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Hur gör jag databastransaktioner med psycopg2/python db api?

Använd db.set_isolation_level(n) , med antagande av db är ditt anslutningsobjekt. Som Federico skrev här, innebörden av n är:

0 -> autocommit
1 -> read committed
2 -> serialized (but not officially supported by pg)
3 -> serialized

Som dokumenterats här, psycopg2.extensions ger dig symboliska konstanter för syftet:

Setting transaction isolation levels
====================================

psycopg2 connection objects hold informations about the PostgreSQL `transaction
isolation level`_.  The current transaction level can be read from the
`.isolation_level` attribute.  The default isolation level is ``READ
COMMITTED``.  A different isolation level con be set through the
`.set_isolation_level()` method.  The level can be set to one of the following
constants, defined in `psycopg2.extensions`:

`ISOLATION_LEVEL_AUTOCOMMIT`
    No transaction is started when command are issued and no
    `.commit()`/`.rollback()` is required.  Some PostgreSQL command such as
    ``CREATE DATABASE`` can't run into a transaction: to run such command use
    `.set_isolation_level(ISOLATION_LEVEL_AUTOCOMMIT)`.

`ISOLATION_LEVEL_READ_COMMITTED`
    This is the default value.  A new transaction is started at the first
    `.execute()` command on a cursor and at each new `.execute()` after a
    `.commit()` or a `.rollback()`.  The transaction runs in the PostgreSQL
    ``READ COMMITTED`` isolation level.

`ISOLATION_LEVEL_SERIALIZABLE`
    Transactions are run at a ``SERIALIZABLE`` isolation level.


.. _transaction isolation level: 
   http://www.postgresql.org/docs/8.1/static/transaction-iso.html


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