python - 如何在不破坏 sqlalchemy 相关依赖项的情况下（重新）安排此 python 代码？

Question

在将 sqlalchemy 映射设置到 python 中的 sqlite 数据库时，我有一个关于“安排代码，这取决于彼此”的小问题。

目标是编写一个脚本，它满足以下条件：

1. 获取一个filename参数作为命令行参数。
2. 基于filename它应该创建一个 SQLite 数据库的绝对路径。
3. 它应该连接到数据库并创建一个engine
4. 它应反映该数据库中的表。
5. 它应该将表中monkey patch的列作为主键列，因为该表没有主键并且 sqlalchemy 需要一个。idmytable

所以我想出了这个：

from sqlalchemy import create_engine, Column, Integer
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base  

def create_path(file_name):
    # generate absolute path to file_name

path_to_file = create_path("my_file_name.sqlite")
engine = create_engine('sqlite:///{path}'.format(path=path_to_file), echo=False)
Base = declarative_base()
Base.metadata.create_all(engine)

class MyTable(Base):
    __tablename__ = 'my_table'
    __table_args__ = {'autoload': True}

    id = Column(Integer, primary_key=True) # Monkey patching the id column as primary key.

def main(argv):
    # parse file_name here from argv
    Session = sessionmaker(bind=engine)
    session = Session()
    for row in session.query(MyTable).all():
        print row
    return "Stop!"

if __name__ == '__main__':
    sys.exit(main())

But this is a doomed construction and I don't see how I could rearrange my code without breaking the dependencies.

1. To be able to create MyClass I need Base to be defined before MyClass.
2. To be able to create Base I need engine to be defined before Base.
3. To be able to create engine I need path_to_file to be defined before engine.
4. To be able to create path_to_file outside of main() I need create_file() to be defined before path_to_file.
5. And so on...

Hopefully you see where I am stuck...

Any suggestions?

Edit: By the way, the code works, but only with a hardcoded filename in the top of the script.

Answer 1

I solved my problem using the classical mapping approach of SQLAlchemy:

from sqlalchemy import create_engine, Column, Integer, MetaData, Table
from sqlalchemy.orm import sessionmaker, mapper

class MyTable(object):    
    pass    

def main():
    path_to_file = create_path("my_file_name.sqlite")
    engine = create_engine('sqlite:///{path}'.format(path=path_to_file), echo=False)

    metadata = MetaData()
    metadata.bind = engine
    my_table = Table('mytable',
                     metadata,
                     Column('id', Integer, primary_key=True), # Monkey patching the id column as primary key.
                     autoload=True,
                     autoload_with=engine)
    mapper(MyTable, my_table)

    Session = sessionmaker(bind=engine)
    session = Session()

    # Do Stuff!

Answer 2

I do not see why you could not use the declarative mapping still complety. I think the key to the proper dependencies is to know what to put in the module and what in the script/function. With python, you can easily define the class inside the run_script function.

from sqlalchemy import create_engine, Column, Integer
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

def create_path(filename):
    import os
    fn = os.path.abspath(os.path.join(os.path.dirname(__file__), 'sqlite_files', filename))
    return fn

def run_script(filename):
    path_to_file = create_path(filename)

    engine = create_engine('sqlite:///{path}'.format(path=path_to_file), echo=True)
    Session = sessionmaker(bind=engine)
    Base = declarative_base(bind=engine)

    # object model
    class MyTable(Base):
        __tablename__ = 'my_table'
        __table_args__ = {'autoload': True}
        id = Column(Integer, primary_key=True) # Monkey patching the id column as primary key

    # script itself
    session = Session()
    for row in session.query(MyTable).all():
        print row
    return "Stop!"

def main(argv):
    assert argv, "must specify a database file name"
    run_script(argv[0])

if __name__ == '__main__':
    import sys
    sys.exit(main(sys.argv))

Answer 3

I don't have the capability to do any testing at my current location. But I suggest moving all of that code to main() as follows. Also, MyTable subclasses declarative_base.

from sqlalchemy import create_engine, Column, Integer 
from sqlalchemy.orm import sessionmaker from sqlalchemy.ext.declarative import declarative_base    

class MyTable(declarative_base):     
    __tablename__ = 'my_table'     
    __table_args__ = {'autoload': True}      

    id = Column(Integer, primary_key=True) # Monkey patching the id column as primary key.  

def create_path(file_name):     
    # generate absolute path to file_name  


def main(argv):     # parse file_name here from argv   
    path_to_file = create_path("my_file_name.sqlite") 
    engine = create_engine('sqlite:///{path}'.format(path=path_to_file), echo=False) 
    Base = MyTable()
    Base.metadata.create_all(engine)  

    Session = sessionmaker(bind=engine)     
    session = Session()     
    for row in session.query(MyTable).all():         
        print row     
    return "Stop!"  

if __name__ == '__main__':     
    sys.exit(main())