code: def checksum(self, start=None, length=None):
      if self.size is not None:
          start = start if start else 0
          length = (
              length
              if length and start + length < self.size
              else self.size
          )
          lookup_key = "%s:%d:%d" % (self.key, start, length)
          if lookup_key not in self.cached_digests:
              if self._data is None:
                  self._make_data()
              if length < 0:
                  self.cached_digests[lookup_key] = sha1(
                      self._data[length:]
                  ).hexdigest()
              else:
                  self.cached_digests[lookup_key] = sha1(
                      self._data[start : start + length]
                  ).hexdigest()
          return self.cached_digests[lookup_key]
  
imports: import os
  import sys
  import zlib
  import json
  from uuid import uuid4
  from hashlib import sha1
  from collections import namedtuple
  from metaflow.plugins.datatools.s3 import S3PutObject
  from metaflow.util import to_fileobj, to_bytes, url_quote
  import numpy
  from ..None import s3client, S3ROOT
label_desc: Use image node_id 3 for calling the RandomFile obj's underlying member method code with example usage: obj.checksum(start, length) and returns: self