code: def _init_read_gz(self):
          
          self.cmp = self.zlib.decompressobj(-self.zlib.MAX_WBITS)
          self.dbuf = b""
  
          # taken from gzip.GzipFile with some alterations
          if self.__read(2) != b"\037\213":
              raise ReadError("not a gzip file")
          if self.__read(1) != b"\010":
              raise CompressionError("unsupported compression method")
  
          flag = ord(self.__read(1))
          self.__read(6)
  
          if flag & 4:
              xlen = ord(self.__read(1)) + 256 * ord(self.__read(1))
              self.read(xlen)
          if flag & 8:
              while True:
                  s = self.__read(1)
                  if not s or s == NUL:
                      break
          if flag & 16:
              while True:
                  s = self.__read(1)
                  if not s or s == NUL:
                      break
          if flag & 2:
              self.__read(2)
docs: Initialize for reading a gzip compressed fileobj.