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SQLite Python tutorial

This is a Python programming tutorial for the SQLite database. It covers the basics of SQLite programming with the Python language. You might also want to check the Python tutorial, SQLite tutorial or MySQL Python tutorial or PostgreSQL Python tutorial on ZetCode.

Prerequisites

To work with this tutorial, we must have Python language, SQLite database, pysqlite language binding and the sqlite3 command line tool installed on the system. If we have Python 2.5+ then we only need to install the sqlite3 command line tool. Both the SQLite library and the pysqlite language binding are built into the Python languge.

$ python
Python 2.7.2+ (default, Jul 20 2012, 22:12:53) 
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sqlite3
>>> sqlite3.version
'2.6.0'
>>> sqlite3.sqlite_version
'3.7.7'

In the shell, we launch the Python interactive interpreter. We can see the Python version. In our case it is Python 2.7.2. The sqlite.version is the version of the pysqlite (2.6.0), which is the binding of the Python language to the SQLite database. The sqlite3.sqlite_version gives us the version of the SQLite database library. In our case it is 3.7.7.

Now we are going to use the sqlite3 command line tool to create a new database.

$ sqlite3 test.db 
SQLite version 3.7.7 2011-06-23 19:49:22
Enter ".help" for instructions
Enter SQL statements terminated with a ";"

We provide a parameter to the sqlite3 tool. A "test.db" is a database name. It is a file on our disk. If it is present, it is opened. If not, it is created.

sqlite> .tables
sqlite> .exit
$ ls
test.db

The .tables command gives a list of tables in the test.db database. There are currently no tables. The .exit command terminates the interactive session of the sqlite3 command line tool. The ls Unix command shows the contents of the current working directory. We can see the test.db file. All data will be stored in this single file.

Version

In the first code example, we will get the version of the SQLite database.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

con = None

try:
    con = lite.connect('test.db')
    
    cur = con.cursor()    
    cur.execute('SELECT SQLITE_VERSION()')
    
    data = cur.fetchone()
    
    print "SQLite version: %s" % data                
    
except lite.Error, e:
    
    print "Error %s:" % e.args[0]
    sys.exit(1)
    
finally:
    
    if con:
        con.close()

In the above Python script we connect to the previously created test.db database. We execute an SQL statement which returns the version of the SQLite database.

import sqlite3 as lite

The sqlite3 module is used to work with the SQLite database.

con = None

We initialize the con variable to None. In case we could not create a connection to the database (for example the disk is full), we would not have a connection variable defined. This would lead to an error in the finally clause.

con = lite.connect('test.db')

Here we connect to the test.db database. The connect() method returns a connection object.

cur = con.cursor()    
cur.execute('SELECT SQLITE_VERSION()')

From the connection, we get the cursor object. The cursor is used to traverse the records from the result set. We call the execute() method of the cursor and execute the SQL statement.

data = cur.fetchone()

We fetch the data. Since we retrieve only one record, we call the fetchone() method.

print "SQLite version: %s" % data

We print the data that we have retrieved to the console.

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

In case of an exception, we print an error message and exit the script with an error code 1.

finally:
    
    if con:
        con.close() 

In the final step, we release the resources.


In the second example, we again get the version of the SQLite database. This time we will use the with keyword.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

con = lite.connect('test.db')

with con:
    
    cur = con.cursor()    
    cur.execute('SELECT SQLITE_VERSION()')
    
    data = cur.fetchone()
    
    print "SQLite version: %s" % data  

The script returns the current version of the SQLite database. With the use of the with keyword. The code is more compact.

with con:

With the with keyword, the Python interpreter automatically releases the resources. It also provides error handling.

Inserting data

We will create a Cars table and insert several rows to it.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

con = lite.connect('test.db')

with con:
    
    cur = con.cursor()    
    cur.execute("CREATE TABLE Cars(Id INT, Name TEXT, Price INT)")
    cur.execute("INSERT INTO Cars VALUES(1,'Audi',52642)")
    cur.execute("INSERT INTO Cars VALUES(2,'Mercedes',57127)")
    cur.execute("INSERT INTO Cars VALUES(3,'Skoda',9000)")
    cur.execute("INSERT INTO Cars VALUES(4,'Volvo',29000)")
    cur.execute("INSERT INTO Cars VALUES(5,'Bentley',350000)")
    cur.execute("INSERT INTO Cars VALUES(6,'Citroen',21000)")
    cur.execute("INSERT INTO Cars VALUES(7,'Hummer',41400)")
    cur.execute("INSERT INTO Cars VALUES(8,'Volkswagen',21600)")

The above script creates a Cars table and inserts 8 rows into the table.

cur.execute("CREATE TABLE Cars(Id INT, Name TEXT, Price INT)")

This SQL statement creates a new Cars table. The table has three columns.

cur.execute("INSERT INTO Cars VALUES(1,'Audi',52642)")
cur.execute("INSERT INTO Cars VALUES(2,'Mercedes',57127)")

These two lines insert two cars into the table. Using the with keyword, the changes are automatically committed. Otherwise, we would have to commit them manually.

sqlite> .mode column  
sqlite> .headers on

We verify the written data with the sqlite3 tool. First we modify the way the data is displayed in the console. We use the column mode and turn on the headers.

sqlite> SELECT * FROM Cars;
Id          Name        Price     
----------  ----------  ----------
1           Audi        52642     
2           Mercedes    57127     
3           Skoda       9000      
4           Volvo       29000     
5           Bentley     350000    
6           Citroen     21000     
7           Hummer      41400     
8           Volkswagen  21600 

This is the data that we have written to the Cars table.


We are going to create the same table. This time using the convenience executemany() method.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

cars = (
    (1, 'Audi', 52642),
    (2, 'Mercedes', 57127),
    (3, 'Skoda', 9000),
    (4, 'Volvo', 29000),
    (5, 'Bentley', 350000),
    (6, 'Hummer', 41400),
    (7, 'Volkswagen', 21600)
)


con = lite.connect('test.db')

with con:
    
    cur = con.cursor()    
    
    cur.execute("DROP TABLE IF EXISTS Cars")
    cur.execute("CREATE TABLE Cars(Id INT, Name TEXT, Price INT)")
    cur.executemany("INSERT INTO Cars VALUES(?, ?, ?)", cars)

This script drops a Cars table if it exists and (re)creates it.

cur.execute("DROP TABLE IF EXISTS Cars")
cur.execute("CREATE TABLE Cars(Id INT, Name TEXT, Price INT)")

The first SQL statement drops the Cars table, if it exists. The second SQL statement creates the Cars table.

cur.executemany("INSERT INTO Cars VALUES(?, ?, ?)", cars)

We insert 8 rows into the table using the convenience executemany() method. The first parameter of this method is a parameterized SQL statement. The second parameter is the data, in the form of tuple of tuples.


We provide another way to create our Cars table. We commit the changes manually and provide our own error handling.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

try:
    con = lite.connect('test.db')

    cur = con.cursor()  

    cur.executescript("""
        DROP TABLE IF EXISTS Cars;
        CREATE TABLE Cars(Id INT, Name TEXT, Price INT);
        INSERT INTO Cars VALUES(1,'Audi',52642);
        INSERT INTO Cars VALUES(2,'Mercedes',57127);
        INSERT INTO Cars VALUES(3,'Skoda',9000);
        INSERT INTO Cars VALUES(4,'Volvo',29000);
        INSERT INTO Cars VALUES(5,'Bentley',350000);
        INSERT INTO Cars VALUES(6,'Citroen',21000);
        INSERT INTO Cars VALUES(7,'Hummer',41400);
        INSERT INTO Cars VALUES(8,'Volkswagen',21600);
        """)

    con.commit()
    
except lite.Error, e:
    
    if con:
        con.rollback()
        
    print "Error %s:" % e.args[0]
    sys.exit(1)
    
finally:
    
    if con:
        con.close() 

In the above script we (re)create the Cars table using the executescript() method.

cur.executescript("""
    DROP TABLE IF EXISTS Cars;
    CREATE TABLE Cars(Id INT, Name TEXT, Price INT);
    INSERT INTO Cars VALUES(1,'Audi',52642);
    INSERT INTO Cars VALUES(2,'Mercedes',57127);
...

The executescript() method allows us to execute the whole SQL code in one step.

con.commit()

Without the with keyword, the changes must be committed using the commit() method.

except lite.Error, e:
    
    if con:
        con.rollback()
        
    print "Error %s:" % e.args[0]
    sys.exit(1)

In case of an error, the changes are rolled back and an error message is printed to the terminal.

The last inserted row id

Sometimes, we need to determine the id of the last inserted row. In Python SQLite, we use the lastrowid attribute of the cursor object.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

con = lite.connect(':memory:')

with con:
    
    cur = con.cursor()    
    cur.execute("CREATE TABLE Friends(Id INTEGER PRIMARY KEY, Name TEXT);")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Tom');")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Rebecca');")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Jim');")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Robert');")
        
    lid = cur.lastrowid
    print "The last Id of the inserted row is %d" % lid

We create a Friends table in memory. The Id is automatically incremented.

cur.execute("CREATE TABLE Friends(Id INTEGER PRIMARY KEY, Name TEXT);")

In SQLite, INTEGER PRIMARY KEY column is auto incremented. There is also an AUTOINCREMENT keyword. When used in INTEGER PRIMARY KEY AUTOINCREMENT a slightly different algorithm for Id creation is used.

cur.execute("INSERT INTO Friends(Name) VALUES ('Tom');")
cur.execute("INSERT INTO Friends(Name) VALUES ('Rebecca');")
cur.execute("INSERT INTO Friends(Name) VALUES ('Jim');")
cur.execute("INSERT INTO Friends(Name) VALUES ('Robert');")

These four SQL statements insert four rows into the Friends table.

lid = cur.lastrowid

Using the lastrowid we get the last inserted row id.

$ ./autoinc.py
The last Id of the inserted row is 4

We see the output of the script.

Retrieving data

Now, that we have inserted some data into the database, we want to get it back.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys


con = lite.connect('test.db')

with con:    
    
    cur = con.cursor()    
    cur.execute("SELECT * FROM Cars")

    rows = cur.fetchall()

    for row in rows:
        print row

In this example, we retrieve all data from the Cars table.

cur.execute("SELECT * FROM Cars")

This SQL statement selects all data from the Cars table.

rows = cur.fetchall()

The fetchall() method gets all records. It returns a result set. Technically, it is a tuple of tuples. Each of the inner tuples represent a row in the table.

for row in rows:
    print row

We print the data to the console, row by row.

$ ./retrieveall.py
(1, u'Audi', 52642)
(2, u'Mercedes', 57127)
(3, u'Skoda', 9000)
(4, u'Volvo', 29000)
(5, u'Bentley', 350000)
(6, u'Citroen', 21000)
(7, u'Hummer', 41400)
(8, u'Volkswagen', 21600)

This is the output of the example.


Returning all data at a time may not be feasible. We can fetch rows one by one.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

con = lite.connect('test.db')

with con:
    
    cur = con.cursor()    
    cur.execute("SELECT * FROM Cars")

    while True:
      
        row = cur.fetchone()
        
        if row == None:
            break
            
        print row[0], row[1], row[2]

In this script we connect to the database and fetch the rows of the Cars table one by one.

while True:

We access the data from the while loop. When we read the last row, the loop is terminated.

row = cur.fetchone()

if row == None:
    break

The fetchone() method returns the next row from the table. If there is no more data left, it returns None. In this case we break the loop.

print row[0], row[1], row[2]

The data is returned in the form of a tuple. Here we select records from the tuple. The first is the Id, the second is the car name and the third is the price of the car.

$ ./retrieveonebyone.py
1 Audi 52642
2 Mercedes 57127
3 Skoda 9000
4 Volvo 29000
5 Bentley 350000
6 Citroen 21000
7 Hummer 41400
8 Volkswagen 21600

This is the output of the example.

The dictionary cursor

The default cursor returns the data in a tuple of tuples. When we use a dictionary cursor, the data is sent in the form of Python dictionaries. This way we can refer to the data by their column names.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite


con = lite.connect('test.db')    

with con:
    
    con.row_factory = lite.Row
       
    cur = con.cursor() 
    cur.execute("SELECT * FROM Cars")

    rows = cur.fetchall()

    for row in rows:
        print "%s %s %s" % (row["Id"], row["Name"], row["Price"])

In this example, we print the contents of the Cars table using the dictionary cursor.

con.row_factory = lite.Row

We select a dictionary cursor. Now we can access records by the names of columns.

for row in rows:
    print "%s %s %s" % (row["Id"], row["Name"], row["Price"])  

The data is accessed by the column names.

Parameterized queries

Now we will concern ourselves with parameterized queries. When we use parameterized queries, we use placeholders instead of directly writing the values into the statements. Parameterized queries increase security and performance.

The Python sqlite3 module supports two types of placeholders. Question marks and named placeholders.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

uId = 1
uPrice = 62300 

con = lite.connect('test.db')

with con:

    cur = con.cursor()    

    cur.execute("UPDATE Cars SET Price=? WHERE Id=?", (uPrice, uId))        
    con.commit()
    
    print "Number of rows updated: %d" % cur.rowcount

We update a price of one car. In this code example, we use the question mark placeholders.

cur.execute("UPDATE Cars SET Price=? WHERE Id=?", (uPrice, uId)) 

The question marks (?) are placeholders for values. The values are added to the placeholders.

print "Number of rows updated: %d" % cur.rowcount

The rowcount property returns the number of updated rows. In our case one row was updated.

$ ./prepared.py
Number of rows updated: 1

Id          Name        Price     
----------  ----------  ----------
1           Audi        62300 

The price of the car was updated.


The second example uses parameterized statements with named placeholders.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

uId = 4

con = lite.connect('test.db')

with con:

    cur = con.cursor()    

    cur.execute("SELECT Name, Price FROM Cars WHERE Id=:Id", 
        {"Id": uId})        
    con.commit()
    
    row = cur.fetchone()
    print row[0], row[1]

We select a name and a price of a car using named placeholders.

cur.execute("SELECT Name, Price FROM Cars WHERE Id=:Id", 
    {"Id": uId})   

The named placeholders start with a colon character.

Inserting images

In this section, we are going to insert an image to the SQLite database. Note that some people argue against putting images into databases. Here we only show how to do it. We do not dwell into technical issues of whether to save images in databases or not.

sqlite> CREATE TABLE Images(Id INTEGER PRIMARY KEY, Data BLOB);

For this example, we create a new table called Images. For the images, we use the BLOB data type, which stands for Binary Large Objects.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys


def readImage():

    try:
        fin = open("woman.jpg", "rb")
        img = fin.read()
        return img
        
    except IOError, e:

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

    finally:
        
        if fin:
            fin.close()


try:
    con = lite.connect('test.db')
    
    cur = con.cursor()
    data = readImage()
    binary = lite.Binary(data)
    cur.execute("INSERT INTO Images(Data) VALUES (?)", (binary,) )

    con.commit()    
    
except lite.Error, e:
    
    if con:
        con.rollback()
        
    print "Error %s:" % e.args[0]
    sys.exit(1)
    
finally:
    
    if con:
        con.close()  

In this script, we read an image from the current working directory and write it into the Images table of the SQLite test.db database.

try:
    fin = open("woman.jpg", "rb")
    img = fin.read()
    return img

We read binary data from the filesystem. We have a jpg image called woman.jpg.

binary = lite.Binary(data)

The data is encoded using the SQLite Binary object.

cur.execute("INSERT INTO Images(Data) VALUES (?)", (binary,) )

This SQL statement is used to insert the image into the database.

Reading images

In this section, we are going to perform the reverse operation. We will read an image from the database table.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys


def writeImage(data):
    
    try:
        fout = open('woman2.jpg','wb')
        fout.write(data)
    
    except IOError, e:    
        print "Error %d: %s" % (e.args[0], e.args[1])
        sys.exit(1)
        
    finally:
        
        if fout:
            fout.close()       
    

try:
    con = lite.connect('test.db')
    
    cur = con.cursor()    
    cur.execute("SELECT Data FROM Images LIMIT 1")
    data = cur.fetchone()[0]
    
    writeImage(data)

    
except lite.Error, e:
    
    print "Error %s:" % e.args[0]
    sys.exit(1)
    
finally:
    
    if con:
        con.close()      

We read image data from the Images table and write it to another file, which we call woman2.jpg.

try:
    fout = open('woman2.jpg','wb')
    fout.write(data)

We open a binary file in a writing mode. The data from the database is written to the file.

cur.execute("SELECT Data FROM Images LIMIT 1")
data = cur.fetchone()[0]

These two lines select and fetch data from the Images table. We obtain the binary data from the first row.

Metadata

Metadata is information about the data in the database. Metadata in a SQLite contains information about the tables and columns, in which we store data. Number of rows affected by an SQL statement is a metadata. Number of rows and columns returned in a result set belong to metadata as well.

Metadata in SQLite can be obtained using the PRAGMA command. SQLite objects may have attributes, which are metadata. Finally, we can also obtain specific metatada from querying the SQLite system sqlite_master table.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

con = lite.connect('test.db')

with con:
    
    cur = con.cursor()    
    
    cur.execute('PRAGMA table_info(Cars)')
    
    data = cur.fetchall()
    
    for d in data:
        print d[0], d[1], d[2]

In this example, we issue the PRAGMA table_info(tableName) command, to get some metadata info about our Cars table.

cur.execute('PRAGMA table_info(Cars)')

The PRAGMA table_info(tableName) command returns one row for each column in the Cars table. Columns in the result set include the column order number, column name, data type, whether or not the column can be NULL, and the default value for the column.

for d in data:
    print d[0], d[1], d[2]

From the provided information, we print the column order number, column name and column data type.

$ ./colnames1.py
0 Id INT
1 Name TEXT
2 Price INT

Output of the example.


Next we will print all rows from the Cars table with their column names.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

con = lite.connect('test.db')

with con:
    
    cur = con.cursor()    
    cur.execute('SELECT * FROM Cars')
    
    col_names = [cn[0] for cn in cur.description]
    
    rows = cur.fetchall()
    
    print "%s %-10s %s" % (col_names[0], col_names[1], col_names[2])

    for row in rows:    
        print "%2s %-10s %s" % row

We print the contents of the Cars table to the console. Now, we include the names of the columns too. The records are aligned with the column names.

col_names = [cn[0] for cn in cur.description]

We get the column names from the description property of the cursor object.

print "%s %-10s %s" % (col_names[0], col_names[1], col_names[2])

This line prints three column names of the Cars table.

for row in rows:    
    print "%2s %-10s %s" % row

We print the rows using the for loop. The data is aligned with the column names.

$ ./colnames2.py
Id Name       Price
 1 Audi       52642
 2 Mercedes   57127
 3 Skoda      9000
 4 Volvo      29000
 5 Bentley    350000
 6 Citroen    21000
 7 Hummer     41400
 8 Volkswagen 21600

Output.


In our last example related to the metadata, we will list all tables in the test.db database.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

con = lite.connect('test.db')

with con:
    
    cur = con.cursor()    
    cur.execute("SELECT name FROM sqlite_master WHERE type='table'")

    rows = cur.fetchall()

    for row in rows:
        print row[0]

The code example prints all available tables in the current database to the terminal.

cur.execute("SELECT name FROM sqlite_master WHERE type='table'")

The table names are stored inside the system sqlite_master table.

$ ./listtables.py
Images
sqlite_sequence
Salaries
Cars

These were the tables on my system.

Export and import of data

We can dump data in an SQL format to create a simple backup of our database tables.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys

cars = (
    (1, 'Audi', 52643),
    (2, 'Mercedes', 57642),
    (3, 'Skoda', 9000),
    (4, 'Volvo', 29000),
    (5, 'Bentley', 350000),
    (6, 'Hummer', 41400),
    (7, 'Volkswagen', 21600)
)

def writeData(data):
    
    f = open('cars.sql', 'w')
    
    with f:
        f.write(data)


con = lite.connect(':memory:')

with con:
    
    cur = con.cursor()
    
    cur.execute("DROP TABLE IF EXISTS Cars")
    cur.execute("CREATE TABLE Cars(Id INT, Name TEXT, Price INT)")
    cur.executemany("INSERT INTO Cars VALUES(?, ?, ?)", cars)
    cur.execute("DELETE FROM Cars WHERE Price < 30000")
    
    data = '\n'.join(con.iterdump())
    
    writeData(data)

In the above example, we recreate the Cars table in the memory. We delete some rows from the table and dump the current state of the table into a cars.sql file. This file can serve as a current backup of the table.

def writeData(data):
    
    f = open('cars.sql', 'w')
    
    with f:
        f.write(data)

The data from the table is being written to the file.

con = lite.connect(':memory:')

We create a temporary table in the memory.

cur.execute("DROP TABLE IF EXISTS Cars")
cur.execute("CREATE TABLE Cars(Id INT, Name TEXT, Price INT)")
cur.executemany("INSERT INTO Cars VALUES(?, ?, ?)", cars)
cur.execute("DELETE FROM Cars WHERE Price < 30000")

These lines create a Cars table, insert values and delete rows, where the Price is less than 30000 units.

data = '\n'.join(con.iterdump())

The con.iterdump() returns an iterator to dump the database in an SQL text format. The built-in join() function takes the iterator and joins all the strings in the iterator separated by a new line. This data is written to the cars.sql file in the writeData() function.

$ cat cars.sql 
BEGIN TRANSACTION;
CREATE TABLE Cars(Id INT, Name TEXT, Price INT);
INSERT INTO "Cars" VALUES(1,'Audi',52643);
INSERT INTO "Cars" VALUES(2,'Mercedes',57642);
INSERT INTO "Cars" VALUES(5,'Bentley',350000);
INSERT INTO "Cars" VALUES(6,'Hummer',41400);
COMMIT;

The contents of the dumped in-memory Cars table.


Now we are going to perform a reverse operation. We will import the dumped table back into memory.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys


def readData():
    
    f = open('cars.sql', 'r')
    
    with f:
        data = f.read()
        return data
        

con = lite.connect(':memory:')

with con:   

    cur = con.cursor()
    
    sql = readData()
    cur.executescript(sql)
    
    cur.execute("SELECT * FROM Cars")
    
    rows = cur.fetchall()
    
    for row in rows:
        print row    

In this script, we read the contents of the cars.sql file and execute it. This will recreate the dumped table.

def readData():
    
    f = open('cars.sql', 'r')
    
    with f:
        data = f.read()
        return data

Inside the readData() method we read the contents of the cars.sql table.

cur.executescript(sql)

We call the executescript() method to launch the SQL script.

cur.execute("SELECT * FROM Cars")

rows = cur.fetchall()

for row in rows:
    print row    

We verify the data.

$ ./import.py
(1, u'Audi', 52643)
(2, u'Mercedes', 57642)
(5, u'Bentley', 350000)
(6, u'Hummer', 41400)

The output shows, that we have successfully recreated the saved Cars table.

Transactions

A transaction is an atomic unit of database operations against the data in one or more databases. The effects of all the SQL statements in a transaction can be either all committed to the database or all rolled back.

In SQLite, any command other than the SELECT will start an implicit transaction. Also, within a transaction a command like CREATE TABLE ..., VACUUM, PRAGMA, will commit previous changes before executing.

Manual transactions are started with the BEGIN TRANSACTION statement and finished with the COMMIT OR ROLLBACK statements.

SQLite supports three non-standard transaction levels. DEFERRED, IMMEDIATE and EXCLUSIVE. SQLite Python module also supports an autocommit mode, where all changes to the tables are immediately effective.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys


try:
    con = lite.connect('test.db')
    cur = con.cursor()    
    cur.execute("DROP TABLE IF EXISTS Friends")
    cur.execute("CREATE TABLE Friends(Id INTEGER PRIMARY KEY, Name TEXT)")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Tom')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Rebecca')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Jim')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Robert')")
    
    #con.commit()
            
except lite.Error, e:
    
    if con:
        con.rollback()
    
    print "Error %s:" % e.args[0]
    sys.exit(1)
    
finally:
    
    if con:
        con.close() 

We create a Friends table and try to fill it with data. However, as we will see, the data is not committed.

#con.commit()

The commit() method is commented. If we uncomment the line, the data will be written to the table.

sqlite> .tables
Cars       Friends    Images     Salaries   Temporary

sqlite> SELECT Count(Id) FROM Friends;
Count(Id) 
----------
0     

The table is created but the data is not written to the table.


In the second example we demonstrate, that some commands implicitly commit previous changes to the database.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys


try:
    con = lite.connect('test.db')
    cur = con.cursor()    
    cur.execute("DROP TABLE IF EXISTS Friends")
    cur.execute("CREATE TABLE Friends(Id INTEGER PRIMARY KEY, Name TEXT)")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Tom')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Rebecca')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Jim')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Robert')")

    cur.execute("CREATE TABLE IF NOT EXISTS Temporary(Id INT)")
            
except lite.Error, e:
    
    if con:
        con.rollback()
    
    print "Error %s:" % e.args[0]
    sys.exit(1)
    
finally:
    
    if con:
        con.close() 

Again, we do not call the commit() command explicitly. But this time, the data is written to the Friends table.

cur.execute("CREATE TABLE IF NOT EXISTS Temporary(Id INT)")

This SQL statement will create a new table. It also commits the previous changes.

$ ./implcommit.py 

sqlite> SELECT * FROM Friends;
Id          Name      
----------  ----------
1           Tom       
2           Rebecca   
3           Jim       
4           Robert    

The data has been written to the Friends table.


In the autocommit mode, an SQL statement is executed immediately.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sqlite3 as lite
import sys


try:
    con = lite.connect('test.db', isolation_level=None)
    cur = con.cursor()    
    cur.execute("DROP TABLE IF EXISTS Friends")
    cur.execute("CREATE TABLE Friends(Id INTEGER PRIMARY KEY, Name TEXT)")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Tom')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Rebecca')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Jim')")
    cur.execute("INSERT INTO Friends(Name) VALUES ('Robert')")

    
except lite.Error, e:    
    
    print "Error %s:" % e.args[0]
    sys.exit(1)
    
finally:
    
    if con:
        con.close() 

In this example, we connect to the database in the autocommit mode.

con = lite.connect('test.db', isolation_level=None)

We have an autocommit mode, when we set the isolation_level to None.

$ ./autocommit.py

sqlite> SELECT * FROM Friends;
Id          Name      
----------  ----------
1           Tom       
2           Rebecca   
3           Jim       
4           Robert  

The data was successfully committed to the Friends table.

This was SQLite Python tutorial. ZetCode has a complete e-book for SQLite Python:
SQLite Python e-book.