def download( self, tarDir = None, imgIds = [] ):
        '''
        Download COCO images from mscoco.org server.
        :param tarDir (str): COCO results directory name
               imgIds (list): images to be downloaded
        :return:
        '''
        if tarDir is None:
            print('Please specify target directory')
            return -1
        if len(imgIds) == 0:
            imgs = self.imgs.values()
        else:
            imgs = self.loadImgs(imgIds)
        N = len(imgs)
        if not os.path.exists(tarDir):
            os.makedirs(tarDir)
        for i, img in enumerate(imgs):
            tic = time.time()
            fname = os.path.join(tarDir, img['file_name'])
            if not os.path.exists(fname):
                urllib.urlretrieve(img['coco_url'], fname)
            print('downloaded %d/%d images (t=%.1fs)'%(i, N, time.time()- tic))

        Download COCO images from mscoco.org server.
        :param tarDir (str): COCO results directory name
               imgIds (list): images to be downloaded
        :return:
        
lib/pycocotools/coco.py