The 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 −
- GadFly
- mSQL
- MySQL
- PostgreSQL
- Microsoft SQL Server 2000
- Informix
- Interbase
- Oracle
- Sybase
Here is the list of available Python database interfaces: Python Database Interfaces and APIs
.You must download a separate DB API module for each database you need
to access. For example, if you need to access an Oracle database as well
as a MySQL database, you must download both the Oracle and the MySQL
database modules.
The DB API provides a minimal standard for working with databases
using Python structures and syntax wherever possible. This API includes
the following:
- Importing the API module.
- Acquiring a connection with the database.
- Issuing SQL statements and stored procedures.
- Closing the connection
We would learn all the concepts using MySQL, so let us talk about MySQLdb module.
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, use the following command:
For Ubuntu, use the following command -
$ sudo apt-get install python-pip python-dev libmysqlclient-dev
For Fedora, use the following command -
$ sudo dnf install python python-devel mysql-devel redhat-rpm-config gcc
For Python command prompt, use the following command -
pip install MySQL-python
Note: Make sure you have root privilege to install above module.
Database Connection
Before connecting to a MySQL database, make sure of the followings −
- You have created a database TESTDB.
- You have created a table EMPLOYEE in TESTDB.
- This table has fields FIRST_NAME, LAST_NAME, AGE, SEX and INCOME.
- User ID "testuser" and password "test123" are set to access TESTDB.
- Python module MySQLdb is installed properly on your machine.
Example
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 −
..................................
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 fetech multiple values from a database table.
- fetchone(): It fetches the next row of a query result set.
A result set is an object that is returned when a cursor object is used
to query a table.
- fetchall(): It fetches all the rows in a result set. If some rows have already been extracted from the result set, then it retrieves
the remaining rows from the result set.
- rowcount: This is a read-only attribute and returns the number of rows that were affected by an execute() method.
Example
The following procedure queries 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:
- Atomicity: Either a transaction completes or nothing happens at all.
- Consistency: A transaction must start in a consistent state and leave the system in a consistent state.
- Isolation: Intermediate results of a transaction are not visible outside the current transaction.
- 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.
