code: def _header(self, pam=False):
          
          if pam or self.magicnum == b'P7':
              header = "\n".join((
                  "P7",
                  "HEIGHT %i" % self.height,
                  "WIDTH %i" % self.width,
                  "DEPTH %i" % self.depth,
                  "MAXVAL %i" % self.maxval,
                  "\n".join("TUPLTYPE %s" % unicode(i) for i in self.tupltypes),
                  "ENDHDR\n"))
          elif self.maxval == 1:
              header = "P4 %i %i\n" % (self.width, self.height)
          elif self.depth == 1:
              header = "P5 %i %i %i\n" % (self.width, self.height, self.maxval)
          else:
              header = "P6 %i %i %i\n" % (self.width, self.height, self.maxval)
          if sys.version_info[0] > 2:
              header = bytes(header, 'ascii')
          return header
docs: Return file header as byte string.