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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def as_dict(self):
 """ Returns a dict representation of the resource """

        result = {}
        for key in self._valid_properties:
            val = getattr(self, key)
            if isinstance(val, datetime):
                val = val.isoformat()
            # Parse custom classes
            elif val and not Model._is_builtin(val):
                val = val.as_dict()
            # Parse lists of objects
            elif isinstance(val, list):
                # We only want to call as_dict in the case where the item
                # isn't a builtin type.
                for i in range(len(val)):
                    if Model._is_builtin(val[i]):
                        continue
                    val[i] = val[i].as_dict()
            # If it's a boolean, add it regardless of the value
            elif isinstance(val, bool):
                result[key] = val

            # Add it if it's not None
            if val:
                result[key] = val
        return result