1. Hong Minhee
  2. sqlalchemy-timezone


sqlalchemy-timezone / doc / build / core / engines.rst

Engine Configuration

The Engine is the starting point for any SQLAlchemy application. It's "home base" for the actual database and its DBAPI, delivered to the SQLAlchemy application through a connection pool and a Dialect, which describes how to talk to a specific kind of database/DBAPI combination.

The general structure can be illustrated as follows:


Where above, an :class:`~sqlalchemy.engine.base.Engine` references both a :class:`~sqlalchemy.engine.base.Dialect` and a :class:`~sqlalchemy.pool.Pool`, which together interpret the DBAPI's module functions as well as the behavior of the database.

Creating an engine is just a matter of issuing a single call, :func:`.create_engine()`:

from sqlalchemy import create_engine
engine = create_engine('postgresql://scott:tiger@localhost:5432/mydatabase')

The above engine invokes the postgresql dialect and a connection pool which references localhost:5432.

The :class:`.Engine`, once created, can either be used directly to interact with the database, or can be passed to a :class:`.Session` object to work with the ORM. This section covers the details of configuring an :class:`.Engine`. The next section, :ref:`connections_toplevel`, will detail the usage API of the :class:`.Engine` and similar, typically for non-ORM applications.

Supported Databases

SQLAlchemy includes many :class:`~sqlalchemy.engine.base.Dialect` implementations for various backends; each is described as its own package in the :ref:`sqlalchemy.dialects_toplevel` package. A SQLAlchemy dialect always requires that an appropriate DBAPI driver is installed.

The table below summarizes the state of DBAPI support in SQLAlchemy 0.7. The values translate as:

  • yes / Python platform - The SQLAlchemy dialect is mostly or fully operational on the target platform.
  • yes / OS platform - The DBAPI supports that platform.
  • no / Python platform - The DBAPI does not support that platform, or there is no SQLAlchemy dialect support.
  • no / OS platform - The DBAPI does not support that platform.
  • partial - the DBAPI is partially usable on the target platform but has major unresolved issues.
  • development - a development version of the dialect exists, but is not yet usable.
  • thirdparty - the dialect itself is maintained by a third party, who should be consulted for information on current support.
  • * - indicates the given DBAPI is the "default" for SQLAlchemy, i.e. when just the database name is specified
Driver Connect string Py2K Py3K Jython Unix Windows
DB2/Informix IDS            
ibm-db thirdparty thirdparty thirdparty thirdparty thirdparty thirdparty
mysql-python drizzle+mysqldb* yes development no yes yes
Firebird / Interbase            
kinterbasdb firebird+kinterbasdb* yes development no yes yes
informixdb informix+informixdb* yes development no unknown unknown
sapdb maxdb+sapdb* development development no yes unknown
Microsoft Access            
pyodbc access+pyodbc* development development no unknown yes
Microsoft SQL Server            
adodbapi mssql+adodbapi development development no no yes
jTDS JDBC Driver mssql+zxjdbc no no development yes yes
mxodbc mssql+mxodbc yes development no yes with FreeTDS yes
pyodbc mssql+pyodbc* yes development no yes with FreeTDS yes
pymssql mssql+pymssql yes development no yes yes
MySQL Connector/J mysql+zxjdbc no no yes yes yes
MySQL Connector/Python mysql+mysqlconnector yes yes no yes yes
mysql-python mysql+mysqldb* yes development no yes yes
OurSQL mysql+oursql yes yes no yes yes
pymysql mysql+pymysql yes development no yes yes
cx_oracle oracle+cx_oracle* yes development no yes yes
Oracle JDBC Driver oracle+zxjdbc no no yes yes yes
pg8000 postgresql+pg8000 yes yes no yes yes
PostgreSQL JDBC Driver postgresql+zxjdbc no no yes yes yes
psycopg2 postgresql+psycopg2* yes yes no yes yes
pypostgresql postgresql+pypostgresql no yes no yes yes
pysqlite sqlite+pysqlite* yes yes no yes yes
sqlite3 sqlite+pysqlite* yes yes no yes yes
Sybase ASE            
mxodbc sybase+mxodbc development development no yes yes
pyodbc sybase+pyodbc* partial development no unknown unknown
python-sybase sybase+pysybase yes [1] development no yes yes
[1]The Sybase dialect currently lacks the ability to reflect tables.

Further detail on dialects is available at :ref:`dialect_toplevel`.

Engine Creation API

Keyword options can also be specified to :func:`~sqlalchemy.create_engine`, following the string URL as follows:

db = create_engine('postgresql://...', encoding='latin1', echo=True)

Database Urls

SQLAlchemy indicates the source of an Engine strictly via RFC-1738 style URLs, combined with optional keyword arguments to specify options for the Engine. The form of the URL is:


Dialect names include the identifying name of the SQLAlchemy dialect which include sqlite, mysql, postgresql, oracle, mssql, and firebird. The drivername is the name of the DBAPI to be used to connect to the database using all lowercase letters. If not specified, a "default" DBAPI will be imported if available - this default is typically the most widely known driver available for that backend (i.e. cx_oracle, pysqlite/sqlite3, psycopg2, mysqldb). For Jython connections, specify the zxjdbc driver, which is the JDBC-DBAPI bridge included with Jython.

# postgresql - psycopg2 is the default driver.
pg_db = create_engine('postgresql://scott:tiger@localhost/mydatabase')
pg_db = create_engine('postgresql+psycopg2://scott:tiger@localhost/mydatabase')
pg_db = create_engine('postgresql+pg8000://scott:tiger@localhost/mydatabase')
pg_db = create_engine('postgresql+pypostgresql://scott:tiger@localhost/mydatabase')

# postgresql on Jython
pg_db = create_engine('postgresql+zxjdbc://scott:tiger@localhost/mydatabase')

# mysql - MySQLdb (mysql-python) is the default driver
mysql_db = create_engine('mysql://scott:tiger@localhost/foo')
mysql_db = create_engine('mysql+mysqldb://scott:tiger@localhost/foo')

# mysql on Jython
mysql_db = create_engine('mysql+zxjdbc://localhost/foo')

# mysql with pyodbc (buggy)
mysql_db = create_engine('mysql+pyodbc://scott:tiger@some_dsn')

# oracle - cx_oracle is the default driver
oracle_db = create_engine('oracle://scott:tiger@')

# oracle via TNS name
oracle_db = create_engine('oracle+cx_oracle://scott:tiger@tnsname')

# mssql using ODBC datasource names.  PyODBC is the default driver.
mssql_db = create_engine('mssql://mydsn')
mssql_db = create_engine('mssql+pyodbc://mydsn')
mssql_db = create_engine('mssql+adodbapi://mydsn')
mssql_db = create_engine('mssql+pyodbc://username:password@mydsn')

SQLite connects to file based databases. The same URL format is used, omitting the hostname, and using the "file" portion as the filename of the database. This has the effect of four slashes being present for an absolute file path:

# sqlite://<nohostname>/<path>
# where <path> is relative:
sqlite_db = create_engine('sqlite:///foo.db')

# or absolute, starting with a slash:
sqlite_db = create_engine('sqlite:////absolute/path/to/foo.db')

To use a SQLite :memory: database, specify an empty URL:

sqlite_memory_db = create_engine('sqlite://')

The :class:`.Engine` will ask the connection pool for a connection when the connect() or execute() methods are called. The default connection pool, :class:`~.QueuePool`, will open connections to the database on an as-needed basis. As concurrent statements are executed, :class:`.QueuePool` will grow its pool of connections to a default size of five, and will allow a default "overflow" of ten. Since the :class:`.Engine` is essentially "home base" for the connection pool, it follows that you should keep a single :class:`.Engine` per database established within an application, rather than creating a new one for each connection.


:class:`.QueuePool` is not used by default for SQLite engines. See :ref:`sqlite_toplevel` for details on SQLite connection pool usage.

Custom DBAPI connect() arguments

Custom arguments used when issuing the connect() call to the underlying DBAPI may be issued in three distinct ways. String-based arguments can be passed directly from the URL string as query arguments:

db = create_engine('postgresql://scott:tiger@localhost/test?argument1=foo&argument2=bar')

If SQLAlchemy's database connector is aware of a particular query argument, it may convert its type from string to its proper type.

:func:`~sqlalchemy.create_engine` also takes an argument connect_args which is an additional dictionary that will be passed to connect(). This can be used when arguments of a type other than string are required, and SQLAlchemy's database connector has no type conversion logic present for that parameter:

db = create_engine('postgresql://scott:tiger@localhost/test', connect_args = {'argument1':17, 'argument2':'bar'})

The most customizable connection method of all is to pass a creator argument, which specifies a callable that returns a DBAPI connection:

def connect():
    return psycopg.connect(user='scott', host='localhost')

db = create_engine('postgresql://', creator=connect)

Configuring Logging

Python's standard logging module is used to implement informational and debug log output with SQLAlchemy. This allows SQLAlchemy's logging to integrate in a standard way with other applications and libraries. The echo and echo_pool flags that are present on :func:`~sqlalchemy.create_engine`, as well as the echo_uow flag used on :class:`~sqlalchemy.orm.session.Session`, all interact with regular loggers.

This section assumes familiarity with the above linked logging module. All logging performed by SQLAlchemy exists underneath the sqlalchemy namespace, as used by logging.getLogger('sqlalchemy'). When logging has been configured (i.e. such as via logging.basicConfig()), the general namespace of SA loggers that can be turned on is as follows:

  • sqlalchemy.engine - controls SQL echoing. set to logging.INFO for SQL query output, logging.DEBUG for query + result set output.
  • sqlalchemy.dialects - controls custom logging for SQL dialects. See the documentation of individual dialects for details.
  • sqlalchemy.pool - controls connection pool logging. set to logging.INFO or lower to log connection pool checkouts/checkins.
  • sqlalchemy.orm - controls logging of various ORM functions. set to logging.INFO for information on mapper configurations.

For example, to log SQL queries using Python logging instead of the echo=True flag:

import logging


By default, the log level is set to logging.WARN within the entire sqlalchemy namespace so that no log operations occur, even within an application that has logging enabled otherwise.

The echo flags present as keyword arguments to :func:`~sqlalchemy.create_engine` and others as well as the echo property on :class:`~sqlalchemy.engine.base.Engine`, when set to True, will first attempt to ensure that logging is enabled. Unfortunately, the logging module provides no way of determining if output has already been configured (note we are referring to if a logging configuration has been set up, not just that the logging level is set). For this reason, any echo=True flags will result in a call to logging.basicConfig() using sys.stdout as the destination. It also sets up a default format using the level name, timestamp, and logger name. Note that this configuration has the affect of being configured in addition to any existing logger configurations. Therefore, when using Python logging, ensure all echo flags are set to False at all times, to avoid getting duplicate log lines.

The logger name of instance such as an :class:`~sqlalchemy.engine.base.Engine` or :class:`~sqlalchemy.pool.Pool` defaults to using a truncated hex identifier string. To set this to a specific name, use the "logging_name" and "pool_logging_name" keyword arguments with :func:`sqlalchemy.create_engine`.


The SQLAlchemy :class:`.Engine` conserves Python function call overhead by only emitting log statements when the current logging level is detected as logging.INFO or logging.DEBUG. It only checks this level when a new connection is procured from the connection pool. Therefore when changing the logging configuration for an already-running application, any :class:`.Connection` that's currently active, or more commonly a :class:`~.orm.session.Session` object that's active in a transaction, won't log any SQL according to the new configuration until a new :class:`.Connection` is procured (in the case of :class:`~.orm.session.Session`, this is after the current transaction ends and a new one begins).