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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 GetFilename(self):
 """Retrieves the name of the active file entry. Returns: str: name of the active file entry or None. """

    if not self._file_entry:
      return None

    data_stream = getattr(self._file_entry.path_spec, 'data_stream', None)
    if data_stream:
      return '{0:s}:{1:s}'.format(self._file_entry.name, data_stream)

    return self._file_entry.name