def __init__(self, annotation_file=None):
    """
    Constructor of Microsoft COCO helper class for reading and visualizing annotations.
    :param annotation_file (str): location of annotation file
    :param image_folder (str): location to the folder that hosts images.
    :return:
    """
    # load dataset
    self.dataset,self.anns,self.cats,self.imgs = dict(),dict(),dict(),dict()
    self.imgToAnns, self.catToImgs = defaultdict(list), defaultdict(list)
    if type(annotation_file) == str:
        print('loading annotations into memory...')
        tic = time.time()
        with open(annotation_file, 'r') as f:
            dataset = json.load(f)
        assert type(dataset)==dict, 'annotation file format {} not supported'.format(type(dataset))
        print('Done (t={:0.2f}s)'.format(time.time()- tic))
        self.dataset = dataset
        self.createIndex()

    elif type(annotation_file) == dict:
        tic = time.time()
        self.dataset = annotation_file
        self.createIndex()

def createIndex(self):
    # create index
    # print('creating index...')
    anns, cats, imgs = {}, {}, {}
    imgToAnns,catToImgs = defaultdict(list),defaultdict(list)
    if 'annotations' in self.dataset:
        for ann in self.dataset['annotations']:
            imgToAnns[ann['image_id']].append(ann)
            anns[ann['id']] = ann

    if 'images' in self.dataset:
        for img in self.dataset['images']:
            imgs[img['id']] = img

    if 'categories' in self.dataset:
        for cat in self.dataset['categories']:
            cats[cat['id']] = cat

    if 'annotations' in self.dataset and 'categories' in self.dataset:
        for ann in self.dataset['annotations']:
            catToImgs[ann['category_id']].append(ann['image_id'])

    # print('index created!')

    # create class members
    self.anns = anns
    self.imgToAnns = imgToAnns
    self.catToImgs = catToImgs
    self.imgs = imgs
    self.cats = cats