code: def hash(self):
          
          hashvals = []
          if (self.md5 is not None):
              hashvals.append('md5:' + self.md5)
          if (self.sha1 is not None):
              hashvals.append('sha-1:' + self.sha1)
          if (self.sha256 is not None):
              hashvals.append('sha-256:' + self.sha256)
          if (len(hashvals) > 0):
              return(' '.join(hashvals))
          return(None)
docs: Provide access to the complete hash string.
  
          The hash string may have zero or more hash values with
          appropriate prefixes. All hash values are assumed to be
          strings