472 lines
20 KiB
Python
472 lines
20 KiB
Python
# engine/__init__.py
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# Copyright (C) 2005-2018 the SQLAlchemy authors and contributors
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# <see AUTHORS file>
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#
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# This module is part of SQLAlchemy and is released under
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# the MIT License: http://www.opensource.org/licenses/mit-license.php
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"""SQL connections, SQL execution and high-level DB-API interface.
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The engine package defines the basic components used to interface
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DB-API modules with higher-level statement construction,
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connection-management, execution and result contexts. The primary
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"entry point" class into this package is the Engine and its public
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constructor ``create_engine()``.
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This package includes:
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base.py
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Defines interface classes and some implementation classes which
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comprise the basic components used to interface between a DB-API,
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constructed and plain-text statements, connections, transactions,
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and results.
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default.py
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Contains default implementations of some of the components defined
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in base.py. All current database dialects use the classes in
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default.py as base classes for their own database-specific
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implementations.
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strategies.py
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The mechanics of constructing ``Engine`` objects are represented
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here. Defines the ``EngineStrategy`` class which represents how
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to go from arguments specified to the ``create_engine()``
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function, to a fully constructed ``Engine``, including
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initialization of connection pooling, dialects, and specific
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subclasses of ``Engine``.
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threadlocal.py
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The ``TLEngine`` class is defined here, which is a subclass of
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the generic ``Engine`` and tracks ``Connection`` and
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``Transaction`` objects against the identity of the current
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thread. This allows certain programming patterns based around
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the concept of a "thread-local connection" to be possible.
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The ``TLEngine`` is created by using the "threadlocal" engine
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strategy in conjunction with the ``create_engine()`` function.
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url.py
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Defines the ``URL`` class which represents the individual
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components of a string URL passed to ``create_engine()``. Also
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defines a basic module-loading strategy for the dialect specifier
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within a URL.
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"""
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from .interfaces import (
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Connectable,
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CreateEnginePlugin,
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Dialect,
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ExecutionContext,
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ExceptionContext,
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# backwards compat
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Compiled,
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TypeCompiler
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)
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from .base import (
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Connection,
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Engine,
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NestedTransaction,
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RootTransaction,
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Transaction,
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TwoPhaseTransaction,
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)
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from .result import (
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BaseRowProxy,
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BufferedColumnResultProxy,
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BufferedColumnRow,
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BufferedRowResultProxy,
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FullyBufferedResultProxy,
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ResultProxy,
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RowProxy,
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)
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from .util import (
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connection_memoize
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)
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from . import util, strategies
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# backwards compat
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from ..sql import ddl
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default_strategy = 'plain'
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def create_engine(*args, **kwargs):
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"""Create a new :class:`.Engine` instance.
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The standard calling form is to send the URL as the
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first positional argument, usually a string
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that indicates database dialect and connection arguments::
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engine = create_engine("postgresql://scott:tiger@localhost/test")
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Additional keyword arguments may then follow it which
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establish various options on the resulting :class:`.Engine`
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and its underlying :class:`.Dialect` and :class:`.Pool`
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constructs::
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engine = create_engine("mysql://scott:tiger@hostname/dbname",
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encoding='latin1', echo=True)
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The string form of the URL is
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``dialect[+driver]://user:password@host/dbname[?key=value..]``, where
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``dialect`` is a database name such as ``mysql``, ``oracle``,
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``postgresql``, etc., and ``driver`` the name of a DBAPI, such as
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``psycopg2``, ``pyodbc``, ``cx_oracle``, etc. Alternatively,
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the URL can be an instance of :class:`~sqlalchemy.engine.url.URL`.
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``**kwargs`` takes a wide variety of options which are routed
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towards their appropriate components. Arguments may be specific to
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the :class:`.Engine`, the underlying :class:`.Dialect`, as well as the
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:class:`.Pool`. Specific dialects also accept keyword arguments that
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are unique to that dialect. Here, we describe the parameters
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that are common to most :func:`.create_engine()` usage.
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Once established, the newly resulting :class:`.Engine` will
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request a connection from the underlying :class:`.Pool` once
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:meth:`.Engine.connect` is called, or a method which depends on it
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such as :meth:`.Engine.execute` is invoked. The :class:`.Pool` in turn
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will establish the first actual DBAPI connection when this request
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is received. The :func:`.create_engine` call itself does **not**
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establish any actual DBAPI connections directly.
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.. seealso::
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:doc:`/core/engines`
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:doc:`/dialects/index`
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:ref:`connections_toplevel`
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:param case_sensitive=True: if False, result column names
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will match in a case-insensitive fashion, that is,
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``row['SomeColumn']``.
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.. versionchanged:: 0.8
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By default, result row names match case-sensitively.
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In version 0.7 and prior, all matches were case-insensitive.
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:param connect_args: a dictionary of options which will be
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passed directly to the DBAPI's ``connect()`` method as
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additional keyword arguments. See the example
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at :ref:`custom_dbapi_args`.
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:param convert_unicode=False: if set to True, sets
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the default behavior of ``convert_unicode`` on the
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:class:`.String` type to ``True``, regardless
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of a setting of ``False`` on an individual
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:class:`.String` type, thus causing all :class:`.String`
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-based columns
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to accommodate Python ``unicode`` objects. This flag
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is useful as an engine-wide setting when using a
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DBAPI that does not natively support Python
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``unicode`` objects and raises an error when
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one is received (such as pyodbc with FreeTDS).
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See :class:`.String` for further details on
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what this flag indicates.
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:param creator: a callable which returns a DBAPI connection.
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This creation function will be passed to the underlying
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connection pool and will be used to create all new database
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connections. Usage of this function causes connection
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parameters specified in the URL argument to be bypassed.
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:param echo=False: if True, the Engine will log all statements
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as well as a repr() of their parameter lists to the engines
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logger, which defaults to sys.stdout. The ``echo`` attribute of
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``Engine`` can be modified at any time to turn logging on and
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off. If set to the string ``"debug"``, result rows will be
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printed to the standard output as well. This flag ultimately
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controls a Python logger; see :ref:`dbengine_logging` for
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information on how to configure logging directly.
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:param echo_pool=False: if True, the connection pool will log
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all checkouts/checkins to the logging stream, which defaults to
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sys.stdout. This flag ultimately controls a Python logger; see
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:ref:`dbengine_logging` for information on how to configure logging
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directly.
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:param empty_in_strategy: The SQL compilation strategy to use when
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rendering an IN or NOT IN expression for :meth:`.ColumnOperators.in_`
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where the right-hand side
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is an empty set. This is a string value that may be one of
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``static``, ``dynamic``, or ``dynamic_warn``. The ``static``
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strategy is the default, and an IN comparison to an empty set
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will generate a simple false expression "1 != 1". The ``dynamic``
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strategy behaves like that of SQLAlchemy 1.1 and earlier, emitting
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a false expression of the form "expr != expr", which has the effect
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of evaluting to NULL in the case of a null expression.
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``dynamic_warn`` is the same as ``dynamic``, however also emits a
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warning when an empty set is encountered; this because the "dynamic"
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comparison is typically poorly performing on most databases.
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.. versionadded:: 1.2 Added the ``empty_in_strategy`` setting and
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additionally defaulted the behavior for empty-set IN comparisons
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to a static boolean expression.
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:param encoding: Defaults to ``utf-8``. This is the string
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encoding used by SQLAlchemy for string encode/decode
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operations which occur within SQLAlchemy, **outside of
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the DBAPI.** Most modern DBAPIs feature some degree of
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direct support for Python ``unicode`` objects,
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what you see in Python 2 as a string of the form
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``u'some string'``. For those scenarios where the
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DBAPI is detected as not supporting a Python ``unicode``
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object, this encoding is used to determine the
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source/destination encoding. It is **not used**
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for those cases where the DBAPI handles unicode
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directly.
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To properly configure a system to accommodate Python
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``unicode`` objects, the DBAPI should be
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configured to handle unicode to the greatest
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degree as is appropriate - see
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the notes on unicode pertaining to the specific
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target database in use at :ref:`dialect_toplevel`.
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Areas where string encoding may need to be accommodated
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outside of the DBAPI include zero or more of:
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* the values passed to bound parameters, corresponding to
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the :class:`.Unicode` type or the :class:`.String` type
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when ``convert_unicode`` is ``True``;
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* the values returned in result set columns corresponding
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to the :class:`.Unicode` type or the :class:`.String`
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type when ``convert_unicode`` is ``True``;
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* the string SQL statement passed to the DBAPI's
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``cursor.execute()`` method;
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* the string names of the keys in the bound parameter
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dictionary passed to the DBAPI's ``cursor.execute()``
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as well as ``cursor.setinputsizes()`` methods;
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* the string column names retrieved from the DBAPI's
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``cursor.description`` attribute.
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When using Python 3, the DBAPI is required to support
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*all* of the above values as Python ``unicode`` objects,
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which in Python 3 are just known as ``str``. In Python 2,
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the DBAPI does not specify unicode behavior at all,
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so SQLAlchemy must make decisions for each of the above
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values on a per-DBAPI basis - implementations are
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completely inconsistent in their behavior.
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:param execution_options: Dictionary execution options which will
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be applied to all connections. See
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:meth:`~sqlalchemy.engine.Connection.execution_options`
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:param implicit_returning=True: When ``True``, a RETURNING-
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compatible construct, if available, will be used to
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fetch newly generated primary key values when a single row
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INSERT statement is emitted with no existing returning()
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clause. This applies to those backends which support RETURNING
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or a compatible construct, including PostgreSQL, Firebird, Oracle,
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Microsoft SQL Server. Set this to ``False`` to disable
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the automatic usage of RETURNING.
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:param isolation_level: this string parameter is interpreted by various
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dialects in order to affect the transaction isolation level of the
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database connection. The parameter essentially accepts some subset of
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these string arguments: ``"SERIALIZABLE"``, ``"REPEATABLE_READ"``,
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``"READ_COMMITTED"``, ``"READ_UNCOMMITTED"`` and ``"AUTOCOMMIT"``.
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Behavior here varies per backend, and
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individual dialects should be consulted directly.
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Note that the isolation level can also be set on a per-:class:`.Connection`
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basis as well, using the
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:paramref:`.Connection.execution_options.isolation_level`
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feature.
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.. seealso::
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:attr:`.Connection.default_isolation_level` - view default level
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:paramref:`.Connection.execution_options.isolation_level`
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- set per :class:`.Connection` isolation level
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:ref:`SQLite Transaction Isolation <sqlite_isolation_level>`
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:ref:`PostgreSQL Transaction Isolation <postgresql_isolation_level>`
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:ref:`MySQL Transaction Isolation <mysql_isolation_level>`
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:ref:`session_transaction_isolation` - for the ORM
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:param label_length=None: optional integer value which limits
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the size of dynamically generated column labels to that many
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characters. If less than 6, labels are generated as
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"_(counter)". If ``None``, the value of
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``dialect.max_identifier_length`` is used instead.
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:param listeners: A list of one or more
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:class:`~sqlalchemy.interfaces.PoolListener` objects which will
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receive connection pool events.
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:param logging_name: String identifier which will be used within
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the "name" field of logging records generated within the
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"sqlalchemy.engine" logger. Defaults to a hexstring of the
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object's id.
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:param max_overflow=10: the number of connections to allow in
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connection pool "overflow", that is connections that can be
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opened above and beyond the pool_size setting, which defaults
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to five. this is only used with :class:`~sqlalchemy.pool.QueuePool`.
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:param module=None: reference to a Python module object (the module
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itself, not its string name). Specifies an alternate DBAPI module to
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be used by the engine's dialect. Each sub-dialect references a
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specific DBAPI which will be imported before first connect. This
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parameter causes the import to be bypassed, and the given module to
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be used instead. Can be used for testing of DBAPIs as well as to
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inject "mock" DBAPI implementations into the :class:`.Engine`.
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:param paramstyle=None: The `paramstyle <http://legacy.python.org/dev/peps/pep-0249/#paramstyle>`_
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to use when rendering bound parameters. This style defaults to the
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one recommended by the DBAPI itself, which is retrieved from the
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``.paramstyle`` attribute of the DBAPI. However, most DBAPIs accept
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more than one paramstyle, and in particular it may be desirable
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to change a "named" paramstyle into a "positional" one, or vice versa.
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When this attribute is passed, it should be one of the values
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``"qmark"``, ``"numeric"``, ``"named"``, ``"format"`` or
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``"pyformat"``, and should correspond to a parameter style known
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to be supported by the DBAPI in use.
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:param pool=None: an already-constructed instance of
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:class:`~sqlalchemy.pool.Pool`, such as a
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:class:`~sqlalchemy.pool.QueuePool` instance. If non-None, this
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pool will be used directly as the underlying connection pool
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for the engine, bypassing whatever connection parameters are
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present in the URL argument. For information on constructing
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connection pools manually, see :ref:`pooling_toplevel`.
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:param poolclass=None: a :class:`~sqlalchemy.pool.Pool`
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subclass, which will be used to create a connection pool
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instance using the connection parameters given in the URL. Note
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this differs from ``pool`` in that you don't actually
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instantiate the pool in this case, you just indicate what type
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of pool to be used.
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:param pool_logging_name: String identifier which will be used within
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the "name" field of logging records generated within the
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"sqlalchemy.pool" logger. Defaults to a hexstring of the object's
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id.
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:param pool_pre_ping: boolean, if True will enable the connection pool
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"pre-ping" feature that tests connections for liveness upon
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each checkout.
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.. versionadded:: 1.2
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.. seealso::
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:ref:`pool_disconnects_pessimistic`
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:param pool_size=5: the number of connections to keep open
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inside the connection pool. This used with
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:class:`~sqlalchemy.pool.QueuePool` as
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well as :class:`~sqlalchemy.pool.SingletonThreadPool`. With
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:class:`~sqlalchemy.pool.QueuePool`, a ``pool_size`` setting
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of 0 indicates no limit; to disable pooling, set ``poolclass`` to
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:class:`~sqlalchemy.pool.NullPool` instead.
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:param pool_recycle=-1: this setting causes the pool to recycle
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connections after the given number of seconds has passed. It
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defaults to -1, or no timeout. For example, setting to 3600
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means connections will be recycled after one hour. Note that
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MySQL in particular will disconnect automatically if no
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activity is detected on a connection for eight hours (although
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this is configurable with the MySQLDB connection itself and the
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server configuration as well).
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.. seealso::
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:ref:`pool_setting_recycle`
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:param pool_reset_on_return='rollback': set the "reset on return"
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behavior of the pool, which is whether ``rollback()``,
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``commit()``, or nothing is called upon connections
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being returned to the pool. See the docstring for
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``reset_on_return`` at :class:`.Pool`.
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.. versionadded:: 0.7.6
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:param pool_timeout=30: number of seconds to wait before giving
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up on getting a connection from the pool. This is only used
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with :class:`~sqlalchemy.pool.QueuePool`.
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:param plugins: string list of plugin names to load. See
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:class:`.CreateEnginePlugin` for background.
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.. versionadded:: 1.2.3
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:param strategy='plain': selects alternate engine implementations.
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Currently available are:
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* the ``threadlocal`` strategy, which is described in
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:ref:`threadlocal_strategy`;
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* the ``mock`` strategy, which dispatches all statement
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execution to a function passed as the argument ``executor``.
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See `example in the FAQ
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<http://docs.sqlalchemy.org/en/latest/faq/metadata_schema.html#how-can-i-get-the-create-table-drop-table-output-as-a-string>`_.
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:param executor=None: a function taking arguments
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``(sql, *multiparams, **params)``, to which the ``mock`` strategy will
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dispatch all statement execution. Used only by ``strategy='mock'``.
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"""
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strategy = kwargs.pop('strategy', default_strategy)
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strategy = strategies.strategies[strategy]
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return strategy.create(*args, **kwargs)
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def engine_from_config(configuration, prefix='sqlalchemy.', **kwargs):
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"""Create a new Engine instance using a configuration dictionary.
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The dictionary is typically produced from a config file.
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The keys of interest to ``engine_from_config()`` should be prefixed, e.g.
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``sqlalchemy.url``, ``sqlalchemy.echo``, etc. The 'prefix' argument
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indicates the prefix to be searched for. Each matching key (after the
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prefix is stripped) is treated as though it were the corresponding keyword
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argument to a :func:`.create_engine` call.
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The only required key is (assuming the default prefix) ``sqlalchemy.url``,
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which provides the :ref:`database URL <database_urls>`.
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A select set of keyword arguments will be "coerced" to their
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expected type based on string values. The set of arguments
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is extensible per-dialect using the ``engine_config_types`` accessor.
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:param configuration: A dictionary (typically produced from a config file,
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but this is not a requirement). Items whose keys start with the value
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of 'prefix' will have that prefix stripped, and will then be passed to
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:ref:`create_engine`.
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:param prefix: Prefix to match and then strip from keys
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in 'configuration'.
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:param kwargs: Each keyword argument to ``engine_from_config()`` itself
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overrides the corresponding item taken from the 'configuration'
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dictionary. Keyword arguments should *not* be prefixed.
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"""
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options = dict((key[len(prefix):], configuration[key])
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for key in configuration
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if key.startswith(prefix))
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options['_coerce_config'] = True
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options.update(kwargs)
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url = options.pop('url')
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return create_engine(url, **options)
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__all__ = (
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'create_engine',
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'engine_from_config',
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)
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