sqlalchemy.engine.interfaces.CreateEnginePlugin

A set of hooks intended to augment the construction of an :class:`_engine.Engine` object based on entrypoint names in a URL. The purpose of :class:`_engine.CreateEnginePlugin` is to allow third-party systems to apply engine, pool and dialect level event listeners without the need for the target application to be modified; instead, the plugin names can be added to the database URL. Target applications for :class:`_engine.CreateEnginePlugin` include: * connection and SQL performance tools, e.g. which use events to track number of checkouts and/or time spent with statements * connectivity plugins such as proxies A rudimentary :class:`_engine.CreateEnginePlugin` that attaches a logger to an :class:`_engine.Engine` object might look like:: import logging from sqlalchemy.engine import CreateEnginePlugin from sqlalchemy import event class LogCursorEventsPlugin(CreateEnginePlugin): def __init__(self, url, kwargs): # consume the parameter "log_cursor_logging_name" from the # URL query logging_name = url.query.get("log_cursor_logging_name", "log_cursor") self.log = logging.getLogger(logging_name) def update_url(self, url): "update the URL to one that no longer includes our parameters" return url.difference_update_query(["log_cursor_logging_name"]) def engine_created(self, engine): "attach an event listener after the new Engine is constructed" event.listen(engine, "before_cursor_execute", self._log_event) def _log_event( self, conn, cursor, statement, parameters, context, executemany): self.log.info("Plugin logged cursor event: %s", statement) Plugins are registered using entry points in a similar way as that of dialects:: entry_points={ 'sqlalchemy.plugins': [ 'log_cursor_plugin = myapp.plugins:LogCursorEventsPlugin' ] A plugin that uses the above names would be invoked from a database URL as in:: from sqlalchemy import create_engine engine = create_engine( "mysql+pymysql://scott:tiger@localhost/test?" "plugin=log_cursor_plugin&log_cursor_logging_name=mylogger" ) The ``plugin`` URL parameter supports multiple instances, so that a URL may specify multiple plugins; they are loaded in the order stated in the URL:: engine = create_engine( "mysql+pymysql://scott:tiger@localhost/test?" "plugin=plugin_one&plugin=plugin_twp&plugin=plugin_three") The plugin names may also be passed directly to :func:`_sa.create_engine` using the :paramref:`_sa.create_engine.plugins` argument:: engine = create_engine( "mysql+pymysql://scott:tiger@localhost/test", plugins=["myplugin"]) .. versionadded:: 1.2.3 plugin names can also be specified to :func:`_sa.create_engine` as a list A plugin may consume plugin-specific arguments from the :class:`_engine.URL` object as well as the ``kwargs`` dictionary, which is the dictionary of arguments passed to the :func:`_sa.create_engine` call. "Consuming" these arguments includes that they must be removed when the plugin initializes, so that the arguments are not passed along to the :class:`_engine.Dialect` constructor, where they will raise an :class:`_exc.ArgumentError` because they are not known by the dialect. As of version 1.4 of SQLAlchemy, arguments should continue to be consumed from the ``kwargs`` dictionary directly, by removing the values with a method such as ``dict.pop``. Arguments from the :class:`_engine.URL` object should be consumed by implementing the :meth:`_engine.CreateEnginePlugin.update_url` method, returning a new copy of the :class:`_engine.URL` with plugin-specific parameters removed:: class MyPlugin(CreateEnginePlugin): def __init__(self, url, kwargs): self.my_argument_one = url.query['my_argument_one'] self.my_argument_two = url.query['my_argument_two'] self.my_argument_three = kwargs.pop('my_argument_three', None) def update_url(self, url): return url.difference_update_query( ["my_argument_one", "my_argument_two"] ) Arguments like those illustrated above would be consumed from a :func:`_sa.create_engine` call such as:: from sqlalchemy import create_engine engine = create_engine( "mysql+pymysql://scott:tiger@localhost/test?" "plugin=myplugin&my_argument_one=foo&my_argument_two=bar", my_argument_three='bat' ) .. versionchanged:: 1.4 The :class:`_engine.URL` object is now immutable; a :class:`_engine.CreateEnginePlugin` that needs to alter the :class:`_engine.URL` should implement the newly added :meth:`_engine.CreateEnginePlugin.update_url` method, which is invoked after the plugin is constructed. For migration, construct the plugin in the following way, checking for the existence of the :meth:`_engine.CreateEnginePlugin.update_url` method to detect which version is running:: class MyPlugin(CreateEnginePlugin): def __init__(self, url, kwargs): if hasattr(CreateEnginePlugin, "update_url"): # detect the 1.4 API self.my_argument_one = url.query['my_argument_one'] self.my_argument_two = url.query['my_argument_two'] else: # detect the 1.3 and earlier API - mutate the # URL directly self.my_argument_one = url.query.pop('my_argument_one') self.my_argument_two = url.query.pop('my_argument_two') self.my_argument_three = kwargs.pop('my_argument_three', None) def update_url(self, url): # this method is only called in the 1.4 version return url.difference_update_query( ["my_argument_one", "my_argument_two"] ) .. seealso:: :ref:`change_5526` - overview of the :class:`_engine.URL` change which also includes notes regarding :class:`_engine.CreateEnginePlugin`. When the engine creation process completes and produces the :class:`_engine.Engine` object, it is again passed to the plugin via the :meth:`_engine.CreateEnginePlugin.engine_created` hook. In this hook, additional changes can be made to the engine, most typically involving setup of events (e.g. those defined in :ref:`core_event_toplevel`). .. versionadded:: 1.1