如何使用SQLAlchemy创建新数据库？

Using SQLAlchemy, an Engine object is created like this:

from sqlalchemy import create_engine
engine = create_engine("postgresql://localhost/mydb")

Accessing engine fails if the database specified in the argument to create_engine (in this case, mydb) does not exist. Is it possible to tell SQLAlchemy to create a new database if the specified database doesn’t exist?

On postgres, three databases are normally present by default. If you are able to connect as a superuser (eg, the postgres role), then you can connect to the postgres or template1 databases. The default pg_hba.conf permits only the unix user named postgres to use the postgres role, so the simplest thing is to just become that user. At any rate, create an engine as usual with a user that has the permissions to create a database:

>>> engine = sqlalchemy.create_engine("postgres://postgres@/postgres")

You cannot use engine.execute() however, because postgres does not allow you to create databases inside transactions, and sqlalchemy always tries to run queries in a transaction. To get around this, get the underlying connection from the engine:

>>> conn = engine.connect()

But the connection will still be inside a transaction, so you have to end the open transaction with a commit:

>>> conn.execute("commit")

And you can then proceed to create the database using the proper PostgreSQL command for it.

>>> conn.execute("create database test")
>>> conn.close()

SQLAlchemy-Utils provides custom data types and various utility functions for SQLAlchemy. You can install the most recent official version using pip:

pip install sqlalchemy-utils

The database helpers include a create_database function:

from sqlalchemy import create_engine
from sqlalchemy_utils import database_exists, create_database

engine = create_engine("postgres://localhost/mydb")
if not database_exists(engine.url):
    create_database(engine.url)

print(database_exists(engine.url))

It’s possible to avoid manual transaction management while creating database by providing isolation_level='AUTOCOMMIT' to create_engine function:

import sqlalchemy

with sqlalchemy.create_engine(
    'postgresql:///postgres',
    isolation_level='AUTOCOMMIT'
).connect() as connection:
    connection.execute('CREATE DATABASE my_database')

Also if you are not sure that database doesn’t exist there is a way to ignore database creation error due to existence by suppressing sqlalchemy.exc.ProgrammingError exception:

import contextlib
import sqlalchemy.exc

with contextlib.suppress(sqlalchemy.exc.ProgrammingError):
    # creating database as above

Please note that I couldn’t get the above suggestions with database_exists because whenever I check if the database exists using if not database_exists(engine.url): I get this error:

InterfaceError(‘(pyodbc.InterfaceError) (\’28000\’, u\'[28000] [Microsoft][SQL Server Native Client 11.0][SQL Server]Login failed for user \\’myUser\\’. (18456) (SQLDriverConnect); [28000] [Microsoft][SQL Server Native Client 11.0][SQL Server]Cannot open database “MY_DATABASE” requested by the login. The login failed. (4060); [28000] [Microsoft][SQL Server Native Client 11.0][SQL Server]Login failed for user \\’myUser\\’. (18456); [28000] [Microsoft][SQL Server Native Client 11.0][SQL Server]Cannot open database “MY_DATABASE” requested by the login. The login failed. (4060)\’)’,)

Also contextlib/suppress was not working and I’m not using postgres so I ended up doing this to ignore the exception if the database happens to already exist with SQL Server:

import logging
import sqlalchemy

logging.basicConfig(filename='app.log', format='%(asctime)s-%(levelname)s-%(message)s', level=logging.DEBUG)
engine = create_engine('mssql+pyodbc://myUser:mypwd@localhost:1234/MY_DATABASE?driver=SQL+Server+Native+Client+11.0?trusted_connection=yes', isolation_level = "AUTOCOMMIT")

try: 
    engine.execute('CREATE DATABASE ' + a_database_name)
except Exception as db_exc:
    logging.exception("Exception creating database: " + str(db_exc))