# Copyright (C) 2005-2012 the SQLAlchemy authors and contributors <see AUTHORS file>
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php
"""SQL connections, SQL execution and high-level DB-API interface.
The engine package defines the basic components used to interface
DB-API modules with higher-level statement construction,
connection-management, execution and result contexts. The primary
"entry point" class into this package is the Engine and it's public
This package includes:
Defines interface classes and some implementation classes which
comprise the basic components used to interface between a DB-API,
constructed and plain-text statements, connections, transactions,
Contains default implementations of some of the components defined
in base.py. All current database dialects use the classes in
default.py as base classes for their own database-specific
The mechanics of constructing ``Engine`` objects are represented
here. Defines the ``EngineStrategy`` class which represents how
to go from arguments specified to the ``create_engine()``
function, to a fully constructed ``Engine``, including
initialization of connection pooling, dialects, and specific
subclasses of ``Engine``.
The ``TLEngine`` class is defined here, which is a subclass of
the generic ``Engine`` and tracks ``Connection`` and
``Transaction`` objects against the identity of the current
thread. This allows certain programming patterns based around
the concept of a "thread-local connection" to be possible.
The ``TLEngine`` is created by using the "threadlocal" engine
strategy in conjunction with the ``create_engine()`` function.
Defines the ``URL`` class which represents the individual
components of a string URL passed to ``create_engine()``. Also
defines a basic module-loading strategy for the dialect specifier
within a URL.
# not sure what this was used for
from .interfaces import (
from .base import (
from .result import (
from .util import (
from . import util, strategies
default_strategy = 'plain'
def create_engine(*args, **kwargs):
"""Create a new :class:`.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.
Additional keyword arguments may then follow it which
establish various options on the resulting :class:`.Engine`
and its underlying :class:`.Dialect` and :class:`.Pool`
The string form of the URL is
``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 :class:`~sqlalchemy.engine.url.URL`.
``**kwargs`` takes a wide variety of options which are routed
towards their appropriate components. Arguments may be specific
to the :class:`.Engine`, the underlying :class:`.Dialect`, as well as
the :class:`.Pool`. Specific dialects also accept keyword
arguments that are unique to that dialect. Here, we describe the
parameters that are common to most :func:`.create_engine()` usage.
Once established, the newly resulting :class:`.Engine` will
request a connection from the underlying :class:`.Pool` once
:meth:`.Engine.connect` is called, or a method which depends on it
such as :meth:`.Engine.execute` is invoked. The :class:`.Pool` in turn
will establish the first actual DBAPI connection when this request
is received. The :func:`.create_engine` call itself does **not**
establish any actual DBAPI connections directly.
:param assert_unicode: Deprecated. This flag
sets an engine-wide default value for
the ``assert_unicode`` flag on the
:class:`.String` type - see that
type for further details.
:param case_sensitive=True: if False, result column names
will match in a case-insensitive fashion, that is,
.. versionchanged:: 0.8
By default, result row names match case-sensitively.
In version 0.7 and prior, all matches were case-insensitive.
:param connect_args: a dictionary of options which will be
passed directly to the DBAPI's ``connect()`` method as
additional keyword arguments. See the example
:param convert_unicode=False: if set to True, sets
the default behavior of ``convert_unicode`` on the
:class:`.String` type to ``True``, regardless
of a setting of ``False`` on an individual
:class:`.String` type, thus causing all :class:`.String`
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 :class:`.String` for further details on
what this flag indicates.
:param 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.
:param 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 :ref:`dbengine_logging` for
information on how to configure logging directly.
:param 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
:ref:`dbengine_logging` for information on how to configure logging
:param 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
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 :ref:`dialect_toplevel`.
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 :class:`.Unicode` type or the :class:`.String` type
when ``convert_unicode`` is ``True``;
* the values returned in result set columns corresponding
to the :class:`.Unicode` type or the :class:`.String`
type when ``convert_unicode`` is ``True``;
* the string SQL statement passed to the DBAPI's
* 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
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.
:param execution_options: Dictionary execution options which will
be applied to all connections. See
:param 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.
:param 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.
:param listeners: A list of one or more
:class:`~sqlalchemy.interfaces.PoolListener` objects which will
receive connection pool events.
:param 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
:param 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 :class:`~sqlalchemy.pool.QueuePool`.
:param 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 :class:`.Engine`.
:param pool=None: an already-constructed instance of
:class:`~sqlalchemy.pool.Pool`, such as a
:class:`~sqlalchemy.pool.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 :ref:`pooling_toplevel`.
:param poolclass=None: a :class:`~sqlalchemy.pool.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.
:param 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
:param pool_size=5: the number of connections to keep open
inside the connection pool. This used with
well as :class:`~sqlalchemy.pool.SingletonThreadPool`. With
:class:`~sqlalchemy.pool.QueuePool`, a ``pool_size`` setting
of 0 indicates no limit; to disable pooling, set ``poolclass`` to
:param 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).
:param 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 :class:`.Pool`.
.. versionadded:: 0.7.6
:param pool_timeout=30: number of seconds to wait before giving
up on getting a connection from the pool. This is only used
:param strategy='plain': selects alternate engine implementations.
Currently available are:
* the ``threadlocal`` strategy, which is described in
* the ``mock`` strategy, which dispatches all statement
execution to a function passed as the argument ``executor``.
See `example in the FAQ <http://www.sqlalchemy.org/trac/wiki/FAQ#HowcanIgettheCREATETABLEDROPTABLEoutputasastring>`_.
:param executor=None: a function taking arguments
``(sql, *multiparams, **params)``, to which the ``mock`` strategy will
dispatch all statement execution. Used only by ``strategy='mock'``.
strategy = kwargs.pop('strategy', default_strategy)
strategy = strategies.strategies[strategy]
return strategy.create(*args, **kwargs)
def 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
opts = util._coerce_config(configuration, prefix)
url = opts.pop('url')
return create_engine(url, **opts)
__all__ = (