Test containers with Docker in PyTest - Build an Automation Script

Verify application connects to a PostgreSQL database running in a Docker container

Preconditions (3)

Step 1: Start a PostgreSQL container using testcontainers in the test setup

Step 2: Wait until the database is ready to accept connections

Step 3: Connect to the PostgreSQL database using connection details from the container

Step 4: Create a test table and insert a sample record

Step 5: Query the inserted record to verify data is stored correctly

Step 6: Stop and remove the PostgreSQL container after the test

✅ Expected Result: The test should pass confirming that the application can connect to the PostgreSQL container, create and query data successfully.

Automation Requirements - pytest

Assertions Needed:

Assert that the inserted record is retrieved correctly from the database

Best Practices:

Use testcontainers Python library to manage Docker container lifecycle

Use explicit waits or retries to ensure database readiness

Use pytest fixtures for setup and teardown

Avoid hardcoding connection details; get them dynamically from the container

Keep tests isolated and clean up resources after test

Automated Solution

PyTest

import pytest
import psycopg2
from testcontainers.postgres import PostgresContainer

@pytest.fixture(scope='module')
def postgres_container():
    with PostgresContainer('postgres:15') as postgres:
        yield postgres


def test_postgres_connection(postgres_container):
    conn = psycopg2.connect(
        dbname=postgres_container.POSTGRES_DB,
        user=postgres_container.POSTGRES_USER,
        password=postgres_container.POSTGRES_PASSWORD,
        host=postgres_container.get_container_host_ip(),
        port=postgres_container.get_exposed_port(postgres_container.port)
    )
    cur = conn.cursor()
    cur.execute('CREATE TABLE test_table (id SERIAL PRIMARY KEY, name VARCHAR(50));')
    cur.execute("INSERT INTO test_table (name) VALUES ('testname') RETURNING id;")
    inserted_id = cur.fetchone()[0]
    conn.commit()

    cur.execute('SELECT name FROM test_table WHERE id = %s;', (inserted_id,))
    result = cur.fetchone()[0]

    cur.close()
    conn.close()

    assert result == 'testname'

This test uses the testcontainers library to start a PostgreSQL Docker container automatically before the test and stop it after.

The postgres_container fixture manages the container lifecycle with scope='module' so it runs once per test module.

Inside the test, it connects to the database using connection info provided by the container object, avoiding hardcoded values.

It creates a table, inserts a record, and queries it back to verify the database is working.

Assertions check that the retrieved data matches the inserted data.

This approach ensures the test is isolated, repeatable, and cleans up resources properly.

Common Mistakes - 4 Pitfalls

Hardcoding database connection details instead of using container properties

Not waiting for the database to be ready before connecting

Not cleaning up the container after tests

Using global or session scope for container without isolation

Bonus Challenge

Now add data-driven testing with 3 different names inserted and verified in the database

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