好的，我只是想通了。这不是错误。浏览django / db / models / sql / query.py的源代码：
160 def __str__(self):161 """162 Returns the query as a string of SQL with the parameter values163 substituted in.164 165 Parameter values won't necessarily be quoted correctly, since that is166 done by the database interface at execution time.167 """168 sql, params = self.get_compiler(DEFAULT_DB_ALIAS).as_sql()169 return sql % params
（http://pre.djangoproject.com/browser/django/trunk/django/db/models/sql/query.py）
一切正常。:)
