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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 _init_fields(self):
 """Update fields based on schema options."""

        if self.opts.fields:
            available_field_names = self.set_class(self.opts.fields)
        else:
            available_field_names = self.set_class(self.declared_fields.keys())
            if self.opts.additional:
                available_field_names |= self.set_class(self.opts.additional)

        invalid_fields = self.set_class()

        if self.only is not None:
            # Return only fields specified in only option
            field_names = self.set_class(self.only)

            invalid_fields |= field_names - available_field_names
        else:
            field_names = available_field_names

        # If "exclude" option or param is specified, remove those fields.
        exclude_field_names = set(self.opts.exclude) | set(self.exclude)
        if exclude_field_names:
            # Note that this isn't available_field_names, since we want to
            # apply "only" for the actual calculation.
            field_names = field_names - exclude_field_names

            invalid_fields |= exclude_field_names - available_field_names

        if invalid_fields:
            message = 'Invalid fields for {}: {}.'.format(self, invalid_fields)
            raise ValueError(message)

        fields_dict = self.dict_class()
        for field_name in field_names:
            field_obj = self.declared_fields.get(field_name, ma_fields.Inferred())
            self._bind_field(field_name, field_obj)
            fields_dict[field_name] = field_obj

        dump_data_keys = [
            obj.data_key or name for name, obj in iteritems(fields_dict) if not obj.load_only
        ]
        if len(dump_data_keys) != len(set(dump_data_keys)):
            data_keys_duplicates = {x for x in dump_data_keys if dump_data_keys.count(x) > 1}
            raise ValueError(
                'The data_key argument for one or more fields collides '
                "with another field's name or data_key argument. "
                'Check the following field names and '
                'data_key arguments: {}'.format(list(data_keys_duplicates)),
            )

        load_attributes = [
            obj.attribute or name for name, obj in iteritems(fields_dict) if not obj.dump_only
        ]
        if len(load_attributes) != len(set(load_attributes)):
            attributes_duplicates = {x for x in load_attributes if load_attributes.count(x) > 1}
            raise ValueError(
                'The attribute argument for one or more fields collides '
                "with another field's name or attribute argument. "
                'Check the following field names and '
                'attribute arguments: {}'.format(list(attributes_duplicates)),
            )

        return fields_dict