Python: 定义不初始化的对象变量

Question

我正在尝试将我的代码从 一个大函数 重写为 oop。

如果我有这个，它会在 session.add(a1) # Unresolved reference:

崩溃

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import *
from sqlalchemy.orm import *

Base = declarative_base()

class Address(Base):
    __tablename__ = 'address'
    id = Column(Integer, primary_key=True)
    street = Column(String, nullable=False)
    city = Column(String, nullable=False)
    user = relationship('User', back_populates="address")

class Main():
    def __init__(self):
        engine = create_engine('mysql://test:test@localhost:3306/test', echo=False)
        Session = sessionmaker(bind=engine)
        session = Session()

    def insert(self):
        #   INSERT
        a1 = Address()
        a1.street = "Str 123"
        a1.city = "City WTF"

        session.add(a1) # Unresolved reference
        session.commit()

if __name__ == '__main__':
    Main().run()

我明白了。 session 是构造函数中的局部对象 (__init__)。

但是如何放置对象 "directly to class"？在 Java 我做了类似的事情：

public class Main {
    Engine engine;
    Session session;
    public Main() {}
        engine = create_engine('mysql://test:test@localhost:3306/test', echo=False)
        session = sessionmaker(bind=engine)
    }
    private insert() {
        //...
        session.commit();
    }
}

如何在 python 中完成？ 抱歉提出愚蠢的问题，我是 python 新手。

-------------------- 编辑：

class Main():
    engine = None # What I write here? How declare uninitialized object?
    session = None # What I write here?
    def __init__(self):
        engine = create_engine('mysql://test:test@localhost:3306/test', echo=False)
        Session = sessionmaker(bind=engine)
        session = Session()

    def insert(self):
        #   INSERT
        a1 = Address()
        a1.street = "Str 123"
        a1.city = "City WTF"
        self.session.add(a1) # Is possible to call session without "self"?
        self.session.commit()

Answer 1

在 Java 你会做 this.session = ...；在 Python 中是 self.session = ...。

Answer 2

您正在 __init__ 方法中初始化局部变量 session 并在该变量未知的方法中调用它。

在这两种情况下都使用 self.varialbe

Answer 3

使用self.session
在会话中保存变量

Answer 4

您 Main class 中的方法看起来属于 Address class.

engine = create_engine('mysql://test:test@localhost:3306/test', echo=False)
session = sessionmaker(bind=engine)

class Address(Base):
    __tablename__ = 'address'
    id = Column(Integer, primary_key=True)
    street = Column(String, nullable=False)
    city = Column(String, nullable=False)
    # you are missing some fields you'll need eventually
    state = Column(String, nullable=False)
    zip_code = Column(String, nullable=False)  # this can be an int if you don't have to worry about Canadian zips which have letters in them
    user = relationship('User', back_populates="address")

    def __init__(self, street, city, state, zip_code, user):
        self.street = street
        self.city = city
        self.state = state
        self.zip_code = zip_code
        self.user = user

    def insert(self, session):
        #   INSERT
        session.add(self)
        session.commit()

您不应将会话创建为 class 的一部分，因为这样每次实例化 class 时都会创建一个新会话。将会话保留在全局 space 中并将其传递给需要它作为参数的 methods/functions（ 不要使用 global）。

现在一切都在正确的地方，你可以像这样使用它：

from models import session, Address
addr = Address('123 Test St.', 'Sometown', 'NY', '12345', some_user)
addr.insert(session)

Python: 定义不初始化的对象变量

Python: define object variable without initialization

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