# SQLAlchemy 0.4 Documentation

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Version: 0.4.6 Last Updated: 05/14/08 14:36:43

### module sqlalchemy.interfaces

Interfaces and abstract types.

### class PoolListener(object)

Hooks into the lifecycle of connections in a `Pool`.

All of the standard connection Pool types can accept event listeners for key connection lifecycle events: creation, pool check-out and check-in. There are no events fired when a connection closes.

For any given DB-API connection, there will be one `connect` event, n number of `checkout` events, and either n or n - 1 `checkin` events. (If a `Connection` is detached from its pool via the `detach()` method, it won't be checked back in.)

These are low-level events for low-level objects: raw Python DB-API connections, without the conveniences of the SQLAlchemy `Connection` wrapper, `Dialect` services or `ClauseElement` execution. If you execute SQL through the connection, explicitly closing all cursors and other resources is recommended.

Events also receive a `_ConnectionRecord`, a long-lived internal `Pool` object that basically represents a "slot" in the connection pool. `_ConnectionRecord` objects have one public attribute of note: `info`, a dictionary whose contents are scoped to the lifetime of the DB-API connection managed by the record. You can use this shared storage area however you like.

There is no need to subclass `PoolListener` to handle events. Any class that implements one or more of these methods can be used as a pool listener. The `Pool` will inspect the methods provided by a listener object and add the listener to one or more internal event queues based on its capabilities. In terms of efficiency and function call overhead, you're much better off only providing implementations for the hooks you'll be using.

def checkin(self, dbapi_con, con_record)

Called when a connection returns to the pool.

Note that the connection may be closed, and may be None if the connection has been invalidated. `checkin` will not be called for detached connections. (They do not return to the pool.)

dbapi_con
  A raw DB-API connection
con_record
  The `_ConnectionRecord` that persistently manages the connection

def checkout(self, dbapi_con, con_record, con_proxy)

Called when a connection is retrieved from the Pool.

dbapi_con
  A raw DB-API connection
con_record
  The `_ConnectionRecord` that persistently manages the connection
con_proxy
  The `_ConnectionFairy` which manages the connection for the span of the current checkout.

If you raise an `exceptions.DisconnectionError`, the current connection will be disposed and a fresh connection retrieved. Processing of all checkout listeners will abort and restart using the new connection.

def connect(self, dbapi_con, con_record)

Called once for each new DB-API connection or Pool's `creator()`.

dbapi_con
  A newly connected raw DB-API connection (not a SQLAlchemy `Connection` wrapper).
con_record
  The `_ConnectionRecord` that persistently manages the connection

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