[Sqlalchemy-tickets] Issue #3355: add before_connect event (zzzeek/sqlalchemy)

[Mike B. <iss...@bi...>]

New issue 3355: add before_connect event
https://bitbucket.org/zzzeek/sqlalchemy/issue/3355/add-before_connect-event

Mike Bayer:

this event is to serve use cases that need to affect how the initial connection is created, without the burden of needing to provide the full-blown "creator" callable from scratch (or monkeypatch after the fact).

    def before_connect(connect, connect_arg, connect_kwarg, connection_record):
        # 1. we can manipulate the args:
        # connect_kwarg['foo'] = 'bar'

        # 2. we can do things before the connection happens, time delays, etc.
        log.debug("about to connect to db: %s" % connect_arg)

        # 3. we can do the connect ourselves, if we use retval=True
        new_conn = connect(*connect_arg, **connect_kwarg)
        new_conn.do_something()
        return new_conn

The above "connect" is actually going to be `dialect.connect`.   It would be nice if the connection pool were built to work at the level of connect + args, but at the moment it isn't, and explicit use of "creator" is widespread, so this is necessarily an engine-level event.

Additionally it has to either preclude a system that actually passes "creator" to create_engine(), or work such that connect_arg and connect_kwarg are forced to empty, 
since user-defined creators don't accept or interact with the connect arguments.   

For the case where a custom pool is fully provided, we probably need to skip that case for now as well.

We also should, in DefaultEngineStrategy, set up a flag so that when the event is added, and the event can't be added because "pool" was provided or "creator" was provided and we aren't covering that, we can raise an error that the event can't be used in this mode.

The instrumentation would take place within DefaultEngineStrategy, but also has to be ready for an event added to an Engine after the fact; it needs to check that the event can be used at all and ensure it is called if so, else raise an error.

This can be a 1.0.xx feature.  Targeting at 1.1 for priority.