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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 GetDisplayName(self, file_entry=None):
 """Retrieves the display name for a file entry. Args: file_entry (Optional[dfvfs.FileEntry]):
 file entry object, where None will return the display name of self._file_entry. Returns: str: human readable string that describes the path to the file entry. Raises: ValueError: if the file entry is missing. """

    if file_entry is None:
      file_entry = self._file_entry

    if file_entry is None:
      raise ValueError('Missing file entry')

    path_spec = getattr(file_entry, 'path_spec', None)

    relative_path = path_helper.PathHelper.GetRelativePathForPathSpec(
        path_spec, mount_path=self._mount_path)
    if not relative_path:
      return file_entry.name

    return self.GetDisplayNameForPathSpec(path_spec)