Python Sqlalchemy如何实现select for update

@_generative()
  def with_for_update(self, read=False, nowait=False, of=None):
    """return a new :class:`.Query` with the specified options for the
    ``FOR UPDATE`` clause.
 
    The behavior of this method is identical to that of
    :meth:`.SelectBase.with_for_update`. When called with no arguments,
    the resulting ``SELECT`` statement will have a ``FOR UPDATE`` clause
    appended. When additional arguments are specified, backend-specific
    options such as ``FOR UPDATE NOWAIT`` or ``LOCK IN SHARE MODE``
    can take effect.
 
    E.g.::
 
      q = sess.query(User).with_for_update(nowait=True, of=User)
 
    The above query on a Postgresql backend will render like::
 
      SELECT users.id AS users_id FROM users FOR UPDATE OF users NOWAIT
 
    .. versionadded:: 0.9.0 :meth:`.Query.with_for_update` supersedes
      the :meth:`.Query.with_lockmode` method.
 
    .. seealso::
 
      :meth:`.GenerativeSelect.with_for_update` - Core level method with
      full argument and behavioral description.
 
    """
     
read
  是标识加互斥锁还是共享锁. 当为 True 时, 即 for share 的语句, 是共享锁. 多个事务可以获取共享锁, 互斥锁只能一个事务获取. 有"多个地方"都希望是"这段时间我获取的数据不能被修改, 我也不会改", 那么只能使用共享锁.
nowait
  其它事务碰到锁, 是否不等待直接"报错".
of
  指明上锁的表, 如果不指明, 则查询中涉及的所有表(行)都会加锁.

def query_city_for_update():
  session = get_session()
  with session.begin():
    query = session.query(City).with_for_update().filter(City.ID == 8)
    print 'SQL : %s' % str(query)
    print_city_info(query.first())

SQL : SELECT city."ID" AS "city_ID", city."Name" AS "city_Name", city."CountryCode" AS "city_CountryCode", city."District" AS "city_District", city."Population" AS "city_Population" 
FROM city 
WHERE city."ID" = :ID_1 FOR UPDATE

{'city': {'population': 234323, 'district': u'Utrecht', 'id': 8, 'country_code': u'NLD', 'name': u'Utrecht'}}