クラス関数の最初のパラメータはクラスで、インスタンス関数の最初のパラメータはインスタンスです.

node2:/tmp/20200608#cat Entity.py
class Document():
  WELCOME_STR = 'Welcome! The context for this book is {}.'
  @classmethod    
  def create_empty_book(cls, title, author):        
       print cls
       return cls(title=title, author=author, context='nothing')
  def get_a(a,b):
    print self;
    return a + b
	

 
 node2:/tmp/20200608#cat Entity.py
# !/usr/bin/env python
# -*- coding: utf-8 -*-
class Document():
    
    WELCOME_STR = 'Welcome! The context for this book is {}.'
    
    def __init__(self, title, author, context):
        print('init function called')
        self.title = title
        self.author = author
        self.__context = context
    
    #    
    @classmethod
    def create_empty_book(cls, title, author):
        print '1111111111111111111111'
        print cls
        print '1111111111111111111111'
        return cls(title=title, author=author