Basic usage of the module is not very different from using Twisted's adbapi:
If you want you can use the :class:`~txpostgres.txpostgres.Cursor` class directly, with a interface closer to Psycopg. Note that using this method you have to make sure never to execute a query before the previous one finishes, as that would violate the PostgreSQL asynchronous protocol.
Every query executed by txpostgres is committed immediately. If you need to execute a series of queries in a transaction, use the :meth:`~txpostgres.txpostgres.Connection.runInteraction` method:
Customising the connection and cursor factories
You might want to customise the way txpostgres creates connections and cursors to take advantage of Psycopg features like :psycopg:`dictionary cursors <extras.html#dictionary-like-cursor>`. To do that, define a subclass of :class:`~txpostgres.txpostgres.Connection` and override :attr:`connectionFactory` or :attr:`cursorFactory` class attributes to use your custom code. Here's an example of how to use dict cursors:
Listening for database notifications
Being an asynchronous driver, txpostgres supports the PostgreSQL NOTIFY feature for sending asynchronous notifications to connections. Here is an example script that connects to the database and listens for notifications on the list channel. Every time a notification is received, it interprets the payload as part of the name of a table and outputs a list of tables with names containing that payload.
$ psql postgres psql (9.1.2) Type "help" for help. postgres=> notify list, 'user'; NOTIFY postgres=> notify list, 'auth'; NOTIFY
You should see the example program outputting lists of table names containing the payload:
$ python notify_example.py Listening on the `list' channel Tables with `user' in their name: pg_user_mapping Tables with `auth' in their name: pg_authid pg_auth_members
The module includes provision for automatically reconnecting to the database in case the connection gets broken. To use it, pass a :class:`~txpostgres.reconnection.DeadConnectionDetector` instance to :class:`~txpostgres.txpostgres.Connection`. You can customise the detector instance or subclass it to add custom logic. See the documentation for :class:`~txpostgres.reconnection.DeadConnectionDetector` for details.
When a :class:`~txpostgres.txpostgres.Connection` is configured with a detector, it will automatically start the reconnection process whenever it encounters a certain class of errors indicative of a disconnect. See :func:`~txpostgres.reconnection.defaultDeathChecker` for more.
While the connection is down, all attempts to use it will result in immediate failures with :exc:`~txpostgres.reconnection.ConnectionDead`. This is to prevent sending additional queries down a link that's known to be down.
Here's an example of using automatic reconnection in txpostgres:
You can run this snippet and then try restarting the database. Logging lines should appear, as the connection gets automatically recovered.
Choosing a Psycopg implementation
To use txpostgres, you will need a recent enough version of Psycopg, namely 2.2.0 or later. Since parts of Psycopg are written in C, it is not available on some Python implementations, like PyPy. When first imported, txpostgres will try to detect if an API-compatible implementation of Psycopg is available.
You can force a certain implementation to be used by exporing an environment variable TXPOSTGRES_PSYCOPG_IMPL. Recognized values are: