Catch and elaborate errors for corrupt json or incorrect object hook

[rwxayheee]

There are several types of JSON files in the Meeko world. Many objects can be created from JSON (which is really convenient when put into use), but the errors are not so clear if there's a problem within the JSON file or the incorrect object hook. It might be helpful to catch and elaborate errors (just provide some quick hints) for bad input JSON or incorrect object hook.

There are some situations that the JSON can be loaded into a dict, but cannot be built into a valid Python object like Polymer. I wish there's a way to catch the problem, but I'm not so sure how to do it

[rwxayheee]

Its really really not necessary unless more people want it. Here's a class to inherit from. Something like the following gives some hints on where the problem occurred (parsing, decoder function, object type) and avoids letting go a partial parse silently.

class BaseJSONParsable:

    # Define in Individual Subclasses 
    expected_json_keys = None
    object_hook = None 
    # and some decoder function

    # Inherit from_json and from_json_file
    @classmethod
    def from_json(cls, json_string):
        try: 
            obj = json.loads(json_string, object_hook=cls.object_hook)
            # Log mismatched keys (maybe not a fatal problem)
            if set(obj.keys()) != cls.expected_json_keys:
                logging.error(
                    f"Keys from JSON ({set(obj.keys())}) differ from "
                    f"expected keys ({cls.expected_json_keys})."
                )  
            # Success
            if isinstance(obj, cls): 
                return obj

        except Exception as decoder_error:     
            try: 
                obj = json.loads(json_string)
            # Error occurred while parsing JSON
            except Exception as parser_error: 
                raise RuntimeError(
                    f"Unable to parse the source JSON for {cls.__name__}: {parser_error}"
                )
            # Error occurred within the decoder function
            raise RuntimeError(
                f"An error occurred when creating {cls.__name__}: {decoder_error}"
            )
        # Failed to create obj
        raise ValueError(
            f"Unexpected object type created from JSON: {type(obj)}. "
            f"Expected object type is: {cls.__name__}."
        )
            
    @classmethod
    def from_json_file(cls, json_file): 
        with open(json_file, "r") as f: 
            json_string = f.read()
        return cls.from_json(json_string)