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 values
163 substituted in.
164
165 Parameter values won't necessarily be quoted correctly, since that is
166 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://code.djangoproject.com/browser/django/trunk/django/db/models/sql/query.py）
一切正常。:)
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