Writing MySQL Scripts with Python DB-API

Paul DuBois
paul@kitebird.com

Document revision: 1.02
Last update: 2006-09-17

Table of Contents


Python is one of the more popular Open Source programming languages, owing largely to its own native expressiveness as well as to the variety of support modules that are available to extend its capabilities. One of these modules is DB-API, which, as the name implies, provides a database application programming interface. DB-API is designed to be relatively independent of details specific to any given database engine, to help you write database-access scripts that are portable between engines.

DB-API's design is similar to that used by Perl and Ruby DBI modules, the PHP PEAR DB class, and the Java JDBC interface: It uses a two-level architecture in which the top level provides an abstract interface that is similar for all supported database engines, and a lower level consisting of drivers for specific engines that handle engine-dependent details. This means, of course, that to use DB-API for writing Python scripts, you must have a driver for your particular database system. For MySQL, DB-API provides database access by means of the MySQLdb driver. This document begins by discussing driver installation (in case you don't have MySQLdb), then moves on to cover how to write DB-API scripts.

MySQLdb Installation


To write MySQL scripts that use DB-API, Python itself must be installed. That will almost certainly be true if you're using Unix, but is less likely for Windows. Installers for either platform can be found on the Python web site (see the "Resources" section at the end of this document).

Verify that your version of Python is 2.3.4 or later, and that the MySQLdb module is installed. You can check both of these requirements by running Python in interactive mode from the command line prompt (something like % for Unix or C:\> for Windows):

   % python
   Python 2.4.3 (#1, Aug 29 2006, 14:45:33)
   [GCC 3.4.6 (Gentoo 3.4.6-r1, ssp-3.4.5-1.0, pie-8.7.9)] on linux2
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import MySQLdb
Assuming that you have a recent enough version of Python and that no error occurs when you issue the import MySQLdb statement, you're ready to begin writing database-access scripts and you can skip to the next section. However, if you get the following error, you need to obtain and install MySQLdb first:
   >>> import MySQLdb
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   ImportError: No module named MySQLdb
To obtain MySQLdb, visit the "Resources" section to see where to fetch a distribution appropriate for your system. Precompiled binaries may available for your platform, or you can install from source. If you use a binary distribution, install it using your platform's usual package installation procedure. To build and install from source, move into the top-level directory of the MySQLdb distribution and issue the following commands. (Under Unix, it's likely that you'll need to run the second command as root so that the driver files can be copied into your Python installation.)
   % python setup.py build
   % python setup.py install
If you encounter problems, check the README file included with the MySQLdb distribution.

A Short DB-API Script


Scripts that access MySQL through DB-API using MySQLdb generally perform the following steps:

The rest of this section presents a short DB-API script that illustrates the basic elements of these steps. Later sections discuss specific aspects of script-writing in more detail.

Writing the Script


Use a text editor to create a file named server_version.py that contains the following script. This script uses MySQLdb to interact with the MySQL server, albeit in relatively rudimentary fashion--all it does is ask the server for its version string:

   # server_version.py - retrieve and display database server version

   import MySQLdb

   conn = MySQLdb.connect (host = "localhost",
                           user = "testuser",
                           passwd = "testpass",
                           db = "test")
   cursor = conn.cursor ()
   cursor.execute ("SELECT VERSION()")
   row = cursor.fetchone ()
   print "server version:", row[0]
   cursor.close ()
   conn.close ()
The import statement tells Python that the script needs to use the code in the MySQLdb module. This statement must precede any attempt to connect to the MySQL server. Then the connection is established by invoking the connect() method of the MySQLdb driver with the proper connection parameters. These include the hostname where the server is running, the username and password for your MySQL account, and the name of the database that you want to use. connect() argument list syntax varies among drivers; for MySQLdb, the arguments are allowed to be given in name = value format, which has the advantage that you can specify them in any order. server_version.py makes a connection to the MySQL server on the local host to access the test database with a username and password of testuser and testpass:
   conn = MySQLdb.connect (host = "localhost",
                           user = "testuser",
                           passwd = "testpass",
                           db = "test")
If the connect() call succeeds, it returns a connection object that serves as the basis for further interaction with MySQL. If the call fails, it raises an exception. (server_version.py doesn't handle the exception, so an error at this point terminates the script. Error handling is covered later in this document.)

After the connection object has been obtained, server_version.py invokes its cursor() method to create a cursor object for processing statements. The script uses this cursor to issue a SELECT VERSION() statement, which returns a string containing server version information:

   cursor = conn.cursor ()
   cursor.execute ("SELECT VERSION()")
   row = cursor.fetchone ()
   print "server version:", row[0]
   cursor.close ()
The cursor object's execute() method sends the statement to the server and fetchone() retrieves a row as a tuple. For the statement shown here, the tuple contains a single value, which the script prints. (If no row is available, fetchone() actually will return the value None; server_version.py blithely assumes that this won't happen, an assumption that you normally should not make. In later examples, we'll handle this case.) Cursor objects can be used to issue multiple statements, but server_version.py has no more need for cursor after getting the version string, so it closes it.

Finally, the script invokes the connection object's close() method to disconnect from the server:

   conn.close ()
After that, conn becomes invalid and should not be used to access the server.

Running the Script


To execute the server_version.py script, invoke Python from the command line prompt and tell it the script name. You should see a result something like this:

   % python server_version.py
   server version: 5.1.12-beta-log
This indicates that the MySQL server version is 5.1.12; the -beta and -log suffixes tell us the distribution stability level and that query logging is enabled. (You might see other suffixes than those shown here. For example, if you have debugging enabled, you'll see a -debug suffix.)

It's possible to set up the script so that it can be run by name without invoking Python explicitly. Under Unix, add an initial #! line to the script that specifies the full pathname of the Python interpreter. This tells the system what program should execute the script. For example, if Python lives at /usr/bin/python on your system, add the following as the first line of the script:

   #!/usr/bin/python
Then use chmod to make the script executable, and you'll be able to run it directly:
   % chmod +x server_version.py
   % ./server_version.py
(The leading "./" tells your command interpreter explicitly that the script is located in your current directory. Many Unix accounts are set up not to search the current directory when looking for commands.)

Under Windows, the #! line is unnecessary (although it's harmless, so you need not remove it if you write the script on a Unix system and then move it to a Windows box). Instead, you can set up a filename association so that .py scripts will be associated with Python. Instead of using chmod to make the script executable, open the Folder Options item in the Control Panel and select its File Types tab. File Types enables you to set up an association for files that end with .py to tell Windows to execute them with Python. Then you can invoke the script by name:

   C:\> server_version.py
If you install ActiveState Python on Windows, the ActiveState installer sets up the association automatically as part of the installation process.

A More Extensive DB-API Script


server_version.py has a number of shortcomings. For example, it doesn't catch exceptions or indicate what went wrong if an error occurs, and it doesn't allow for the possibility that the statement it runs might not return any results. This section shows how to address these issues using a more elaborate script, animal.py, that uses a table containing animal names and categories:

   CREATE TABLE animal
   (
     name     CHAR(40),
     category CHAR(40)
   )
If you've read the PEAR DB document available at the Kitebird site (see "Resources"), you might recognize this table and some of the statements issued by animal.py; they were used in that document, too.

The animal.py script begins like this (including the #! line, should you intend to run the script on a Unix system):

   #!/usr/bin/python
   # animal.py - create animal table and
   # retrieve information from it

   import sys
   import MySQLdb
As with server_version.py, the script imports MySQLdb, but it also imports the sys module for use in error handling. (animal.py uses sys.exit() to return 1 to indicate abnormal termination if an error occurs.)

Error Handling


After importing the requisite modules, animal.py establishes a connection to the server using the connect() call. To allow for the possibility of connection failure (for example, so that you can display the reason for the failure), it's necessary to catch exceptions. To handle exceptions in Python, put your code in a try statement and include an except clause that contains the error-handling code. The resulting connection sequence looks like this:

   try:
     conn = MySQLdb.connect (host = "localhost",
                             user = "testuser",
                             passwd = "testpass",
                             db = "test")
   except MySQLdb.Error, e:
     print "Error %d: %s" % (e.args[0], e.args[1])
     sys.exit (1)
The except clause names an exception class (MySQLdb.Error in this example) to obtain the database-specific error information that MySQLdb can provide, as well as a variable (e) in which to store the information. If an exception occurs, MySQLdb makes this information available in e.args, a two-element tuple containing the numeric error code and a string describing the error. The except clause shown in the example prints both values and exits.

Any database-related statements can be placed in a similar try/except structure to trap and report errors; for brevity, the following discussion doesn't show the exception-handling code. (The complete text of animal.py is listed in the Appendix.)

Methods for Issuing Statements


The next section of animal.py creates a cursor object and uses it to issue statements that set up and populate the animal table:

   cursor = conn.cursor ()
   cursor.execute ("DROP TABLE IF EXISTS animal")
   cursor.execute ("""
       CREATE TABLE animal
       (
         name     CHAR(40),
         category CHAR(40)
       )
     """)
   cursor.execute ("""
       INSERT INTO animal (name, category)
       VALUES
         ('snake', 'reptile'),
         ('frog', 'amphibian'),
         ('tuna', 'fish'),
         ('racoon', 'mammal')
     """)
   print "Number of rows inserted: %d" % cursor.rowcount
Note that this code includes no error checking. (Remember that it will be placed in a try statement; errors will trigger exceptions that are caught and handled in the corresponding except clause, which allows the main flow of the code to read more smoothly.) The statements perform the following actions:
Each statement is issued by invoking the cursor object's execute() method. The first two statements produce no result, but the third produces a count indicating the number of rows inserted. The count is available in the cursor's rowcount attribute. (Some database interfaces provide this count as the return value of the statement-execution call, but that is not true for DB-API.)

The animal table is set up at this point, so we can issue SELECT statements to retrieve information from it. As with the preceding statements, SELECT statements are issued using execute(). However, unlike statements such as DROP or INSERT, SELECT statements generate a result set that you must retrieve. That is, execute() only issues the statement, it does not return the result set. You can use fetchone() to get the rows one at a time, or fetchall() to get them all at once. animal.py uses both approaches. Here's how to use fetchone() for row-at-a-time retrieval:

   cursor.execute ("SELECT name, category FROM animal")
   while (1):
     row = cursor.fetchone ()
     if row == None:
       break
     print "%s, %s" % (row[0], row[1])
   print "Number of rows returned: %d" % cursor.rowcount
fetchone() returns the next row of the result set as a tuple, or the value None if no more rows are available. The loop checks for this and exits when the result set has been exhausted. For each row returned, the tuple contains two values (that's how many columns the SELECT statement asked for), which animal.py prints. The print statement shown above accesses the individual tuple elements. However, because they are used in order of occurrence within the tuple, the print statement could just as well have been written like this:
   print "%s, %s" % row
After displaying the statement result, the script also prints the number of rows returned (available as the value of the rowcount attribute).

fetchall() returns the entire result set all at once as a tuple of tuples, or as an empty tuple if the result set is empty. To access the individual row tuples, iterate through the row set that fetchall() returns:

   cursor.execute ("SELECT name, category FROM animal")
   rows = cursor.fetchall ()
   for row in rows:
     print "%s, %s" % (row[0], row[1])
   print "Number of rows returned: %d" % cursor.rowcount
This code prints the row count by accessing rowcount, just as for the fetchone() loop. Another way to determine the row count when you use fetchall() is by taking the length of the value that it returns:
   print "%d rows were returned" % len (rows)
The fetch loops shown thus far retrieve rows as tuples. It's also possible to fetch rows as dictionaries, which enables you to access column values by name. The following code shows how to do this. Note that dictionary access requires a different kind of cursor, so the example closes the cursor and obtains a new one that uses a different cursor class:
   cursor.close ()
   cursor = conn.cursor (MySQLdb.cursors.DictCursor)
   cursor.execute ("SELECT name, category FROM animal")
   result_set = cursor.fetchall ()
   for row in result_set:
     print "%s, %s" % (row["name"], row["category"])
   print "Number of rows returned: %d" % cursor.rowcount
NULL values in a result set are returned as None to your program.

MySQLdb supports a placeholder capability that enables you to bind data values to special markers within the statement string. This provides an alternative to embedding the values directly into the statement. The placeholder mechanism handles adding quotes around data values, and it escapes any special characters that occur within values. The following examples demonstrate an UPDATE statement that changes snake to turtle, first using literal values and then using placeholders. The literal-value statement looks like this:

   cursor.execute ("""
         UPDATE animal SET name = 'turtle'
         WHERE name = 'snake'
       """)
   print "Number of rows updated: %d" % cursor.rowcount
Alternatively, you can issue the same statement by using %s placeholder markers and binding the appropriate values to them:
   cursor.execute ("""
         UPDATE animal SET name = %s
         WHERE name = %s
       """, ("snake", "turtle"))
   print "Number of rows updated: %d" % cursor.rowcount
Note the following points about the form of the preceding execute() call:
After issuing the statements, animal.py closes the cursor, commits the changes, and disconnects from the server:
   cursor.close ()
   conn.commit ()
   conn.close ()
The connection object commit() method commits any outstanding changes in the current transaction to make them permanent in the database. In DB-API, connections begin with autocommit mode disabled, so you must call commit() before disconnecting or changes may be lost.

If the animal table is a MyISAM table, commit() has no effect: MyISAM is a non-transactional storage engine, so changes to MyISAM tables take effect immediately regardless of the autocommit mode. However, if animal uses a transactional storage engine such as InnoDB, failure to invoke commit() results in an implicit transaction rollback when you disconnect. For example, if you add ENGINE=InnoDB to the end of the CREATE TABLE statement and remove the commit() invocation near the end of the script, you'll find that animal is empty after the script runs.

For scripts that only retrieve data, no changes need to be committed and commit() is unnecessary.

Portability Notes


If you want to port a MySQLdb-based DB-API script for use with a different database, sources of non-portability occur anywhere that the driver name might be used:

Another type of non-portability that does not involve the driver name concerns the use of placeholders. The DB-API specification allows for several placeholder syntaxes, and some drivers use a syntax that differs from the one supported by MySQLdb.

Resources


The scripts that are used for examples in this document can be downloaded from the following location:

   http://www.kitebird.com/articles/

You may find the following additional resources helpful for using Python DB-API and the MySQLdb driver:

Appendix


The full source code for the animal.py script is shown here:

   #!/usr/bin/python
   # animal.py - create animal table and
   # retrieve information from it

   import sys
   import MySQLdb

   # connect to the MySQL server

   try:
     conn = MySQLdb.connect (host = "localhost",
                             user = "testuser",
                             passwd = "testpass",
                             db = "test")
   except MySQLdb.Error, e:
     print "Error %d: %s" % (e.args[0], e.args[1])
     sys.exit (1)

   # create the animal table and populate it

   try:
     cursor = conn.cursor ()
     cursor.execute ("DROP TABLE IF EXISTS animal")
     cursor.execute ("""
         CREATE TABLE animal
         (
           name     CHAR(40),
           category CHAR(40)
         )
       """)
     cursor.execute ("""
         INSERT INTO animal (name, category)
         VALUES
           ('snake', 'reptile'),
           ('frog', 'amphibian'),
           ('tuna', 'fish'),
           ('racoon', 'mammal')
       """)
     print "Number of rows inserted: %d" % cursor.rowcount

   # perform a fetch loop using fetchone()

     cursor.execute ("SELECT name, category FROM animal")
     while (1):
       row = cursor.fetchone ()
       if row == None:
         break
       print "%s, %s" % (row[0], row[1])
     print "Number of rows returned: %d" % cursor.rowcount

   # perform a fetch loop using fetchall()

     cursor.execute ("SELECT name, category FROM animal")
     rows = cursor.fetchall ()
     for row in rows:
       print "%s, %s" % (row[0], row[1])
     print "Number of rows returned: %d" % cursor.rowcount

   # issue a statement that changes the name by including data values
   # literally in the statement string, then change the name back
   # by using placeholders

     cursor.execute ("""
           UPDATE animal SET name = 'turtle'
           WHERE name = 'snake'
         """)
     print "Number of rows updated: %d" % cursor.rowcount

     cursor.execute ("""
           UPDATE animal SET name = %s
           WHERE name = %s
         """, ("snake", "turtle"))
     print "Number of rows updated: %d" % cursor.rowcount

   # create a dictionary cursor so that column values
   # can be accessed by name rather than by position

     cursor.close ()
     cursor = conn.cursor (MySQLdb.cursors.DictCursor)
     cursor.execute ("SELECT name, category FROM animal")
     result_set = cursor.fetchall ()
     for row in result_set:
       print "%s, %s" % (row["name"], row["category"])
     print "Number of rows returned: %d" % cursor.rowcount

     cursor.close ()

   except MySQLdb.Error, e:
     print "Error %d: %s" % (e.args[0], e.args[1])
     sys.exit (1)

   conn.commit ()
   conn.close ()

Acknowledgment


The original version of this document was written for NuSphere Corporation. The current version is an updated revision of the original.

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