sqlalchemy 對於行級鎖有兩種實現方式，with_lockmode(self,mode): 和 with_for_update(self,read=False,nowait=False,of=None)，前者在sqlalchemy 0.9.0 被廢棄，用後者代替。所以我們使用with_for_update !

看下函式的定義：

@_generative()
  def with_for_update(self,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
  是標識加互斥鎖還是共享鎖. 當為 True 時,即 for share 的語句,是共享鎖. 多個事務可以獲取共享鎖,互斥鎖只能一個事務獲取. 有"多個地方"都希望是"這段時間我獲取的資料不能被修改,我也不會改",那麼只能使用共享鎖.
nowait
  其它事務碰到鎖,是否不等待直接"報錯".
of
  指明上鎖的表,如果不指明,則查詢中涉及的所有表(行)都會加鎖.

q = sess.query(User).with_for_update(nowait=True,of=User)

對應於sql：

SELECT users.id AS users_id FROM users FOR UPDATE OF users NOWAIT

mysql 不支援這幾個引數，轉成sql都是:

SELECT users.id AS users_id FROM users FOR UPDATE

範例：

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())

結果：

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'}}

SELECT ... FOR UPDATE 的用法，不過鎖定(Lock)的資料是判別就得要注意一下了。由於InnoDB 預設是Row-Level Lock，所以只有「明確」的指定主鍵，MySQL 才會執行Row lock (只鎖住被選取的資料) ，否則mysql 將會執行Table Lock (將整個資料表單給鎖住)。

以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支援我們。