Question

我在这里看了一下教程：http://www.pythoncentral.io/introductory-tutorial-python-sqlalchemy/

我的问题的决定性部分首先是：

import os
import sys
from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy import create_engine

Base = declarative_base()

class Person(Base):
    __tablename__ = 'person'
    # Here we define columns for the table person
    # Notice that each column is also a normal Python instance attribute.
    id = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)

class Address(Base):
    __tablename__ = 'address'
    # Here we define columns for the table address.
    # Notice that each column is also a normal Python instance attribute.
    id = Column(Integer, primary_key=True)
    street_name = Column(String(250))
    street_number = Column(String(250))
    post_code = Column(String(250), nullable=False)
    person_id = Column(Integer, ForeignKey('person.id'))
    person = relationship(Person)

# Create an engine that stores data in the local directory's
# sqlalchemy_example.db file.
engine = create_engine('sqlite:///sqlalchemy_example.db')

# Create all tables in the engine. This is equivalent to "Create Table"
# statements in raw SQL.
Base.metadata.create_all(engine)

和第二：

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

from sqlalchemy_declarative import Address, Base, Person

engine = create_engine('sqlite:///sqlalchemy_example.db')
# Bind the engine to the metadata of the Base class so that the
# declaratives can be accessed through a DBSession instance
Base.metadata.bind = engine

DBSession = sessionmaker(bind=engine)
# A DBSession() instance establishes all conversations with the database
# and represents a "staging zone" for all the objects loaded into the
# database session object. Any change made against the objects in the
# session won't be persisted into the database until you call
# session.commit(). If you're not happy about the changes, you can
# revert all of them back to the last commit by calling
# session.rollback()
session = DBSession()

# Insert a Person in the person table
new_person = Person(name='new person')
session.add(new_person)
session.commit()

# Insert an Address in the address table
new_address = Address(post_code='00000', person=new_person)
session.add(new_address)
session.commit()

如果运行第二部分（例如＆＃34; python second_part＆＃34;），则取消导入声明

from sqlalchemy_declarative import Address, Base, Person

导致第一部分中的所有可执行代码运行？（这将触发每次运行第一部分时在db中创建表的代码）

祝你好运

Answer 1

是的，每次导入第一个文件时，都会再次创建表。

要防止这种情况，请使用以下语句：

if __name__ == '__main__':
    Base.metadata.create_all(engine)

Python导入 - 这些可执行语句是否运行？

1 个答案: