city.py

class City:

    # 1. add type hints, here we see that name shouldn't* be None
    name: str = "vienna"

    # 2. add more type hints
    def create_zoo(self, owner: str) -> Optional[Zoo]:
        zoo_configs: Dict[
            str, ZooConfiguration
        ] = ZOOsConfigBuilder().get_configs()
        
        config = zoo_configs.get(self.name)

        # 3. we know that config is now either None 
        # or a ZooConfiguration instance. If it is 
        # a ZooConfiguration, then we call its property
        # is_open_to_public, which must return a True/False.
        if not config or not config.is_open_to_public:
            # 4. here we are reminded, that the result 
            # of this function can be a None as well as 
            # a Zoo instance.
            return None
        
        # 5. here we know that config is a ZooConfiguration instance
        # we know that it has a Callable variable called `get_config`
        # and that this Callable accepts 2 parameters: 
        #   - a string, zoo owner 
        #   - an integer, zoo size
        zoo_config: Dict[str, Any] = config.get_config(owner, 130)
        return Zoo(zoo_config)