sqlalchemy.event.attr

Attribute implementation for _Dispatch classes.

The various listener targets for a particular event class are represented as attributes, which refer to collections of listeners to be fired off. These collections can exist at the class level as well as at the instance level. An event is fired off using code like this:

some_object.dispatch.first_connect(arg1, arg2)

Above, some_object.dispatch would be an instance of _Dispatch and first_connect is typically an instance of _ListenerCollection if event listeners are present, or _EmptyListener if none are present.

The attribute mechanics here spend effort trying to ensure listener functions are available with a minimum of function call overhead, that unnecessary objects aren't created (i.e. many empty per-instance listener collections), as well as that everything is garbage collectable when owning references are lost. Other features such as "propagation" of listener functions across many _Dispatch instances, "joining" of multiple _Dispatch instances, as well as support for subclass propagation (e.g. events assigned to Pool vs. QueuePool) are all implemented here.