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Getting started with Testcontainers for Python

Learn how to create a Python application and test database interactions using Testcontainers for Python with a real PostgreSQL instance.

Table of contents

In this guide, you will learn how to:

  • Create a Python application that uses PostgreSQL to store customer data
  • Use psycopg to interact with the database
  • Write integration tests using testcontainers-python and pytest
  • Manage container lifecycle with pytest fixtures

Prerequisites

  • Python 3.10+
  • pip
  • A Docker environment supported by Testcontainers
Note

If you're new to Testcontainers, visit the Testcontainers overview to learn more about Testcontainers and the benefits of using it.

Create the Python project

Initialize the project

Start by creating a Python project with a virtual environment:

$ mkdir tc-python-demo
$ cd tc-python-demo
$ python3 -m venv venv
$ source venv/bin/activate

This guide uses psycopg3 to interact with the Postgres database, pytest for testing, and testcontainers-python for running a PostgreSQL database in a container.

Install the dependencies:

$ pip install "psycopg[binary]" pytest testcontainers[postgres]
$ pip freeze > requirements.txt

The pip freeze command generates a requirements.txt file so that others can install the same package versions using pip install -r requirements.txt.

Create the database helper

Create a db/connection.py file with a function to get a database connection:

import os

import psycopg


def get_connection():
    host = os.getenv("DB_HOST", "localhost")
    port = os.getenv("DB_PORT", "5432")
    username = os.getenv("DB_USERNAME", "postgres")
    password = os.getenv("DB_PASSWORD", "postgres")
    database = os.getenv("DB_NAME", "postgres")
    return psycopg.connect(f"host={host} dbname={database} user={username} password={password} port={port}")

Instead of hard-coding the database connection parameters, the function uses environment variables. This makes it possible to run the application in different environments without changing code.

Create the business logic

Create a customers/customers.py file and define the Customer class:

class Customer:
    def __init__(self, cust_id, name, email):
        self.id = cust_id
        self.name = name
        self.email = email

    def __str__(self):
        return f"Customer({self.id}, {self.name}, {self.email})"

Add a create_table() function to create the customers table:

from db.connection import get_connection


def create_table():
    with get_connection() as conn:
        with conn.cursor() as cur:
            cur.execute("""
                CREATE TABLE customers (
                    id serial PRIMARY KEY,
                    name varchar not null,
                    email varchar not null unique)
                """)
            conn.commit()

The function obtains a database connection using get_connection() and creates the customers table. The with statement automatically closes the connection when done.

Add the remaining CRUD functions:

def create_customer(name, email):
    with get_connection() as conn:
        with conn.cursor() as cur:
            cur.execute(
                "INSERT INTO customers (name, email) VALUES (%s, %s)", (name, email))
            conn.commit()


def get_all_customers() -> list[Customer]:
    with get_connection() as conn:
        with conn.cursor() as cur:
            cur.execute("SELECT * FROM customers")
            return [Customer(cid, name, email) for cid, name, email in cur]


def get_customer_by_email(email) -> Customer:
    with get_connection() as conn:
        with conn.cursor() as cur:
            cur.execute("SELECT id, name, email FROM customers WHERE email = %s", (email,))
            (cid, name, email) = cur.fetchone()
            return Customer(cid, name, email)


def delete_all_customers():
    with get_connection() as conn:
        with conn.cursor() as cur:
            cur.execute("DELETE FROM customers")
            conn.commit()
Note

To keep it straightforward for this guide, each function creates a new connection. In a real-world application, use a connection pool to reuse connections.

Write tests with Testcontainers

You'll create a PostgreSQL container using Testcontainers and use it for all the tests. Before each test, you'll delete all customer records so that tests run with a clean database.

Set up pytest fixtures

This guide uses pytest fixtures for setup and teardown logic. A recommended approach is to use finalizers to guarantee cleanup runs even if setup fails:

@pytest.fixture
def setup(request):
    # setup code

    def cleanup():
        # teardown code

    request.addfinalizer(cleanup)
    return some_value

Create the test file

Create a tests/__init__.py file with empty content to enable pytest auto-discovery.

Then create tests/test_customers.py with the fixtures:

import os
import pytest
from testcontainers.postgres import PostgresContainer

from customers import customers

postgres = PostgresContainer("postgres:16-alpine")


@pytest.fixture(scope="module", autouse=True)
def setup(request):
    postgres.start()

    def remove_container():
        postgres.stop()

    request.addfinalizer(remove_container)
    os.environ["DB_CONN"] = postgres.get_connection_url()
    os.environ["DB_HOST"] = postgres.get_container_host_ip()
    os.environ["DB_PORT"] = str(postgres.get_exposed_port(5432))
    os.environ["DB_USERNAME"] = postgres.username
    os.environ["DB_PASSWORD"] = postgres.password
    os.environ["DB_NAME"] = postgres.dbname
    customers.create_table()


@pytest.fixture(scope="function", autouse=True)
def setup_data():
    customers.delete_all_customers()

Here's what the fixtures do:

  • The setup fixture has scope="module", so it runs once for all tests in the file. It starts a PostgreSQL container, sets environment variables with the connection details, and creates the customers table. A cleanup function removes the container after all tests complete.
  • The setup_data fixture has scope="function", so it runs before every test. It deletes all records to give each test a clean database.

Write the tests

Add the test functions to the same file:

def test_get_all_customers():
    customers.create_customer("Siva", "siva@gmail.com")
    customers.create_customer("James", "james@gmail.com")
    customers_list = customers.get_all_customers()
    assert len(customers_list) == 2


def test_get_customer_by_email():
    customers.create_customer("John", "john@gmail.com")
    customer = customers.get_customer_by_email("john@gmail.com")
    assert customer.name == "John"
    assert customer.email == "john@gmail.com"
  • test_get_all_customers() inserts two customer records, fetches all customers, and asserts the count.
  • test_get_customer_by_email() inserts a customer, fetches it by email, and asserts the details.

Because setup_data deletes all records before each test, the tests can run in any order.

Run tests and next steps

Run the tests

Run the tests using pytest:

$ pytest -v

You should see output similar to:

============================= test session starts ==============================
platform linux -- Python 3.13.x, pytest-9.x.x
collected 2 items

tests/test_customers.py::test_get_all_customers PASSED                   [ 50%]
tests/test_customers.py::test_get_customer_by_email PASSED               [100%]

============================== 2 passed in 1.90s ===============================

The tests run against a real PostgreSQL database instead of mocks, which gives more confidence in the implementation.

Summary

The Testcontainers for Python library helps you write integration tests using the same type of database (Postgres) that you use in production, instead of mocks. Because you aren't using mocks and instead talk to real services, you're free to refactor code and still verify that the application works as expected.

In addition to PostgreSQL, Testcontainers for Python provides modules for many SQL databases, NoSQL databases, messaging queues, and more. You can use Testcontainers to run any containerized dependency for your tests.

To learn more about Testcontainers, visit the Testcontainers overview.

Further reading