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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 posix_to_dt_str(posix):
 """Reverse of str_to_datetime. This is used by GCS stub to generate GET bucket XML response. Args: posix: A float of secs from unix epoch. Returns: A datetime str. """

  dt = datetime.datetime.utcfromtimestamp(posix)
  dt_str = dt.strftime(_DT_FORMAT)
  return dt_str + '.000Z'