PostgreSQL Python – Call PostgreSQL Functions

Summary: in this tutorial, you will learn how to call PostgreSQL functions from a Python program.

Calling a PostgreSQL function in Python steps

To call a PostgreSQL function from a Python program, you use the following steps:

First, create a new database connection to the PostgreSQL database server by calling the connect() function of the psycopg2 module.

conn = psycopg2.connect(dsn)

The connect() method returns a new instance of the connection class.

Next, create a new cursor by calling the cursor() method of the connection object.

cur = conn.cursor()

Then, pass the name of the function and the optional input values to the callproc() method of the cursor object.

cur.callproc('function_name', (value1,value2))

Internally, the  callproc() method translates the function call and input values into the following statement

SELECT * FROM function_name(value1,value2);

Both statements have the same effect.

After that, process the result set returned by the function using the fetchone(),  fetchall(), or  fetchmany() method.

Finally, call the close() method of the cursor and connection objects to close the communication with the PostgreSQL database server.

cur.close()
conn.close()

Calling a function example

The following get_parts_by_vendors() function returns a list of parts provided by a specified vendor.

CREATE OR REPLACE FUNCTION get_parts_by_vendor(id integer)
   RETURNS TABLE(part_id INTEGER, part_name VARCHAR) AS
 $$
 BEGIN
  RETURN QUERY
 SELECT parts.part_id, parts.part_name
  FROM parts
  INNER JOIN vendor_parts on vendor_parts.part_id = parts.part_id
  WHERE vendor_id = id;
 END; $$
 LANGUAGE plpgsql;

The following get_parts() function calls the get_parts_by_vendors() function:

!/usr/bin/python
 import psycopg2
 from config import config
 def get_parts(vendor_id):
     """ get parts provided by a vendor specified by the vendor_id """
     conn = None
     try:
         # read database configuration
         params = config()
         # connect to the PostgreSQL database
         conn = psycopg2.connect(**params)
         # create a cursor object for execution
         cur = conn.cursor()
         # another way to call a function
         # cur.execute("SELECT * FROM get_parts_by_vendor( %s); ",(vendor_id,))
         cur.callproc('get_parts_by_vendor', (vendor_id,))
         # process the result set
         row = cur.fetchone()
         while row is not None:
             print(row)
             row = cur.fetchone()
         # close the communication with the PostgreSQL database server
         cur.close()
     except (Exception, psycopg2.DatabaseError) as error:
         print(error)
     finally:
         if conn is not None:
             conn.close()

The following calls the get_parts() function to get a list of parts provided by the vendor with id 1:

if name == 'main':
  get_parts(1)

In this tutorial, you have learned step by step how to call a PostgreSQL function in Python.

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