code: def compile_pattern(self):
          
          if self.PATTERN is not None:
              PC = PatternCompiler()
              self.pattern, self.pattern_tree = PC.compile_pattern(self.PATTERN,
                                                                   with_tree=True)
docs: Compiles self.PATTERN into self.pattern.
  
          Subclass may override if it doesn't want to use
          self.{pattern,PATTERN} in .match().