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Accessing PostgreSQL with Python and Psycopg2

The combination of Python and PostgreSQL is potent, particularly when you use the Psycopg2 driver.


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s an open source database, PostgreSQL has one of the largest libraries of Application Programmable Interfaces (APIs) available, letting you access it easily from various languages. One such language is Python, which happens to be one of my favorite languages; I use it for almost all my hacking. To be honest, I'm not that great a programmer. I'm a database administrator and operating system consultant by trade. But Python ensures that the code that I do write is readable by other, more talented programmers—even six months after I stopped working on it.

Nine times out of 10, I use Python to communicate with a PostgreSQL database. My driver of choice when doing so is called Psycopg2. Like the first release (Psycopg), it's designed to be lightweight and fast. Recently Psycopg2 has been under heavy development and is currently in Beta 4, which is supposed to be the last beta before the official release.

This article discusses how to connect to PostgreSQL with Psycopg2 and illustrates some of the nice features that the driver offers. For the example code I used Psycopg2, Python2.4, and PostgreSQL 8.1dev.

I do not advocate running PostgreSQL 8.1dev in production, it is currently in alpha. The current version of PostgreSQL is 8.0.3, but 8.0.4 should be current by the time you read this article.

Psycopg2 is a DB API 2.0-compliant PostgreSQL driver that is under active development. It is designed for multi-threaded applications and manages its own connection pool. One other interesting feature of the driver is that if you use the PostgreSQL array data type, Psycopg2 will automatically convert a result containing that data type to a Python list.

The following discusses specific uses of Psycopg2. It does not try to implement a lot of object-oriented code, but instead tries to provide clear and concise syntactical examples of using the driver with PostgreSQL.

Making the Initial Connection

#!/usr/bin/python2.4 # # Small script to show PostgreSQL and Psycopg2 together # import psycopg2 try: conn = psycopg2.connect("dbname='template1' user='dbuser' host='localhost' password='dbpass'"); except: print "I am unable to connect to the database"

The above code imports the adapter and tries to connect to the database. If the connection fails, the code prints a statement to STDOUT. You could also use the exception to try the connection again with different parameters if you like.

The next step is to define a cursor to work with. It is important to note that Python/Psycopg cursors are not cursors as defined by PostgreSQL. They are completely different beasts.

cur = conn.cursor()

Executing a Query
After making the connection and defining a cursor, you can execute a query. For example:



cur.execute("""SELECT datname from pg_database""")

After executing your query you will need to assign the results to a variable. The results will be in the form of a Python list.

rows = cur.fetchall()

At this point, the query results are stored in the variable named rows. Using this variable, you can start processing the results. For example, to print all the data selected to the screen, you could write:

print "\nShow me the databases:\n" for row in rows: print " ", row[0]

Psychopg2 and Isolation Level
Everything you've seen so far would work with any database that Python can access. Now, here are some finer points. PostgreSQL does not have an autocommit facility, which means that all queries execute within a transaction.

Execution within a transaction is a very good thing; it ensures data integrity and allows for appropriate error handling. However there are queries that can not be run from within a transaction. Consider the following example:

#/usr/bin/python2.4 # # import psycopg2 import sys # Try to connect try: conn=psycopg2.connect("dbname='template1' user='dbuser' password='mypass'") except: print "I am unable to connect to the database, exiting." sys.exit() cur = conn.cursor() try: cur.execute("""DROP DATABASE foo_test""") except: print "I can't drop our test database, check your isolation level." sys.ext()

The preceding code would fail with the printed message of "I can't drop our test database, check your isolation level" That's because PostgreSQL cannot drop databases within a transaction; dropping a database is an all-or-nothing command. To drop the database you would need to change the isolation level of the database, which you can do using the following command:

conn.set_isolation_level(0)

You would place the conn.set_isolation_level command immediately above the DROP DATABASE cursor execution command in the example.



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