Test containers with Docker in PyTest - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a Docker container for testing using pytest.

PyTest

from testcontainers.mysql import MySqlContainer

def test_mysql_container():
    with MySqlContainer("mysql:8.0") as [1]:
        assert container.is_running

Drag options to blanks, or click blank then click option'

Aservice

Bclient

Cdocker

Dcontainer

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not represent the container instance.

Forgetting to assign the container to a variable.

The variable name 'container' is used to refer to the running MySQL container instance.

2fill in blank

medium

Complete the code to get the connection URL from the running MySQL container.

PyTest

from testcontainers.mysql import MySqlContainer

def test_connection_url():
    with MySqlContainer("mysql:8.0") as container:
        url = container.[1]()
        assert url.startswith("mysql://")

Drag options to blanks, or click blank then click option'

Aconnection_url

Bget_connection_url

Cget_url

Dconnection_string

Attempts:

3 left

💡 Hint

Common Mistakes

Using an attribute instead of a method.

Using incorrect method names like 'connection_url' without parentheses.

The method 'get_connection_url()' returns the connection URL string for the MySQL container.

3fill in blank

hard

Fix the error in the code to properly start a PostgreSQL test container.

PyTest

from testcontainers.postgres import PostgresContainer

def test_postgres():
    container = PostgresContainer("postgres:13")
    container.[1]()
    assert container.is_running
    container.stop()

Drag options to blanks, or click blank then click option'

Astart

Blaunch

Crun

Dbegin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'run()' or 'launch()' which are not valid methods.

Forgetting to call the method with parentheses.

The correct method to start the container is 'start()'. Other options are invalid and cause errors.

4fill in blank

hard

Fill both blanks to create a test that uses a Redis container and checks if it is running.

PyTest

from testcontainers.redis import RedisContainer

def test_redis_container():
    with RedisContainer() as [1]:
        assert [2].is_running

Drag options to blanks, or click blank then click option'

Aredis_container

Bredis

Ccontainer

Dclient

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the 'with' statement and assertion.

Using generic names like 'client' that do not represent the container.

The variable 'redis_container' is used consistently to refer to the Redis container instance inside the 'with' block.

5fill in blank

hard

Fill all three blanks to create a test that starts a MongoDB container, gets its connection URL, and asserts it contains 'mongodb://'.

PyTest

from testcontainers.mongodb import MongoDbContainer

def test_mongodb_connection():
    with MongoDbContainer("mongo:5.0") as [1]:
        url = [2].[3]()
        assert "mongodb://" in url

Drag options to blanks, or click blank then click option'

Amongo_container

Bmongo

Cget_connection_url

Dconnection_url

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for the container instance.

Using 'connection_url' as an attribute instead of a method.

Forgetting parentheses when calling the method.

The variable 'mongo_container' is used to hold the container instance. The method 'get_connection_url()' returns the connection URL string.

Practice

1. What is the main benefit of using test containers with Docker in pytest?

easy

A. They make tests run faster by skipping setup steps.

B. They replace the need for writing any test code.

C. They automatically fix bugs in the application code.

D. They provide real service environments during tests for better reliability.

Solution

1. Step 1: Understand test containers purpose

   Test containers run real services like databases inside Docker during tests.

2. Step 2: Identify benefit in pytest context

   This makes tests more reliable and realistic by using actual service environments.

3. Final Answer:

   They provide real service environments during tests for better reliability. -> Option D

4. Quick Check:

   Real service environment = Better test reliability [OK]

Hint: Remember: test containers run real services inside Docker [OK]

Common Mistakes:

• Thinking test containers replace writing tests
• Believing they fix code bugs automatically
• Assuming tests run faster by skipping setup

2. Which pytest fixture code correctly starts a Docker container for testing?

easy

A. def container():
container = docker.run('redis')
yield container
container.stop()

B. def container():
client = docker.from_env()
container = client.run('redis')
yield container
container.stop()

C. def container():
client = docker.from_env()
container = client.containers.run('redis', detach=True)
yield container
container.stop()

D. def container():
client = docker.from_env()
container = client.containers.run('redis')
container.start()
yield container
container.stop()

Solution

1. Step 1: Check correct Docker client usage

   Use docker.from_env() to get client, then client.containers.run() with detach=True to start container.

2. Step 2: Verify fixture lifecycle management

   Yield container for test, then stop container after test finishes.

3. Final Answer:

   def container(): client = docker.from_env() container = client.containers.run('redis', detach=True) yield container container.stop() -> Option C

4. Quick Check:

   Use client.containers.run with detach=True [OK]

Hint: Use client.containers.run with detach=True to start container [OK]

Common Mistakes:

• Calling client.run instead of client.containers.run
• Missing detach=True causing blocking call
• Not stopping container after test

3. Given this pytest fixture, what will be printed when running the test?

import pytest
import docker

@pytest.fixture
 def redis_container():
    client = docker.from_env()
    container = client.containers.run('redis:alpine', detach=True)
    yield container
    container.stop()

def test_redis_running(redis_container):
    print(redis_container.status)

medium

A. running

B. created

C. exited

D. paused

Solution

1. Step 1: Understand container lifecycle in fixture

   Container is started with detach=True, so status should be 'running' during test.

2. Step 2: Check printed status in test

   redis_container.status returns current container status, expected 'running' while test runs.

3. Final Answer:

   running -> Option A

4. Quick Check:

   Container started = status 'running' [OK]

Hint: Container status is 'running' while test uses it [OK]

Common Mistakes:

• Expecting 'created' before container starts
• Assuming container is 'exited' during test
• Confusing status with container image tag

4. Identify the error in this pytest fixture that manages a Docker container:

@pytest.fixture
def redis_container():
    client = docker.from_env()
    container = client.containers.run('redis', detach=True)
    yield container
    container.remove()

medium

A. Missing container.stop() before container.remove() to stop container properly.

B. Using container.remove() instead of container.delete() which is invalid.

C. Not specifying environment variables for Redis container causes failure.

D. Yielding container before starting it causes runtime error.

Solution

1. Step 1: Check container cleanup steps

   Container must be stopped before removal to avoid errors.

2. Step 2: Identify missing stop call

   Fixture calls container.remove() but misses container.stop() before it.

3. Final Answer:

   Missing container.stop() before container.remove() to stop container properly. -> Option A

4. Quick Check:

   Stop container before remove to clean up [OK]

Hint: Always stop container before removing it [OK]

Common Mistakes:

• Calling remove without stopping container
• Confusing remove() with non-existent delete()
• Assuming environment vars are mandatory for container start

5. You want to write a pytest fixture that starts a PostgreSQL container with Docker, waits until it is ready to accept connections, and then yields it for tests. Which approach correctly combines container management and readiness check?

hard

A. Start container without detach, yield immediately, and rely on test to wait for readiness.

B. Start container with detach=True, then poll container logs until 'database system is ready' appears before yielding.

C. Start container with detach=True and yield immediately without any readiness check.

D. Start container with detach=True, sleep fixed 1 second, then yield container.

Solution

1. Step 1: Manage container lifecycle properly

   Start PostgreSQL container detached to run in background during tests.

2. Step 2: Implement readiness check before yielding

   Poll container logs for 'database system is ready' message to ensure service is ready.

3. Step 3: Yield container after readiness confirmed

   This ensures tests run only after PostgreSQL is ready to accept connections.

4. Final Answer:

   Start container with detach=True, then poll container logs until 'database system is ready' appears before yielding. -> Option B

5. Quick Check:

   Wait for readiness log before yielding container [OK]

Hint: Wait for readiness log, don't guess with sleep [OK]

Common Mistakes:

• Yielding container before it is ready
• Using fixed sleep instead of log polling
• Starting container without detach causing blocking