How to save a dictionary into a file, keeping nice format?

If I have dictionary like:

{
  "cats": {
           "sphinx": 3,
           "british": 2
          },
  "dogs": {}
}

And try to save it to a text file, I get something like this:

{"cats": {"sphinx": 3}, {"british": 2}, "dogs": {}}

How can I save a dictionary in pretty format, so it will be easy to read by human eye?