back by using an attrgetter for the default case

be stated in the query string for a URL.   Custom plugins can
be written which will be given the chance up front to alter and/or
consume the engine's URL and keyword arguments, and then at engine
create time will be given the engine itself to allow additional
modifications or event registration.  Plugins are written as a
subclass of :class:`.CreateEnginePlugin`; see that class for
details.
fixes #3536

pep-249 exception names linked to exception classes of an entirely
different name, preventing SQLAlchemy's own exception wrapping from
wrapping the error appropriately.
The SQLAlchemy dialect in use needs to implement a new
accessor :attr:`.DefaultDialect.dbapi_exception_translation_map`
to support this feature; this is implemented now for the py-postgresql
dialect.
fixes #3421

functionality.   Added a new "soft invalidate" feature to the
connection pool at the level of the checked out connection wrapper
as well as the :class:`._ConnectionRecord`.  This works similarly
to a modern pool invalidation in that connections aren't actively
closed, but are recycled only on next checkout; this is essentially
a per-connection version of that feature.  A new event
:class:`.PoolEvents.soft_invalidate` is added to complement it.
fixes #3379

- Added new flag
:attr:`.ExceptionContext.invalidate_pool_on_disconnect`.
Allows an error handler within :meth:`.ConnectionEvents.handle_error`
to maintain a "disconnect" condition, but to handle calling invalidate
on individual connections in a specific manner within the event.

- Added new event :class:`.DialectEvents.do_connect`, which allows
interception / replacement of when the :meth:`.Dialect.connect`
hook is called to create a DBAPI connection.  Also added
dialect plugin hooks :meth:`.Dialect.get_dialect_cls` and
:meth:`.Dialect.engine_created` which allow external plugins to
add events to existing dialects using entry points.
fixes #3355