code: def _bin_zfill(num, width=None):
      
      s = bin(num)[2:]
      return s if width is None else s.zfill(width)
docs: Convert a base-10 number to a binary string.
  
      Parameters
      num: int
      width: int, optional
          Zero-extend the string to this width.
      Examples
      --------
  
      >>> _bin_zfill(42)
      '101010'
      >>> _bin_zfill(42, 8)
      '00101010'