code: def check_files(self, set_length=True, check_length=True):
          
          total_size = 0  # total size of all files in bytes
          path_prefix = None
          for resource in self.resources:
              if (resource.path is None):
                  # explicit test because exception raised by getsize otherwise
                  # confusing
                  raise DumpError(
                      "No file path defined for resource %s" %
                      resource.uri)
              if (path_prefix is None):
                  path_prefix = os.path.dirname(resource.path)
              else:
                  path_prefix = os.path.commonprefix(
                      [path_prefix, os.path.dirname(resource.path)])
              size = os.path.getsize(resource.path)
              if (resource.length is not None):
                  if (check_length and resource.length != size):
                      raise DumpError("Size of resource %s is %d on disk, not %d as specified" %
                                      (resource.uri, size, resource.length))
              elif (set_length):
                  resource.length = size
              if (size > self.max_size):
                  raise DumpError(
                      "Size of file (%s, %d) exceeds maximum (%d) dump size" %
                      (resource.path, size, self.max_size))
              total_size += size
          self.path_prefix = path_prefix
          self.total_size = total_size
          self.logger.info(
              "Total size of files to include in dump %d bytes" %
              (total_size))
          return True
docs: Check all files in self.resources, find longest common prefix.
  
          Go though and check all files in self.resources, add up size, and find
          longest common path that can be used when writing the dump file. Saved in
          self.path_prefix.
  
          Parameters set_length and check_length control control whether then set_length
          attribute should be set from the file size if not specified, and whether any
          length specified should be checked. By default both are True. In any event, the
          total size calculated is the size of files on disk.