Thanks for the clarification though your example is unpythonic. Here's refactored to be more pythonic and I'd argue it's more readable than pattern match alternative:

    pet = json_pet.get('type')
    if pet == 'cat':
        return Cat(json_pet['name'], json_pet['pattern'])
    if pet == 'dog':
        return Dog(json_pet['name'], json_pet['breed'])

Again it goes against the pep20 which is really the philosophical foundation of the language: "There should be one-- and preferably only one --obvious way to do it."