PYTHON - DATABASE ACCESSThe Python standard for database interfaces is the Python DB-API. Most Python database interfaces adhere to this standard. You can choose the right database for your application. Python Database API supports a wide range of database servers such as:
The DB API provides a minimal standard for working with databases using Python structures and syntax wherever possible. This API includes the following:
What is MySQLdb? MySQLdb is an interface for connecting to a MySQL database server from Python. It implements the Python Database API v2.0 and is built on top of the MySQL C API. How do I Install MySQLdb? Before proceeding, you make sure you have MySQLdb installed on your machine. Just type the following in your Python script and execute it:
#!/usr/bin/python
import MySQLdb If it produces the following result, then it means MySQLdb module is not installed:
Traceback (most recent call last):
File "test.py", line 3, in <module> import MySQLdb ImportError: No module named MySQLdb To install MySQLdb module, download it from MySQLdb Download page and proceed as follows:
$ gunzip MySQL-python-1.2.2.tar.gz
$ tar -xvf MySQL-python-1.2.2.tar $ cd MySQL-python-1.2.2 $ python setup.py build $ python setup.py install Note: Make sure you have root privilege to install above module. Database Connection Before connecting to a MySQL database, make sure of the followings:
Following is the example of connecting with MySQL database "TESTDB"
#!/usr/bin/python
import MySQLdb # Open database connection db = MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # execute SQL query using execute() method. cursor.execute("SELECT VERSION()") # Fetch a single row using fetchone() method. data = cursor.fetchone() print "Database version : %s " % data # disconnect from server db.close() While running this script, it is producing the following result in my Linux machine.
Database version : 5.0.45
If a connection is established with the datasource, then a Connection Object is returned and saved into db for further use, otherwise db is set to None. Next, db object is used to create a cursor object, which in turn is used to execute SQL queries. Finally, before coming out, it ensures that database connection is closed and resources are released. Creating Database Table Once a database connection is established, we are ready to create tables or records into the database tables using execute method of the created cursor. Example Let us create Database table EMPLOYEE:
#!/usr/bin/python
import MySQLdb # Open database connection db = MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Drop table if it already exist using execute() method. cursor.execute("DROP TABLE IF EXISTS EMPLOYEE") # Create table as per requirement sql = """CREATE TABLE EMPLOYEE ( FIRST_NAME CHAR(20) NOT NULL, LAST_NAME CHAR(20), AGE INT, SEX CHAR(1), INCOME FLOAT )""" cursor.execute(sql) # disconnect from server db.close() INSERT Operation It is required when you want to create your records into a database table. Example The following example executes SQL INSERT statement to create a record into EMPLOYEE table:
#!/usr/bin/python
import MySQLdb # Open database connection db = MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to INSERT a record into the database. sql = """INSERT INTO EMPLOYEE(FIRST_NAME, LAST_NAME, AGE, SEX, INCOME) VALUES ('Mac', 'Mohan', 20, 'M', 2000)""" try: # Execute the SQL command cursor.execute(sql) # Commit your changes in the database db.commit() except: # Rollback in case there is any error db.rollback() # disconnect from server db.close() Above example can be written as follows to create SQL queries dynamically:
#!/usr/bin/python
import MySQLdb # Open database connection db = MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to INSERT a record into the database. sql = "INSERT INTO EMPLOYEE(FIRST_NAME, \ LAST_NAME, AGE, SEX, INCOME) \ VALUES ('%s', '%s', '%d', '%c', '%d' )" % \ ('Mac', 'Mohan', 20, 'M', 2000) try: # Execute the SQL command cursor.execute(sql) # Commit your changes in the database db.commit() except: # Rollback in case there is any error db.rollback() # disconnect from server db.close() Example Following code segment is another form of execution where you can pass parameters directly:
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user_id = "test123" password = "password" con.execute('insert into Login values("%s", "%s")' % \ (user_id, password)) .................................. READ Operation READ Operation on any database means to fetch some useful information from the database. Once our database connection is established, you are ready to make a query into this database. You can use either fetchone() method to fetch single record or fetchall() method to fetch multiple values from a database table.
The following procedure querries all the records from EMPLOYEE table having salary more than 1000:
#!/usr/bin/python
import MySQLdb # Open database connection db = MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to INSERT a record into the database. sql = "SELECT * FROM EMPLOYEE \ WHERE INCOME > '%d'" % (1000) try: # Execute the SQL command cursor.execute(sql) # Fetch all the rows in a list of lists. results = cursor.fetchall() for row in results: fname = row[0] lname = row[1] age = row[2] sex = row[3] income = row[4] # Now print fetched result print "fname=%s,lname=%s,age=%d,sex=%s,income=%d" % \ (fname, lname, age, sex, income ) except: print "Error: unable to fecth data" # disconnect from server db.close() This will produce the following result:
fname=Mac, lname=Mohan, age=20, sex=M, income=2000
Update Operation UPDATE Operation on any database means to update one or more records, which are already available in the database. The following procedure updates all the records having SEX as 'M'. Here, we increase AGE of all the males by one year. Example
#!/usr/bin/python
import MySQLdb # Open database connection db = MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to UPDATE required records sql = "UPDATE EMPLOYEE SET AGE = AGE + 1 WHERE SEX = '%c'" % ('M') try: # Execute the SQL command cursor.execute(sql) # Commit your changes in the database db.commit() except: # Rollback in case there is any error db.rollback() # disconnect from server db.close() DELETE Operation DELETE operation is required when you want to delete some records from your database. Following is the procedure to delete all the records from EMPLOYEE where AGE is more than 20: Example
#!/usr/bin/python
import MySQLdb # Open database connection db = MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to DELETE required records sql = "DELETE FROM EMPLOYEE WHERE AGE > '%d'" % (20) try: # Execute the SQL command cursor.execute(sql) # Commit your changes in the database db.commit() except: # Rollback in case there is any error db.rollback() # disconnect from server db.close() Performing Transactions Transactions are a mechanism that ensures data consistency. Transactions have the following four properties: 1. Atomicity: Either a transaction completes or nothing happens at all. 2. Consistency: A transaction must start in a consistent state and leave the system in a consistent state. 3. Isolation: Intermediate results of a transaction are not visible outside the current transaction. 4. Durability: Once a transaction was committed, the effects are persistent, even after a system failure. The Python DB API 2.0 provides two methods to either commit or rollback a transaction. Example You already know how to implement transactions. Here is again similar example:
# Prepare SQL query to DELETE required records
sql = "DELETE FROM EMPLOYEE WHERE AGE > '%d'" % (20) try # Execute the SQL command cursor.execute(sql) # Commit your changes in the database db.commit() except: # Rollback in case there is any error db.rollback() COMMIT Operation Commit is the operation, which gives a green signal to database to finalize the changes, and after this operation, no change can be reverted back. Here is a simple example to call commit method.
db.commit()
ROLLBACK Operation If you are not satisfied with one or more of the changes and you want to revert back those changes completely, then use rollback() method. Here is a simple example to call rollback() method.
db.rollback()
Disconnecting Database To disconnect Database connection, use close() method.
db.close()
If the connection to a database is closed by the user with the close() method, any outstanding transactions are rolled back by the DB. However, instead of depending on any of DB lower level implementation details, your application would be better off calling commit or rollback explicitly. Handling Errors There are many sources of errors. A few examples are a syntax error in an executed SQL statement, a connection failure, or calling the fetch method for an already canceled or finished statement handle. The DB API defines a number of errors that must exist in each database module. The following table lists these exceptions.
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