Python 为什么有两个不同的输出
这是第一个输出:Python 为什么有两个不同的输出,python,sql,sqlalchemy,Python,Sql,Sqlalchemy,这是第一个输出: from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey,create_engine engine = create_engine('sqlite:///college.db',echo = True) metadata = MetaData() users = Table('users', metadata, Column('id', Integer, primary_k
from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey,create_engine
engine = create_engine('sqlite:///college.db',echo = True)
metadata = MetaData()
users = Table('users', metadata,
Column('id', Integer, primary_key=True),
Column('name', String(50)),
Column('fullname', String(50)),
)
addresses = Table('addresses', metadata,
Column('id', Integer, primary_key=True),
Column('user_id', None, ForeignKey('users.id')),
Column('email_address', String(50), nullable=False))
metadata.create_all(engine)
然后,我再次点击run,输出发生了变化:
CREATE TABLE addresses (
id INTEGER NOT NULL,
user_id INTEGER,
email_address VARCHAR NOT NULL,
PRIMARY KEY (id),
FOREIGN KEY(user_id) REFERENCES users (id)
能否解释一下为什么在我第二次运行代码后会有一些变化?当您第二次运行代码时,表已经创建好了,这就是为什么您会看到输出。如果要再次获得相同的输出,必须先删除表:
2019-04-18 21:06:57,881 INFO sqlalchemy.engine.base.Engine SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
2019-04-18 21:06:57,886 INFO sqlalchemy.engine.base.Engine ()
2019-04-18 21:06:57,892 INFO sqlalchemy.engine.base.Engine SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
2019-04-18 21:06:57,899 INFO sqlalchemy.engine.base.Engine ()
2019-04-18 21:06:57,904 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("users")
2019-04-18 21:06:57,909 INFO sqlalchemy.engine.base.Engine ()
2019-04-18 21:06:57,923 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("addresses")
2019-04-18 21:06:57,928 INFO sqlalchemy.engine.base.Engine ()
metadata.create_all(engine)
2019-04-18 21:07:17,156 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("users")
2019-04-18 21:07:17,157 INFO sqlalchemy.engine.base.Engine ()
当然,这会给你留下一个空数据库!
要在重新创建表时获得异常,可以使用checkfirst
参数:
from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey,create_engine
engine = create_engine('sqlite:///college.db',echo = True)
metadata = MetaData()
users = Table('users', metadata,
Column('id', Integer, primary_key=True),
Column('name', String(50)),
Column('fullname', String(50)),
)
addresses = Table('addresses', metadata,
Column('id', Integer, primary_key=True),
Column('user_id', None, ForeignKey('users.id')),
Column('email_address', String(50), nullable=False))
metadata.create_all(engine)
users.drop(engine)
addresses.drop(engine)
您看到的输出对于获取表的某些元数据非常有用:
metadata.create_all(engine, checkfirst=False)
另请参见。当您第二次运行代码时,表已经被创建,这就是您获得所看到的输出的原因。如果要再次获得相同的输出,必须先删除表:
2019-04-18 21:06:57,881 INFO sqlalchemy.engine.base.Engine SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
2019-04-18 21:06:57,886 INFO sqlalchemy.engine.base.Engine ()
2019-04-18 21:06:57,892 INFO sqlalchemy.engine.base.Engine SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
2019-04-18 21:06:57,899 INFO sqlalchemy.engine.base.Engine ()
2019-04-18 21:06:57,904 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("users")
2019-04-18 21:06:57,909 INFO sqlalchemy.engine.base.Engine ()
2019-04-18 21:06:57,923 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("addresses")
2019-04-18 21:06:57,928 INFO sqlalchemy.engine.base.Engine ()
metadata.create_all(engine)
2019-04-18 21:07:17,156 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("users")
2019-04-18 21:07:17,157 INFO sqlalchemy.engine.base.Engine ()
当然,这会给你留下一个空数据库!
要在重新创建表时获得异常,可以使用checkfirst
参数:
from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey,create_engine
engine = create_engine('sqlite:///college.db',echo = True)
metadata = MetaData()
users = Table('users', metadata,
Column('id', Integer, primary_key=True),
Column('name', String(50)),
Column('fullname', String(50)),
)
addresses = Table('addresses', metadata,
Column('id', Integer, primary_key=True),
Column('user_id', None, ForeignKey('users.id')),
Column('email_address', String(50), nullable=False))
metadata.create_all(engine)
users.drop(engine)
addresses.drop(engine)
您看到的输出对于获取表的某些元数据非常有用:
metadata.create_all(engine, checkfirst=False)
另见