How to download COCO dataset images?

I am trying to download the COCO dataset images using the following COCO API command:

from pycocotools.coco import COCO
import requests
catIds = COCO.getCatIds(catNms=['person','dog', 'car'])

…but am getting the following error message. Any idea why this is happening?

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-42-9cc4e2f62a0e> in <module>
----> 1 catIds = COCO.getCatIds(catNms=['person','dog', 'car'])

TypeError: getCatIds() missing 1 required positional argument: 'self'

Previously, I configured pycocotools using these instructions.

EDIT:

Still some strange error messages. Looking into code it seems as if there are no such categories as a ‘person’,’dog’ or ‘car’. Why is it so?

My code:

a = COCO()  
catIds = a.getCatIds(catNms=['person','dog', 'car'])

Received error message:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-12-57400207fde1> in <module>
      1 a = COCO() # calling init
----> 2 catIds = a.getCatIds(catNms=['person','dog', 'car'])

~Anaconda3libsite-packagespycocotoolscoco.py in getCatIds(self, catNms, supNms, catIds)
    171             cats = self.dataset['categories']
    172         else:
--> 173             cats = self.dataset['categories']
    174             cats = cats if len(catNms) == 0 else [cat for cat in cats if cat['name']          in catNms]
    175             cats = cats if len(supNms) == 0 else [cat for cat in cats if cat['supercategory'] in supNms]

KeyError: 'categories'

Extract from the file ‘cocoapi/PythonAPI/pycocotools/coco.py’:

a = COCO() # calling init
catIds = a.getCatIds(catNms=['person','dog', 'car']) # calling the method from the class

from pycocotools.coco import COCO

# After downloading images from http://cocodataset.org/#download

# Define location of annotations
dataDir = '..'
dataType = 'val2017'
annFile = f'{dataDir}/annotations/instances_{dataType}.json'

# Create instance
coco = COCO(annFile)

# Filter for specific categories 
catIds = coco.getCatIds(catNms=['person','dog', 'car'])