = []
26
27        for r in v['regions']:
28            attr = r['shape_attributes']
29            width = attr['width']
30            height = attr['height']
31
32            xmin = attr['x']
33            ymin = attr['y']
34            xmax = xmin + width
35            ymax = ymin + height
36            bboxes.append([ymin, xmin, ymax, xmax])
37
38
39            wh.append([width,height])
40            if not has_size_fitting:
41                img.append(trim_img(arr,[ymin, xmin, ymax, xmax]))#arr:フライの元画像
42        if len(bboxes) == 0:
43            continue
44        if has_size_fitting:
45            stride = 5
46            maxw,maxh = check_maximum_width_and_height(wh)
47            for index,(ymin,xmin,ymax,xmax) in enumerate(bboxes):
48                width_tmp = xmin-xmax
49                xmin -= stride
50                xmax = xmin + maxw+stride
51                bboxes[index][1] = xmin
52                bboxes[index][3] = xmax
53                height_tmp = xmin-xmax
54                ymax = ymin + maxh+stride
55                ymin -= stride
56                bboxes[index][2] = ymax
57                bboxes[index][0] = ymin
58
59            for coords in bboxes:
60                img.append(trim_img(arr,coords))
61
62        bboxes = np.array(bboxes, np.float32)
63        result.append((filename,bboxes,wh,img))
64    sys.stdout.write('\n')
65    sys.stdout.flush()
66    return result
67
68def main():
69    a = make_dataset_for_VIA('size_sample_expand.json','data',has_size_fitting=False)
70    for i in range(len(a)):#aの長さだけ回す
71       img = a[i][3]#img画
72       filename = 'grabcut_result_id=%d.png' % (i)
73       cv2.imwrite(os.path.join('good_data',filename),img)
74
75if __name__ == '__main__':
76    a = make_dataset_for_VIA('size_sample_expand.json','data',has_size_fitting=False)
77    for i in range(len(a)):#aの長さだけ回す
78       img = a[i][3]#img画像
79       filename = 'result_id=%d.png' % (i)
80       cv2.imwrite(os.path.join('good_data',filename),img)