Release: 1.0.0 | Release Date: Not released

SQLAlchemy 1.0 Documentation

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:

../_images/sqla_engine_arch.png

Where above, an Engine references both a Dialect and a 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, create_engine():

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

The above engine creates a Dialect object tailored towards PostgreSQL, as well as a Pool object which will establish a DBAPI connection at localhost:5432 when a connection request is first received. Note that the Engine and its underlying Pool do not establish the first actual DBAPI connection until the Engine.connect() method is called, or an operation which is dependent on this method such as Engine.execute() is invoked. In this way, Engine and Pool can be said to have a lazy initialization behavior.

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

Supported Databases

SQLAlchemy includes many Dialect implementations for various backends. Dialects for the most common databases are included with SQLAlchemy; a handful of others require an additional install of a separate dialect.

See the section Dialects for information on the various backends available.

Database Urls

The create_engine() function produces an Engine object based on a URL. These URLs follow RFC-1738, and usually can include username, password, hostname, database name as well as optional keyword arguments for additional configuration. In some cases a file path is accepted, and in others a “data source name” replaces the “host” and “database” portions. The typical form of a database URL is:

dialect+driver://username:password@host:port/database

Dialect names include the identifying name of the SQLAlchemy dialect, a name such as sqlite, mysql, postgresql, oracle, or mssql. 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.

Examples for common connection styles follow below. For a full index of detailed information on all included dialects as well as links to third-party dialects, see Dialects.

Postgresql

The Postgresql dialect uses psycopg2 as the default DBAPI. pg8000 is also available as a pure-Python substitute:

# default
engine = create_engine('postgresql://scott:tiger@localhost/mydatabase')

# psycopg2
engine = create_engine('postgresql+psycopg2://scott:tiger@localhost/mydatabase')

# pg8000
engine = create_engine('postgresql+pg8000://scott:tiger@localhost/mydatabase')

More notes on connecting to Postgresql at PostgreSQL.

MySQL

The MySQL dialect uses mysql-python as the default DBAPI. There are many MySQL DBAPIs available, including MySQL-connector-python and OurSQL:

# default
engine = create_engine('mysql://scott:tiger@localhost/foo')

# mysql-python
engine = create_engine('mysql+mysqldb://scott:tiger@localhost/foo')

# MySQL-connector-python
engine = create_engine('mysql+mysqlconnector://scott:tiger@localhost/foo')

# OurSQL
engine = create_engine('mysql+oursql://scott:tiger@localhost/foo')

More notes on connecting to MySQL at MySQL.

Oracle

The Oracle dialect uses cx_oracle as the default DBAPI:

engine = create_engine('oracle://scott:tiger@127.0.0.1:1521/sidname')

engine = create_engine('oracle+cx_oracle://scott:tiger@tnsname')

More notes on connecting to Oracle at Oracle.

Microsoft SQL Server

The SQL Server dialect uses pyodbc as the default DBAPI. pymssql is also available:

# pyodbc
engine = create_engine('mssql+pyodbc://scott:tiger@mydsn')

# pymssql
engine = create_engine('mssql+pymssql://scott:tiger@hostname:port/dbname')

More notes on connecting to SQL Server at Microsoft SQL Server.

SQLite

SQLite connects to file-based databases, using the Python built-in module sqlite3 by default.

As SQLite connects to local files, the URL format is slightly different. The “file” portion of the URL is the filename of the database. For a relative file path, this requires three slashes:

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

And for an absolute file path, four slashes are used:

engine = create_engine('sqlite:////absolute/path/to/foo.db')

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

engine = create_engine('sqlite://')

More notes on connecting to SQLite at SQLite.

Others

See Dialects, the top-level page for all additional dialect documentation.

Engine Creation API

sqlalchemy.create_engine(*args, **kwargs)

Create a new Engine instance.

The standard calling form is to send the URL as the first positional argument, usually a string that indicates database dialect and connection arguments:

engine = create_engine("postgresql://scott:tiger@localhost/test")

Additional keyword arguments may then follow it which establish various options on the resulting Engine and its underlying Dialect and Pool constructs:

engine = create_engine("mysql://scott:tiger@hostname/dbname",
                            encoding='latin1', echo=True)

The string form of the URL is dialect[+driver]://user:password@host/dbname[?key=value..], where dialect is a database name such as mysql, oracle, postgresql, etc., and driver the name of a DBAPI, such as psycopg2, pyodbc, cx_oracle, etc. Alternatively, the URL can be an instance of URL.

**kwargs takes a wide variety of options which are routed towards their appropriate components. Arguments may be specific to the Engine, the underlying Dialect, as well as the Pool. Specific dialects also accept keyword arguments that are unique to that dialect. Here, we describe the parameters that are common to most create_engine() usage.

Once established, the newly resulting Engine will request a connection from the underlying Pool once Engine.connect() is called, or a method which depends on it such as Engine.execute() is invoked. The Pool in turn will establish the first actual DBAPI connection when this request is received. The create_engine() call itself does not establish any actual DBAPI connections directly.

Parameters:
  • case_sensitive=True

    if False, result column names will match in a case-insensitive fashion, that is, row['SomeColumn'].

    Changed in version 0.8: By default, result row names match case-sensitively. In version 0.7 and prior, all matches were case-insensitive.

  • connect_args – a dictionary of options which will be passed directly to the DBAPI’s connect() method as additional keyword arguments. See the example at Custom DBAPI connect() arguments.
  • convert_unicode=False

    if set to True, sets the default behavior of convert_unicode on the String type to True, regardless of a setting of False on an individual String type, thus causing all String -based columns to accommodate Python unicode objects. This flag is useful as an engine-wide setting when using a DBAPI that does not natively support Python unicode objects and raises an error when one is received (such as pyodbc with FreeTDS).

    See String for further details on what this flag indicates.

  • creator – a callable which returns a DBAPI connection. This creation function will be passed to the underlying connection pool and will be used to create all new database connections. Usage of this function causes connection parameters specified in the URL argument to be bypassed.
  • echo=False – if True, the Engine will log all statements as well as a repr() of their parameter lists to the engines logger, which defaults to sys.stdout. The echo attribute of Engine can be modified at any time to turn logging on and off. If set to the string "debug", result rows will be printed to the standard output as well. This flag ultimately controls a Python logger; see Configuring Logging for information on how to configure logging directly.
  • echo_pool=False – if True, the connection pool will log all checkouts/checkins to the logging stream, which defaults to sys.stdout. This flag ultimately controls a Python logger; see Configuring Logging for information on how to configure logging directly.
  • encoding

    Defaults to utf-8. This is the string encoding used by SQLAlchemy for string encode/decode operations which occur within SQLAlchemy, outside of the DBAPI. Most modern DBAPIs feature some degree of direct support for Python unicode objects, what you see in Python 2 as a string of the form u'some string'. For those scenarios where the DBAPI is detected as not supporting a Python unicode object, this encoding is used to determine the source/destination encoding. It is not used for those cases where the DBAPI handles unicode directly.

    To properly configure a system to accommodate Python unicode objects, the DBAPI should be configured to handle unicode to the greatest degree as is appropriate - see the notes on unicode pertaining to the specific target database in use at Dialects.

    Areas where string encoding may need to be accommodated outside of the DBAPI include zero or more of:

    • the values passed to bound parameters, corresponding to the Unicode type or the String type when convert_unicode is True;
    • the values returned in result set columns corresponding to the Unicode type or the String type when convert_unicode is True;
    • the string SQL statement passed to the DBAPI’s cursor.execute() method;
    • the string names of the keys in the bound parameter dictionary passed to the DBAPI’s cursor.execute() as well as cursor.setinputsizes() methods;
    • the string column names retrieved from the DBAPI’s cursor.description attribute.

    When using Python 3, the DBAPI is required to support all of the above values as Python unicode objects, which in Python 3 are just known as str. In Python 2, the DBAPI does not specify unicode behavior at all, so SQLAlchemy must make decisions for each of the above values on a per-DBAPI basis - implementations are completely inconsistent in their behavior.

  • execution_options – Dictionary execution options which will be applied to all connections. See execution_options()
  • implicit_returning=True – When True, a RETURNING- compatible construct, if available, will be used to fetch newly generated primary key values when a single row INSERT statement is emitted with no existing returning() clause. This applies to those backends which support RETURNING or a compatible construct, including Postgresql, Firebird, Oracle, Microsoft SQL Server. Set this to False to disable the automatic usage of RETURNING.
  • isolation_level

    this string parameter is interpreted by various dialects in order to affect the transaction isolation level of the database connection. The parameter essentially accepts some subset of these string arguments: "SERIALIZABLE", "REPEATABLE_READ", "READ_COMMITTED", "READ_UNCOMMITTED" and "AUTOCOMMIT". Behavior here varies per backend, and individual dialects should be consulted directly.

  • label_length=None – optional integer value which limits the size of dynamically generated column labels to that many characters. If less than 6, labels are generated as “_(counter)”. If None, the value of dialect.max_identifier_length is used instead.
  • listeners – A list of one or more PoolListener objects which will receive connection pool events.
  • logging_name – String identifier which will be used within the “name” field of logging records generated within the “sqlalchemy.engine” logger. Defaults to a hexstring of the object’s id.
  • max_overflow=10 – the number of connections to allow in connection pool “overflow”, that is connections that can be opened above and beyond the pool_size setting, which defaults to five. this is only used with QueuePool.
  • module=None – reference to a Python module object (the module itself, not its string name). Specifies an alternate DBAPI module to be used by the engine’s dialect. Each sub-dialect references a specific DBAPI which will be imported before first connect. This parameter causes the import to be bypassed, and the given module to be used instead. Can be used for testing of DBAPIs as well as to inject “mock” DBAPI implementations into the Engine.
  • pool=None – an already-constructed instance of Pool, such as a QueuePool instance. If non-None, this pool will be used directly as the underlying connection pool for the engine, bypassing whatever connection parameters are present in the URL argument. For information on constructing connection pools manually, see Connection Pooling.
  • poolclass=None – a Pool subclass, which will be used to create a connection pool instance using the connection parameters given in the URL. Note this differs from pool in that you don’t actually instantiate the pool in this case, you just indicate what type of pool to be used.
  • pool_logging_name – String identifier which will be used within the “name” field of logging records generated within the “sqlalchemy.pool” logger. Defaults to a hexstring of the object’s id.
  • pool_size=5 – the number of connections to keep open inside the connection pool. This used with QueuePool as well as SingletonThreadPool. With QueuePool, a pool_size setting of 0 indicates no limit; to disable pooling, set poolclass to NullPool instead.
  • pool_recycle=-1 – this setting causes the pool to recycle connections after the given number of seconds has passed. It defaults to -1, or no timeout. For example, setting to 3600 means connections will be recycled after one hour. Note that MySQL in particular will disconnect automatically if no activity is detected on a connection for eight hours (although this is configurable with the MySQLDB connection itself and the server configuration as well).
  • pool_reset_on_return=’rollback’

    set the “reset on return” behavior of the pool, which is whether rollback(), commit(), or nothing is called upon connections being returned to the pool. See the docstring for reset_on_return at Pool.

    New in version 0.7.6.

  • pool_timeout=30 – number of seconds to wait before giving up on getting a connection from the pool. This is only used with QueuePool.
  • strategy=’plain’

    selects alternate engine implementations. Currently available are:

  • executor=None – a function taking arguments (sql, *multiparams, **params), to which the mock strategy will dispatch all statement execution. Used only by strategy='mock'.
sqlalchemy.engine_from_config(configuration, prefix='sqlalchemy.', **kwargs)

Create a new Engine instance using a configuration dictionary.

The dictionary is typically produced from a config file where keys are prefixed, such as sqlalchemy.url, sqlalchemy.echo, etc. The ‘prefix’ argument indicates the prefix to be searched for.

A select set of keyword arguments will be “coerced” to their expected type based on string values. In a future release, this functionality will be expanded and include dialect-specific arguments.

sqlalchemy.engine.url.make_url(name_or_url)

Given a string or unicode instance, produce a new URL instance.

The given string is parsed according to the RFC 1738 spec. If an existing URL object is passed, just returns the object.

class sqlalchemy.engine.url.URL(drivername, username=None, password=None, host=None, port=None, database=None, query=None)

Represent the components of a URL used to connect to a database.

This object is suitable to be passed directly to a create_engine() call. The fields of the URL are parsed from a string by the make_url() function. the string format of the URL is an RFC-1738-style string.

All initialization parameters are available as public attributes.

Parameters:
  • drivername – the name of the database backend. This name will correspond to a module in sqlalchemy/databases or a third party plug-in.
  • username – The user name.
  • password – database password.
  • host – The name of the host.
  • port – The port number.
  • database – The database name.
  • query – A dictionary of options to be passed to the dialect and/or the DBAPI upon connect.
get_dialect()

Return the SQLAlchemy database dialect class corresponding to this URL’s driver name.

translate_connect_args(names=, []**kw)

Translate url attributes into a dictionary of connection arguments.

Returns attributes of this url (host, database, username, password, port) as a plain dictionary. The attribute names are used as the keys by default. Unset or false attributes are omitted from the final dictionary.

Parameters:
  • **kw – Optional, alternate key names for url attributes.
  • names – Deprecated. Same purpose as the keyword-based alternate names, but correlates the name to the original positionally.

Pooling

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

Note

QueuePool is not used by default for SQLite engines. See SQLite for details on SQLite connection pool usage.

For more information on connection pooling, see Connection Pooling.

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.

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 create_engine(), as well as the echo_uow flag used on 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

logging.basicConfig()
logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)

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 create_engine() and others as well as the echo property on 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 Engine or 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 sqlalchemy.create_engine().

Note

The SQLAlchemy 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 Connection that’s currently active, or more commonly a Session object that’s active in a transaction, won’t log any SQL according to the new configuration until a new Connection is procured (in the case of Session, this is after the current transaction ends and a new one begins).