PostgreSQL in Python Using Psycopg2

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Are you a Python programmer learning to work with PostgreSQL? If so, this tutorial on psycopg2, the PostgreSQL connector for Python, is for you. You can connect to PostgreSQL databases and run queries—all from within your Python script—using the psycopg2 adapter.

In this tutorial, you’ll learn the basics of using psycopg2 in Python to do the following:

• Connect to a PostgreSQL database
• Run SQL queries against a database: create tables, insert, retrieve, and delete records
• Use context managers in Python to run queries

Let’s get started!

Prerequisites

To follow along with this tutorial, you need to have the following installed in your development environment:

• PostgreSQL and Python 3.6 or a later version
• Psycopg2, the Postgres connector for Python. It’s available as a PyPI package that you can install using pip:

pip3 install psycopg2

How to Connect to a PostgreSQL Database

The connect() Function in Psycopg2

Download the code and follow along.

After you’ve installed the psycopg2 library, you can import it into your working environment. As a first step, let’s create a main.py file, import psycopg2, and connect to a PostgreSQL database.

To connect to a database, you can use the connect() function from psycopg2. The connect() function takes in the information needed to connect to the database, such as the name of the database, username, and password, as keyword arguments. It returns a connection object if the connection succeeds. You can use the connect() function as shown:

# psycopg2-tutorial/main.py

import psycopg2

db_connection = psycopg2.connect(dbname='test',
                                 user=<username>,
                                 password=<password>,
                                 host='localhost',
                                 port=5432)

print("Successfully connected to the database.")

In the above code snippet:

• dbname is the name of the database that you’d like to connect to. Here, I’ve connected to the test database.
• user and password are the username and the password required for authentication.
• host is the IP address of the server on which your database is running (‘localhost’ in this case).
• port refers to the port number that the server listens to for incoming connection requests to the database. The default value is 5432.

How to Parse Config Files in Python

The configparser module, built into the Python standard library, lets you parse configuration files of the INI file format.

A typical config file consists of multiple sections, with each section having a set of key-value pairs. The keys and values are separated by a : or =. Here’s the general structure of a config file with one section and n key-value pairs:

[section-name]
key1=value1
key2=value2
.
.
.
keyn=valuen

Let’s create the config file db_info.ini that stores the details required to connect to the PostgreSQL database. The names of the keyword arguments, dbname, user, password, host, and port, should be the keys. And the values used for these arguments in the connect() function call should be the values in the config file:

# psycopg2-tutorial/db_info.ini

[postgres-sample-db]
dbname=test
user=<username>
password=<password>
host=localhost
port=5432

Parsing the Config File with ConfigParser

Let’s create db_config.py containing the definition of get_db_info(), a function that parses the config file and returns the info we need to connect to the database.

# psycopg2-tutorial/db_config.py

from configparser import ConfigParser

def get_db_info(filename,section):
    # instantiating the parser object
    parser=ConfigParser()
    parser.read(filename)

    db_info={}
    if parser.has_section(section):
         # items() method returns (key,value) tuples
         key_val_tuple = parser.items(section) 
         for item in key_val_tuple:
             db_info[item[0]]=item[1] # index 0: key & index 1: value

    return db_info

The function get_db_info() takes in the names of the config file and the section as arguments. It returns the details of the database as a Python dictionary. Under the hood, it works by instantiating a ConfigParser object that reads in the config file, taps into the section, and retrieves the key-value pairs.

Now we do the following inside main.py:

• Import the get_db_info() function from db_config,
• Call the get_db_info() function with the correct arguments and store the returned dictionary in db_info, and
• Use db_info in the call to the connect() function.

# psycopg2-tutorial/main.py

import psycopg2
from db_config import get_db_info

filename='db_info.ini'
section='postgres-sample-db'
db_info = get_db_info(filename,section)

db_connection = psycopg2.connect(**db_info)
print("Successfully connected to the database.")

Handling Connection Errors

Let’s recall what we’ve done so far. We stored the details of the database in a config file, parsed it using ConfigParser, and connected to the database using the connect() function from psycopg2. However, we haven’t accounted for the errors that may arise during the connection process.

Suppose you enter an incorrect password (yeah, that happens often!). Open the config file db_info.ini, update the password to something invalid, and run the script again.

There’ll be a runtime error. Focusing on the relevant information in the traceback, you’ll see that an OperationalError exception is thrown as password authentication failed. Psycopg2 has an implementation of the OperationalError class.

Traceback (most recent call last):
...
psycopg2.OperationalError: connection to server at "localhost" (::1), \
port 5432 failed: FATAL:  password authentication failed for user "postgres"

What happens when you try connecting to a database that does not exist? For example, I updated the dbname field in the config file from test to test1 (the test1 database does not exist). It’s an OperationalError exception (again).

Traceback (most recent call last):
...
psycopg2.OperationalError: connection to server at "localhost" (::1), \
port 5432 failed: FATAL:  database "test1" does not exist

You can run the script a few more times by changing one or more fields to invalid values. And you’ll see that an OperationalError exception is thrown in all of the runs.

Therefore, such connection errors that arise due to incorrect or invalid values can all be subsumed under the OperationalError exception.

Exception Handling Using try and except

Now, let’s import the OperationalError class from psycopg2 and handle the OperationalError exception using Python’s try and except blocks.

# psycopg2-tutorial/main.py

import psycopg2
from psycopg2 import OperationalError
from db_config import get_db_info


filename='db_info.ini'
section='postgres-sample-db'
db_info = get_db_info(filename,section)

try:
    db_connection = psycopg2.connect(**db_info)
    print("Successfully connected to the database.")

except OperationalError:
    print("Error connecting to the database :/")

Let’s parse what the above code snippet does.

• If the try block succeeds, the connect() function returns a database connection instance, db_connection.
• If an OperationalError exception is raised during the connection process, the except block is triggered, and we get notified that there was an error connecting to the database.

In essence: we try to connect to the database and run queries when there are no OperationalError exceptions.

A Note on Closing the Connection 🤨

We haven’t queried the database yet, so why close the connection? We’ll discuss this to get a high-level overview of what we’re trying to accomplish. Also, it’s easier to think coherently about the try, except, and finally blocks here.

We know how to connect to a database and handle connection errors. After connecting to the database and querying it, we should finally close the connection by calling the close() method on the connection object.

You can do this inside a finally block. The statements inside the finally block are always executed, regardless of whether or not the try block succeeds. So you may come up with the following:

from psycopg2 import OperationalError

try:
    # connecting to the db
    # querying the db

except OperationalError:
    # print out the error message

finally:
    # close the db connection

This is almost correct, but there’s a small problem. To close a connection, it should exist. If the connection fails, then the connection object is never returned. So you’ll run into a NameError exception trying to close a connection that does not even exist.

To account for this, let’s set db_connection to None initially and close the connection only if it exists: if db_connection: is equivalent to if db_connection is not None:.

Putting it all together, here’s our main.py file:

# psycopg2-tutorial/main.py

import psycopg2
from psycopg2 import OperationalError
from db_config import get_db_info


filename='db_info.ini'
section='postgres-sample-db'
db_info = get_db_info(filename,section)

db_connection = None
try:
    db_connection = psycopg2.connect(**db_info)
    print("Successfully connected to the database.")

except OperationalError:
    print("Error connecting to the database :/")

finally:
    if db_connection:
        db_connection.close()
        print("Closed connection.")

How To Query the Database

So far, we’ve learned to connect to the database and handle connection errors. Can we start querying the database yet?

The Cursor Object

There’s one more step before you can start running queries: creating a database cursor.

Cursors are analogous to file handlers. They let you query databases and fetch results just the way file handlers let you perform I/O operations on files. Once you have a cursor object, you can call methods on it to query the database and fetch the results of the query.

To create a cursor object, you can call the cursor() method on the connection object.

db_cursor = db_connection.cursor()

In the test database, let’s create a table, people. Each record in the table has the fields: name, city, and profession. The SQL query is defined in the string, create_table.

To execute the query, call the execute() method on the cursor object, db_cursor, and pass in the query string, create_table.

create_table = '''CREATE TABLE people(
                          id SERIAL PRIMARY KEY,
                          name varchar(50) NOT NULL,
                          city varchar(40),
                          profession varchar(60));'''
db_cursor.execute(create_table)

After creating the table, let’s insert a record: Jane Lee who works as a Rust programmer in the fictional city of Rustmore.

insert_record = "INSERT INTO people (name,city,profession) \
VALUES (%s, %s, %s);"
insert_value = ('Jane Lee','Rustmore','Rust programmer')
db_cursor.execute(insert_record, insert_value)

Committing Transactions

Now run main.py. The script will run without errors. However, if you look up the database, you’ll not be able to find the people table with Jane Lee’s record. Why? Well, for the changes to persist in the database, you’ll have to commit the transaction.

To do so you can either call the commit() method on the connection object or set the autocommit attribute of the connection object to True.

db_connection.autocommit = True

Re-run main.py.The changes we’ve made will now persist in the test database.

Inserting Records Into a Table

In the test database, we now have a table named people containing only one record - not very interesting. We need to insert more records into the table. I can spend the rest of the day coming up with names and fictional cities. Or I can generate synthetic data using Faker. You can install Faker using pip: pip install Faker.

The fake_data.py file contains the generate_fake_data() function that’ll return a list of (name, city, job) tuples. After instantiating a Faker object, I’ve set the seed for reproducibility. Without the seed, you’ll get a different set of records every time you run the script.

# psycopg2-tutorial/fake_data.py

from faker import Faker

fake = Faker()
Faker.seed(42)

def generate_fake_data(num):
    records = []

    for i in range(num):
        name, city, job = fake.name(), fake.city(), fake.job()
        records.append((name,city,job))
    return records

Inside the main.py file, import generate_fake_data from the fake_data module. Let’s use the generate_fake_data() function to generate 100 more records that we can insert into the people table.

Here, records is a tuple of records. We can loop through it and insert the records into the table by calling the execute() method on db_cursor.

from fake_data import generate_fake_data
records = tuple(generate_fake_data(100)) #cast into a tuple for immutability
insert_record = "INSERT INTO people (name,city,profession)\
VALUES (%s, %s, %s);"

for record in records:
    db_cursor.execute(insert_record,record)

How to Retrieve Data from Tables

Next, let’s read data from the people table. We’ll run a simple select query to read all the records from the table.

Executing the select query, SELECT * FROM people; returns all the records in the table. To fetch the results of the query, you can call a few fetch methods on the cursor object. These methods return records from the result as tuples:

Fetch the Next Record with fetchone()

Calling the fetchone() method on db_cursor fetches the next record in the result.

db_cursor.execute("SELECT * FROM people;")
print(db_cursor.fetchone())

As this is the first time we’re fetching the record, it fetches the first record.

(1, 'Jane Lee', 'Rustmore', 'Rust programmer')

Fetch the Next n Records with fetchmany()

The fetchmany() method takes in the number of records to fetch (n) and fetches the next n records from the result.

for record in db_cursor.fetchmany(10):
    print(record)

As we’ve mentioned 10 in the fetchmany() method call, we get the next 10 records: records in rows 2 to 11 in the people table.

(2, 'Allison Hill', 'East Jill', 'Sports administrator')
(3, 'Javier Johnson', 'East William', 'Aid worker')
(4, 'Michelle Miles', 'Robinsonshire', 'Health physicist')
(5, 'Abigail Shaffer', 'Petersonberg', 'Engineer, structural')
(6, 'Gabrielle Davis', 'West Melanieview', 'Armed forces logistics/ \
support/administrative officer')
(7, 'Kimberly Dudley', 'Millerport', 'Water engineer')
(8, 'Heidi Lee', 'North Donnaport', 'Cartographer')
(9, 'Sharon James', 'Reidstad', 'Designer, textile')
(10, 'Daniel Adams', 'New Cynthiaside', 'Financial risk analyst')
(11, 'James Mayo', 'Lake Mark', 'Banker')

Fetch All the Remaining Rows with fetchall()

Calling the fetchall() method on the cursor object returns all the remaining records in the result.

for record in db_cursor.fetchall():
    print(record)

We’ve already fetched the first 11 records in the result. So fetchall() fetches all the remaining records.

# output (truncated)
(12, 'Andrew Stewart', 'Carlshire', 'International aid/development worker')
(13, 'Jonathan Wilkerson', 'Thomasberg', 'Fine artist')
(14, 'Kimberly Burgess', 'Hurstfurt', 'Medical illustrator')
.
.
.
(98, 'David Grant', 'Obrienbury', 'Scientist, research (medical)')
(99, 'Terry Evans', 'Torreston', 'Travel agency manager')
(100, 'Annette Farmer', 'West Donna', 'Designer, graphic')
(101, 'Juan Moore', 'Hayesfort', 'Horticulturist, amenity')

Updating and Deleting Records

From the cities in the table, I’d like to get the list of all cities that occur more than once.

get_count ='''SELECT city, COUNT(*)
              FROM people
              GROUP BY city HAVING COUNT(*)>1;'''
db_cursor.execute(get_count)
print(db_cursor.fetchall())

We see that ‘Johnsonmouth’ is the only city that appears more than once.

# Output
[('Johnsonmouth', 2)]

Let’s update both the occurrences of ‘Johnsonmouth’ to another fictional city, say, ‘Mathville’.

update_query = "UPDATE people SET city=%s WHERE city=%s"
values = ('Mathville','Johnsonmouth')
db_cursor.execute(update_query,values)

Now let’s delete the records where the city is ‘Mathville’.

delete_record = "DELETE FROM people WHERE city=%s;"
record = ('Mathville',) # pass in as a tuple
db_cursor.execute(delete_record,record)

With that, we’ve run a basic set of queries to insert, retrieve, update, and delete records. So what do we do next? We’ve already accounted for closing the database connection (finally) after running the queries.

It’s a good practice to also close the database cursor. To do this, you can call the close() method on the cursor object. But this has to be inside the finally block too, before closing the database connection. This is because if the connection object does not exist, the cursor doesn’t exist either!

If needed, look up the main.py file to make sure you’ve understood it all.

Simplifying Querying Using Context Managers

So far we’ve implemented exception handling for errors that arise when trying to connect to a database. We then ran SQL queries to create a table and insert records, retrieve data from the table, and update and delete records. In all of the above, the queries ran without errors and returned the desired results. ✅

In practice, however, some queries may not run as expected! We may run into errors: from syntax errors in the query string to errors arising from attempting invalid transactions.

For example: retrieving, updating, and deleting records make sense only when there is a table with records. Psycopg2 provides several exception classes such as DatabaseError and ProgrammingError that are triggered when the SQL queries fail to run successfully.

In summary: you’re not only trying to connect to the database and run a set of queries. Rather, for each of the queries, you try to run the query (inside a try block) and catch any exception (inside the except block).

Suppose you need to run n queries. If you implement error handling for each of the n queries, your script will look like this:

# all imports
from psycopg2 import OperationalError

try:
    # connecting to the db
      try:
          # running query #1
            
      except <ErrorType1>:
            # roll back changes

      try:
          # running query #2
            
      except <ErrorType2>:
            # roll back changes
     
      ...

except OperationalError:
    # print out the error message

finally:
    # close the db connection

Implementing such error handling for each of the n queries can be difficult. And here’s where context managers can help.

Connection and Cursor Objects as Context Managers

In Python, context managers can be used to control the execution of a block of statements and the with statement can be used to create an execution context.

In general, context managers mitigate resource leakage and help in efficient resource handling. They do this by setting up and tearing down resources when the execution enters and exits the context, respectively.

The connection and cursor objects we’ve used so far are both context managers. You can use them in with statements using the following general syntax:

# psycopg2-tutorial/main2.py
try:
    with psycopg2.connect(**db_info) as db_connection:
        print("Successfully connected to the database.")

        with db_connection.cursor() as db_cursor:
        # run queries

except OperationalError:
    # print out the error message

finally:
    # close the db connection

In the above snippet:

• The outer with statement wraps a block that executes with the database connection instance as the context manager.
• The inner with statement wraps a set of queries that run with the database cursor as the context manager.

But how does this help?

When you use the connection object as a context manager:

• All successful transactions are automatically committed when the execution exits the outer with block. Note that the outer with block also wraps the creation of the cursor context manager to run the queries. So the set of queries represents a single transaction. If you need to run several hundred queries, you can create multiple connection context managers, each wrapping a database transaction.
• If there are exceptions due to one or more failing queries, changes are rolled back. Without the context manager, you’ll have to call the rollback() method on the connection object to do this.
• When the cursor object exits a with block, it is automatically closed. So you don’t have to close the cursor.

The version of the code using context managers can be found in the main2.py file.

Working with Psycopg2: A Quick Review

Let’s review all that we’ve learned in this tutorial:

• To connect to databases, psycopg2 provides the connect() convenience function. The connect() function takes in the details of the database—as keyword arguments—and returns a connection object if there are no errors.
• You can create a database cursor by calling the cursor() method on the connection object.
• To run database queries, call the execute() method on the cursor object. Specify the SQL query as a string, use placeholders for values, and pass in the values as tuples. To run multiple queries, you can use executemany(), but there are no known performance improvements.
• Fetch the results of the query by calling one of the fetch methods: fetchone(), fetchmany(), or fetchall() on the connection object.
• You can call the commit() and rollback() methods on the connection object to commit transactions to the database and roll back changes if there’s an error, respectively.
• When you run queries using the database connection as a context manager, successful transactions are automatically committed to the database. And changes are rolled back in the event of errors.

Conclusion

In this tutorial, you’ve learned the basics of querying PostgreSQL databases using psycopg2, including connection establishment, running queries, and exception handling. Now, you can expand your knowledge by adding more tables to your database, defining table relationships, and querying data from multiple tables.

And as you continue to build and expand your Python applications, you might encounter the need for consistent builds across different environments. For this, don’t forget to check out Earthly, a tool designed to ensure reproducibility and efficiency in your build process.