原生session:

from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine
from sqlalchemy應用.models import Users

engine = create_engine(
    "mysql+pymysql://root:[email protected]:3306/pro6?charset=utf8",
    max_overflow=0,  # 超過連接池大小外最多創建的連接
    pool_size=5,  # 連接池大小
)

#from sqlalchemy.orm.session import Session
 


SessionF = sessionmaker(bind=engine)
session = SessionF()

print(session)

obj1 = Users(name=‘ctz‘, email=‘[email protected]‘, extra=‘aaaa‘)

session.add(obj1)

session.commit()
session.close()

問題：由於無法提供線程共享功能，所以在開發時要註意,要給每個線程都創建自己的session

打印sesion可知他是sqlalchemy.orm.session.Session的對象

查看Session的源碼 可得到:

class Session(_SessionClassMethods):
    """Manages persistence operations for ORM-mapped objects.

    The Session‘s usage paradigm is described at :doc:`/orm/session`.


    """

    public_methods = (
        ‘__contains__‘, ‘__iter__‘, ‘add‘, ‘add_all‘, ‘begin‘, ‘begin_nested‘,
        ‘close‘, ‘
 
commit‘, ‘connection‘, ‘delete‘, ‘execute‘, ‘expire‘,
        ‘expire_all‘, ‘expunge‘, ‘expunge_all‘, ‘flush‘, ‘get_bind‘,
        ‘is_modified‘, ‘bulk_save_objects‘, ‘bulk_insert_mappings‘,
        ‘bulk_update_mappings‘,
        ‘merge‘, ‘query‘, ‘refresh‘, ‘rollback‘,
        ‘scalar‘)

2.scoped_session

from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy應用.models import Users
from sqlalchemy.orm import scoped_session


engine=create_engine(
    "mysql+pymysql://root:[email protected]:3306/pro6?charset=utf8",
    max_overflow=0,  # 超過連接池大小外最多創建的連接
    pool_size=5,  # 連接池大小
)

SessionF=sessionmaker(bind=engine)

#scoped_session封裝了兩個值 Session 和 registry,registry加括號就執行了ThreadLocalRegistry的__call__方法,如果

# 當前本地線程中有session就返回session,沒有就將session添加到了本地線程

#self.registry()=session
session=scoped_session(SessionF)

print(session)
obj1=Users(name=‘ctz‘,email=‘[email protected]‘,extra=‘aaaa‘)

session.remove()
session.query_property()

優點:支持線程安全,為每個線程都創建一個session:

兩種方式:通過本地線程Threading.Local()和創建唯一標識的方法(參考flask請求源碼)

源碼分析：

首先我們在scoped_session中放入了Sesion對象

SessionF=sessionmaker(bind=engine)

session=scoped_session(Session)

一、scoped_session類中

class scoped_session(object):
    session_factory = None
    def __init__(self, session_factory, scopefunc=None):
     
        #傳遞過來的那個Session對象
        self.session_factory = session_factory
        #scopefunc唯一標示函數
        if scopefunc:
            self.registry = ScopedRegistry(session_factory, scopefunc)
        else:
            self.registry = ThreadLocalRegistry(session_factory)

第一次進來scopefunc唯一標識為None，我們將Session作為參數傳遞到ThreadLocalRegistry中，

ThreadLocalRegistry類中

class ThreadLocalRegistry(ScopedRegistry):
    """A :class:`.ScopedRegistry` that uses a ``threading.local()``
    variable for storage.

    """

    def __init__(self, createfunc):
        #傳遞過來的那個Session對象
        self.createfunc = createfunc
        self.registry = threading.local()
     #scoped_session.registry()後執行
    def __call__(self):
        try:
            #如果本地線程中有值的話直接將值返回，
            return self.registry.value
        except AttributeError:
            #沒有值的話就示例話Session(),並將他存到本地線程中，並把實例的對象返回
            #相當於Session()後的對象加到了本地線程中
            val = self.registry.value = self.createfunc()
            return val

其中__call__()只有當scoped_session.registry加括號執行

那我們怎麽調用那些方法呢？

def instrument(name):
    def do(self, *args, **kwargs):
        return getattr(self.registry(), name)(*args, **kwargs)
    return do

for meth in Session.public_methods:
    setattr(scoped_session, meth, instrument(meth))

其中 Session就是sqlalchemy.orm.session.Session

 public_methods = (
        ‘__contains__‘, ‘__iter__‘, ‘add‘, ‘add_all‘, ‘begin‘, ‘begin_nested‘,
        ‘close‘, ‘commit‘, ‘connection‘, ‘delete‘, ‘execute‘, ‘expire‘,
        ‘expire_all‘, ‘expunge‘, ‘expunge_all‘, ‘flush‘, ‘get_bind‘,
        ‘is_modified‘, ‘bulk_save_objects‘, ‘bulk_insert_mappings‘,
        ‘bulk_update_mappings‘,
        ‘merge‘, ‘query‘, ‘refresh‘, ‘rollback‘,
        ‘scalar‘)

在instrument函數中

self.registry()幫我們執行了ThreadLocalRegistry中的___call__方法，拿到了sesion對象


方法源碼示例:

 def has(self):
        return hasattr(self.registry, "value")

    def remove(self):
        if self.registry.has():
            self.registry().close()
        self.registry.clear()

實際兩種方式原理都是一樣的都是第一種，只是第二種將session放到了本地線程中，為每一個進程都設置了一個session，實現了線程安全

SqlAlchemy 中操作數據庫時session和scoped_session的區別