The ``connection`` class ======================== .. sectionauthor:: Daniele Varrazzo .. testsetup:: from pprint import pprint import psycopg2.extensions drop_test_table('foo') .. class:: connection Handles the connection to a PostgreSQL database instance. It encapsulates a database session. Connections are created using the factory function `~psycopg2.connect()`. Connections are thread safe and can be shared among many thread. See :ref:`thread-safety` for details. .. method:: cursor([name] [, cursor_factory]) Return a new `cursor` object using the connection. If `name` is specified, the returned cursor will be a *server side* (or *named*) cursor. Otherwise the cursor will be *client side*. See :ref:`server-side-cursors` for further details. The `cursor_factory` argument can be used to create non-standard cursors. The class returned should be a subclass of `psycopg2.extensions.cursor`. See :ref:`subclassing-cursor` for details. .. extension:: The `name` and `cursor_factory` parameters are Psycopg extensions to the |DBAPI|. .. index:: pair: Transaction; Commit .. method:: commit() Commit any pending transaction to the database. Psycopg can be set to perform automatic commits at each operation, see `~connection.set_isolation_level()`. .. index:: pair: Transaction; Rollback .. method:: rollback() Roll back to the start of any pending transaction. Closing a connection without committing the changes first will cause an implicit rollback to be performed. .. method:: close() Close the connection now (rather than whenever `__del__()` is called). The connection will be unusable from this point forward; an `~psycopg2.InterfaceError` will be raised if any operation is attempted with the connection. The same applies to all cursor objects trying to use the connection. Note that closing a connection without committing the changes first will cause any pending change to be discarded as if a :sql:`ROLLBACK` was performed (unless a different isolation level has been selected: see `~connection.set_isolation_level()`). .. index:: single: PgBouncer; unclean server .. versionchanged:: 2.2 previously an explicit :sql:`ROLLBACK` was issued by Psycopg on `!close()`. The command could have been sent to the backend at an inappropriate time, so Psycopg currently relies on the backend to implicitly discard uncommitted changes. Some middleware are known to behave incorrectly though when the connection is closed during a transaction (when `~connection.status` is `~psycopg2.extensions.STATUS_IN_TRANSACTION`), e.g. PgBouncer_ reports an ``unclean server`` and discards the connection. To avoid this problem you can ensure to terminate the transaction with a `~connection.commit()`/`~connection.rollback()` before closing. .. _PgBouncer: http://pgbouncer.projects.postgresql.org/ .. index:: single: Exceptions; In the connection class .. rubric:: Exceptions as connection class attributes The `!connection` also exposes as attributes the same exceptions available in the `psycopg2` module. See :ref:`dbapi-exceptions`. .. index:: single: Two-phase commit; methods .. rubric:: Two-phase commit support methods .. versionadded:: 2.3 .. seealso:: :ref:`tpc` for an introductory explanation of these methods. Note that PostgreSQL supports two-phase commit since release 8.1: these methods raise `~psycopg2.NotSupportedError` if used with an older version server. .. _tpc_methods: .. method:: xid(format_id, gtrid, bqual) Returns a `~psycopg2.extensions.Xid` instance to be passed to the `!tpc_*()` methods of this connection. The argument types and constraints are explained in :ref:`tpc`. The values passed to the method will be available on the returned object as the members `!format_id`, `!gtrid`, `!bqual`. The object also allows accessing to these members and unpacking as a 3-items tuple. .. method:: tpc_begin(xid) Begins a TPC transaction with the given transaction ID *xid*. This method should be called outside of a transaction (i.e. nothing may have executed since the last `~connection.commit()` or `~connection.rollback()` and `connection.status` is `~psycopg2.extensions.STATUS_READY`). Furthermore, it is an error to call `!commit()` or `!rollback()` within the TPC transaction: in this case a `~psycopg2.ProgrammingError` is raised. The *xid* may be either an object returned by the `~connection.xid()` method or a plain string: the latter allows to create a transaction using the provided string as PostgreSQL transaction id. See also `~connection.tpc_recover()`. .. index:: pair: Transaction; Prepare .. method:: tpc_prepare() Performs the first phase of a transaction started with `~connection.tpc_begin()`. A `~psycopg2.ProgrammingError` is raised if this method is used outside of a TPC transaction. After calling `!tpc_prepare()`, no statements can be executed until `~connection.tpc_commit()` or `~connection.tpc_rollback()` will be called. The `~connection.reset()` method can be used to restore the status of the connection to `~psycopg2.extensions.STATUS_READY`: the transaction will remain prepared in the database and will be possible to finish it with `!tpc_commit(xid)` and `!tpc_rollback(xid)`. .. seealso:: the |PREPARE TRANSACTION|_ PostgreSQL command. .. |PREPARE TRANSACTION| replace:: :sql:`PREPARE TRANSACTION` .. _PREPARE TRANSACTION: http://www.postgresql.org/docs/9.0/static/sql-prepare-transaction.html .. index:: pair: Commit; Prepared .. method:: tpc_commit([xid]) When called with no arguments, `!tpc_commit()` commits a TPC transaction previously prepared with `~connection.tpc_prepare()`. If `!tpc_commit()` is called prior to `!tpc_prepare()`, a single phase commit is performed. A transaction manager may choose to do this if only a single resource is participating in the global transaction. When called with a transaction ID *xid*, the database commits the given transaction. If an invalid transaction ID is provided, a `~psycopg2.ProgrammingError` will be raised. This form should be called outside of a transaction, and is intended for use in recovery. On return, the TPC transaction is ended. .. seealso:: the |COMMIT PREPARED|_ PostgreSQL command. .. |COMMIT PREPARED| replace:: :sql:`COMMIT PREPARED` .. _COMMIT PREPARED: http://www.postgresql.org/docs/9.0/static/sql-commit-prepared.html .. index:: pair: Rollback; Prepared .. method:: tpc_rollback([xid]) When called with no arguments, `!tpc_rollback()` rolls back a TPC transaction. It may be called before or after `~connection.tpc_prepare()`. When called with a transaction ID *xid*, it rolls back the given transaction. If an invalid transaction ID is provided, a `~psycopg2.ProgrammingError` is raised. This form should be called outside of a transaction, and is intended for use in recovery. On return, the TPC transaction is ended. .. seealso:: the |ROLLBACK PREPARED|_ PostgreSQL command. .. |ROLLBACK PREPARED| replace:: :sql:`ROLLBACK PREPARED` .. _ROLLBACK PREPARED: http://www.postgresql.org/docs/9.0/static/sql-rollback-prepared.html .. index:: pair: Transaction; Recover .. method:: tpc_recover() Returns a list of `~psycopg2.extensions.Xid` representing pending transactions, suitable for use with `tpc_commit()` or `tpc_rollback()`. If a transaction was not initiated by Psycopg, the returned Xids will have attributes `~psycopg2.extensions.Xid.format_id` and `~psycopg2.extensions.Xid.bqual` set to `None` and the `~psycopg2.extensions.Xid.gtrid` set to the PostgreSQL transaction ID: such Xids are still usable for recovery. Psycopg uses the same algorithm of the `PostgreSQL JDBC driver`__ to encode a XA triple in a string, so transactions initiated by a program using such driver should be unpacked correctly. .. __: http://jdbc.postgresql.org/ Xids returned by `!tpc_recover()` also have extra attributes `~psycopg2.extensions.Xid.prepared`, `~psycopg2.extensions.Xid.owner`, `~psycopg2.extensions.Xid.database` populated with the values read from the server. .. seealso:: the |pg_prepared_xacts|_ system view. .. |pg_prepared_xacts| replace:: `pg_prepared_xacts` .. _pg_prepared_xacts: http://www.postgresql.org/docs/9.0/static/view-pg-prepared-xacts.html .. extension:: The above methods are the only ones defined by the |DBAPI| protocol. The Psycopg connection objects exports the following additional methods and attributes. .. attribute:: closed Read-only attribute reporting whether the database connection is open (0) or closed (1). .. method:: cancel Cancel the current database operation. The method interrupts the processing of the current operation. If no query is being executed, it does nothing. You can call this function from a different thread than the one currently executing a database operation, for instance if you want to cancel a long running query if a button is pushed in the UI. Interrupting query execution will cause the cancelled method to raise a `~psycopg2.extensions.QueryCanceledError`. Note that the termination of the query is not guaranteed to succeed: see the documentation for |PQcancel|_. .. |PQcancel| replace:: `!PQcancel()` .. _PQcancel: http://www.postgresql.org/docs/8.4/static/libpq-cancel.html#AEN34765 .. versionadded:: 2.3 .. method:: reset Reset the connection to the default. The method rolls back an eventual pending transaction and executes the PostgreSQL |RESET|_ and |SET SESSION AUTHORIZATION|__ to revert the session to the default values. A two-phase commit transaction prepared using `~connection.tpc_prepare()` will remain in the database available for recover. .. |RESET| replace:: :sql:`RESET` .. _RESET: http://www.postgresql.org/docs/9.0/static/sql-reset.html .. |SET SESSION AUTHORIZATION| replace:: :sql:`SET SESSION AUTHORIZATION` .. __: http://www.postgresql.org/docs/9.0/static/sql-set-session-authorization.html .. versionadded:: 2.0.12 .. attribute:: dsn Read-only string containing the connection string used by the connection. .. index:: pair: Transaction; Autocommit pair: Transaction; Isolation level .. _autocommit: .. attribute:: isolation_level .. method:: set_isolation_level(level) Read or set the `transaction isolation level`_ for the current session. The level defines the different phenomena that can happen in the database between concurrent transactions. The value set or read is an integer: symbolic constants are defined in the module `psycopg2.extensions`: see :ref:`isolation-level-constants` for the available values. The default level is :sql:`READ COMMITTED`: at this level a transaction is automatically started the first time a database command is executed. If you want an *autocommit* mode, switch to `~psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT` before executing any command:: >>> conn.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT) See also :ref:`transactions-control`. .. index:: pair: Client; Encoding .. attribute:: encoding .. method:: set_client_encoding(enc) Read or set the client encoding for the current session. The default is the encoding defined by the database. It should be one of the `characters set supported by PostgreSQL`__ .. __: http://www.postgresql.org/docs/9.0/static/multibyte.html .. index:: pair: Client; Logging .. attribute:: notices A list containing all the database messages sent to the client during the session. .. doctest:: :options: NORMALIZE_WHITESPACE >>> cur.execute("CREATE TABLE foo (id serial PRIMARY KEY);") >>> pprint(conn.notices) ['NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "foo_pkey" for table "foo"\n', 'NOTICE: CREATE TABLE will create implicit sequence "foo_id_seq" for serial column "foo.id"\n'] To avoid a leak in case excessive notices are generated, only the last 50 messages are kept. You can configure what messages to receive using `PostgreSQL logging configuration parameters`__ such as ``log_statement``, ``client_min_messages``, ``log_min_duration_statement`` etc. .. __: http://www.postgresql.org/docs/9.0/static/runtime-config-logging.html .. attribute:: notifies List of `~psycopg2.extensions.Notify` objects containing asynchronous notifications received by the session. For other details see :ref:`async-notify`. .. versionchanged:: 2.3 Notifications are instances of the `!Notify` object. Previously the list was composed by 2 items tuples :samp:`({pid},{channel})` and the payload was not accessible. To keep backward compatibility, `!Notify` objects can still be accessed as 2 items tuples. .. index:: pair: Backend; PID .. method:: get_backend_pid() Returns the process ID (PID) of the backend server process handling this connection. Note that the PID belongs to a process executing on the database server host, not the local host! .. seealso:: libpq docs for `PQbackendPID()`__ for details. .. __: http://www.postgresql.org/docs/9.0/static/libpq-status.html#LIBPQ-PQBACKENDPID .. versionadded:: 2.0.8 .. index:: pair: Server; Parameters .. method:: get_parameter_status(parameter) Look up a current parameter setting of the server. Potential values for ``parameter`` are: ``server_version``, ``server_encoding``, ``client_encoding``, ``is_superuser``, ``session_authorization``, ``DateStyle``, ``TimeZone``, ``integer_datetimes``, and ``standard_conforming_strings``. If server did not report requested parameter, return ``None``. .. seealso:: libpq docs for `PQparameterStatus()`__ for details. .. __: http://www.postgresql.org/docs/9.0/static/libpq-status.html#LIBPQ-PQPARAMETERSTATUS .. versionadded:: 2.0.12 .. index:: pair: Transaction; Status .. method:: get_transaction_status() Return the current session transaction status as an integer. Symbolic constants for the values are defined in the module `psycopg2.extensions`: see :ref:`transaction-status-constants` for the available values. .. seealso:: libpq docs for `PQtransactionStatus()`__ for details. .. __: http://www.postgresql.org/docs/9.0/static/libpq-status.html#LIBPQ-PQTRANSACTIONSTATUS .. index:: pair: Protocol; Version .. attribute:: protocol_version A read-only integer representing frontend/backend protocol being used. Currently Psycopg supports only protocol 3, which allows connection to PostgreSQL server from version 7.4. Psycopg versions previous than 2.3 support both protocols 2 and 3. .. seealso:: libpq docs for `PQprotocolVersion()`__ for details. .. __: http://www.postgresql.org/docs/9.0/static/libpq-status.html#LIBPQ-PQPROTOCOLVERSION .. versionadded:: 2.0.12 .. index:: pair: Server; Version .. attribute:: server_version A read-only integer representing the backend version. The number is formed by converting the major, minor, and revision numbers into two-decimal-digit numbers and appending them together. For example, version 8.1.5 will be returned as ``80105``. .. seealso:: libpq docs for `PQserverVersion()`__ for details. .. __: http://www.postgresql.org/docs/9.0/static/libpq-status.html#LIBPQ-PQSERVERVERSION .. versionadded:: 2.0.12 .. index:: pair: Connection; Status .. attribute:: status A read-only integer representing the status of the connection. Symbolic constants for the values are defined in the module `psycopg2.extensions`: see :ref:`connection-status-constants` for the available values. .. method:: lobject([oid [, mode [, new_oid [, new_file [, lobject_factory]]]]]) Return a new database large object. See :ref:`large-objects` for an overview. :param oid: The OID of the object to read or write. 0 to create a new large object and and have its OID assigned automatically. :param mode: Access mode to the object: can be ``r``, ``w``, ``rw`` or ``n`` (meaning don't open it). :param new_oid: Create a new object using the specified OID. The function raises `OperationalError` if the OID is already in use. Default is 0, meaning assign a new one automatically. :param new_file: The name of a file to be imported in the the database (using the |lo_import|_ function) :param lobject_factory: Subclass of `~psycopg2.extensions.lobject` to be instantiated. :rtype: `~psycopg2.extensions.lobject` .. |lo_import| replace:: `!lo_import()` .. _lo_import: http://www.postgresql.org/docs/9.0/static/lo-interfaces.html#LO-IMPORT .. versionadded:: 2.0.8 .. rubric:: Methods related to asynchronous support. .. versionadded:: 2.2.0 .. seealso:: :ref:`async-support` and :ref:`green-support`. .. attribute:: async Read only attribute: 1 if the connection is asynchronous, 0 otherwise. .. method:: poll() Used during an asynchronous connection attempt, or when a cursor is executing a query on an asynchronous connection, make communication proceed if it wouldn't block. Return one of the constants defined in :ref:`poll-constants`. If it returns `~psycopg2.extensions.POLL_OK` then the connection has been estabilished or the query results are available on the client. Otherwise wait until the file descriptor returned by `fileno()` is ready to read or to write, as explained in :ref:`async-support`. `poll()` should be also used by the function installed by `~psycopg2.extensions.set_wait_callback()` as explained in :ref:`green-support`. `poll()` is also used to receive asynchronous notifications from the database: see :ref:`async-notify` from further details. .. method:: fileno() Return the file descriptor underlying the connection: useful to read its status during asynchronous communication. .. method:: isexecuting() Return `True` if the connection is executing an asynchronous operation. .. testcode:: :hide: conn.rollback()