'Creates an instance of the InputReader for the given input shard state.
Args:
json: The InputReader state as a dict-like object.
_reader: For dependency injection.
Returns:
An instance of the InputReader configured using the values of json.'

@classmethod
def from_json(cls, json, _reader=blobstore.BlobReader):

    return cls(json[cls.BLOB_KEY_PARAM], json[cls.START_FILE_INDEX_PARAM], json[cls.END_FILE_INDEX_PARAM], json[cls.OFFSET_PARAM], _reader)
