code: def walk(prev, inital_path, *args, **kw):
      
      for dir_path, dir_names, filenames in os.walk(inital_path):
          for filename in filenames:
              yield os.path.join(dir_path, filename)
docs: This pipe wrap os.walk and yield absolute path one by one.
  
      :param prev: The previous iterator of pipe.
      :type prev: Pipe
      :param args: The end-of-line symbol for each output.
      :type args: list of string.
      :param kw: The end-of-line symbol for each output.
      :type kw: dictionary of options. Add 'endl' in kw to specify end-of-line symbol.
      :returns: generator