Connecting and using PostgreSQL database with psycopg2 [Python 2.7]

A presentation on
psycopg2 python
module
by Dinesh Neupane [@neupanedinesh_] | Kathmandu
University

Psycopg2
A python driver for PostGreSQL
PostgreSQL
Python
Script

Background
 PostGreSQL database adapter for Python
Programming language
 Version 2 : original code completely rewritten for
better styling of classes for connection and cursor
 Postgres is type sensitive so we have to declare
types on each of our columns.
1

Background
● Robust engine that is implemented as a server
rather than a single file. As a server, Postgres
accepts connections from clients who can request a
SELECT, INSERT, or any other type of SQL query.
● This type of a design is called a client-server model,
where clients can interact with the server.
2

Requirements
 PostGreSQL 9.3 or higher
 Python 2.0 or higher
3

Installation
● Install python package installer (pip)
● For windows machine (to install package with all it’s
dependencies)
pip install psycopg2
4

Classes and Methods
● Connect()
Create a new database connection and returns a new
connection instance
connect("dbname=‘test_database’user=‘postgres'host='localhost'
password=‘italy'")
5

Connection Parameters
The basic connection parameters are:
● dbname – the database name (database is a
deprecated alias) user – user name used to
authenticate
● password – password used to authenticate
● host – database host address
● port – connection port number
6

Connection Example
Example:
import psycopg2
try:
conn = psycopg2.connect("dbname='test_database'
user='postgres' host='localhost' password='italy’”)
print "Connection Sucessful !! "
except:
print "unable to connect to the database"
7

Cursor()
Cursors are created by the connection.cursor()
method: they are bound to the connection for the
entire lifetime and all the commands are executed in
the context of the database session wrapped by the
connection.
8

Cursor()
The class cursor allows interaction with the database:
● Send commands to the database using using
methods like execute() and executemany()
● Retrieve data from the database by iteration or
using methods such as fetchone(), fetchmany() or
fetchall()
● cursor.description to get the metadata
9

Cursor()
● mogrify() - Return a query string after arguments
binding. The string returned is exactly the one that
would be sent to the database running the execute()
method or similar. Example:
>>> cur.mogrify("INSERT INTO table_one VALUES(6, 'Sarad
Chaudhary', 'Banke Nepalgunj’)”)
"INSERT INTO table_one VALUES(6, 'Sarad Chaudhary', 'Banke
Nepalgunj’)”
10

Create and Drop table
CREATE TABLE student (
student_id INTEGER UNIQUE,
student_name VARCHAR(50),
phone CHAR(10),
birth_date DATE,
);
This creates a new table costumers with different
attribute fields.
11

Adaptation of Python Values to
SQL Types
Python PostgreSQL
None Null
Bool Bool
Float Real, Double
Int, Long Smallint, integer, bigint
Decimal Numeric
Str Varchar
List Array
12

Closing a database session
Make the changes to the database persistent
● Conn.commit()
Close the communication with database
● Cur.close()
● Conn.close()
13

The student table is created in the database that you
are connected to. [first schema in search path]
If you want the table to be created in some other
schema, you can prefix the table name with the
schema qualifier, for example:
CREATE TABLE another_schema.student( ... );
14

Dropping a table is much easier than creating a table.
The syntax for the DROP TABLE command is:
DROP TABLE student;
The rentals table existed in some schema other than your current
schema, you would qualify the table name:
DROP TABLE other_schema.student;
15

SQL Transactions
● If you check the postgres
database now you would
notice that there actually
isn't a users table inside it.
● This isn't a bug – it's
because of a concept
called SQL transactions.
16import psycopg2
conn=psycopg2.connect("host=loca
lhost dbname=postgres
user=postgres") cur =
conn.cursor()
cur.execute(""“
CREATE TABLE users(
id integer PRIMARY KEY,
email text,
name text,
address text )
""")

SQL Transactions
● With Postgres, we're dealing
with multiple users who could
be changing the database at
the same time.
● Let's imagine a simple
scenario where we're keeping
track of accounts for different
customers of a bank.
17import psycopg2
conn=psycopg2.connect("host=loca
lhost dbname=postgres
user=postgres") cur =
conn.cursor()
cur.execute("""
CREATE TABLE accounts(
id integer PRIMARY KEY,
name text,
balance float )
""")

SQL Transactions
● Our table looks like this:
● Let’s say Madhav gave 100 rupees to Sanjib. We could model this with
two queries:
UPDATE accounts SET balance=200 WHERE name=“Sanjib";
UPDATE accounts SET balance=100 WHERE name=“Madhav";
18
id name balance
1 Sanjib 100
2 Madhav 200

SQL Transactions
● We remove 100 rupees from Madhav, and add 100 rupees to Sanjib.
What if the second UPDATE statement has an error in it, the database
fails, or another user has a conflicting query?
● The first query would run properly, but the second would fail. That
would result in the following:
19
id name balance
1 Sanjib 200
2 Madhav 200

SQL Transactions
● Sanjib would be credited 100 rupees, but 100 rupees would not be
deducted from Madhav. This would cause the bank to lose money.
● Transactions prevent this type of behaviour by ensuring that all the
queries in a transaction block are executed at the same time. If any of
the transactions fail, the whole group fails, and no changes are made
to the database at all.
● When commit() is called, the PostgreSQL engine will run all the
queries at once.
20

DML: Insert
● Pass data to fill a query placeholders and let
Psycopg perform the correct conversion
● No more SQL injections!
>> "INSERT INTO table_one VALUES(6, 'Sarad Chaudhary', 'Banke
Nepalgunj’)”
21

DML: Insert
Example:
cur = conn.cursor()
insert_query="INSERT INTO table_one VALUES(6, 'Sarad
Chaudhary', 'Banke Nepalgunj’)”
cur.execute(insert_query)
conn.commit()
22

DML: Insert (copy_from() method)
● Method to load a file like .csv into the table of running
database
● Like the execute() method, it is attached to the Cursor object
but it contains different parameters.
● Arguments:
copy_from(f, ‘table_name’, sep=‘,’)
f=file to load(without header), table_name=table name, it should
load into, sep=delimeter
23

copy_from() method
Example:
import psycopg2
conn = psycopg2.connect("dbname='test_database' user='postgres'
host='localhost' password='italy’”)
cur = conn.cursor()
with open('name_list.csv', 'r') as f:
# no need to any other module to load csv
next(f)
# Skip the header row
cur.copy_from(f, 'table_one', sep=',’)
conn.commit()
24

DML: Delete
If you want to pass values to the DELETE statement, you use the
placeholders ( %s) in the DELETE statement and pass input
values to the second parameter of the execute() method.
The DELETE statement with a placeholder for the value of the id
field is as follows:
DELETE FROM test_table WHERE id = %s;
25

DML: Update
Execute the UPDATE statement with the input values
by calling the execute() method of the cursor object.
cur.execute(update_sql, (value1,value2))
26

Connecting and using PostgreSQL database with psycopg2 [Python 2.7]

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