pytest-testcontainers 0.1.0

Named pytest fixtures and a maker convention on top of testcontainers-python.

pytest-testcontainers

Named pytest fixtures + a maker convention on top of testcontainers-python, plus clean-session "fresh DB per test" fixtures that get you per-test isolation without per-test container start.

testcontainers-python is a great library, but it ships no pytest fixtures and no pytest entry point. Every project that wants def test_db(pg): … ends up rewriting the same ~150 lines of conftest.py: session-scoped fixtures times five services, times correct xdist worker handling, times reuse mode with stable names and Ryuk disabled, times a normalized "Docker is not running" error.

pytest-testcontainers ships those 150 lines.

Why you might want this

• Trivial defaults: def test_x(tc_psql): … boots postgres:16 once per worker, gives you back a PostgresContainer, and tears it down at session end.
• Custom images in one line: write your own fixture against make_postgres(image="acme/pg-with-extensions:16") — the rest of your test code is unchanged because the maker returns the same upstream class your test already knows.
• Clean-session "fresh DB per test": tc_psql_db creates a test_<hex> database before the test and drops it after — same isolation as a per-test container, ~480× faster.
• Reuse mode for fast dev loops: --testcontainers-reuse keeps named containers alive between pytest runs (Ryuk disabled, stable per-worker names) so iterative dev doesn't pay the container-start tax every iteration.
• Normalized errors: "Docker daemon not reachable?" gets you a human-readable pytest.UsageError with remediation, not a wall of docker-py traceback.
• No magic: just @pytest.fixture decorators on top of the upstream classes. No env-var injection, no conftest re-import dance, no pytest_load_initial_conftests cleverness. Read the source in 15 minutes.

Installation

Using uv (recommended)

uv add --group dev pytest-testcontainers

Using pip

pip install pytest-testcontainers

For the clean-session fast path (auto-detects clients; no install needed for basic functionality):

# all clean-session fast paths in one shot
pip install pytest-testcontainers[clients]

# or pick the ones you need
pip install pytest-testcontainers[clients-postgres]
pip install pytest-testcontainers[clients-mysql]
pip install pytest-testcontainers[clients-mongo]

You also need a working Docker daemon — Docker Desktop, colima, Rancher Desktop, or Podman with the Docker socket all work. No platform-specific extra steps; if docker.from_env().ping() works, this plugin works.

Quickstart

def test_my_thing(tc_psql):
    host = tc_psql.get_container_host_ip()
    port = int(tc_psql.get_exposed_port(5432))
    # connect with your library of choice (psycopg, SQLAlchemy, asyncpg, ...)

That's the whole "hello world." tc_psql is the upstream PostgresContainer — every method/attribute on it works the same as when you build it yourself.

Services × variants

Each service ships three fixture variants plus a maker function:

Service	Maker	Session fixture	Function-scoped fixture	Clean-session fixture
Postgres	make_postgres	tc_psql	tc_psql_func	tc_psql_db
Redis	make_redis	tc_redis	tc_redis_func	tc_redis_clean
MySQL	make_mysql	tc_mysql	tc_mysql_func	tc_mysql_db
MongoDB	make_mongo	tc_mongo	tc_mongo_func	tc_mongo_db
RabbitMQ	make_rabbitmq	tc_rabbitmq	tc_rabbitmq_func	—
(any)	make_container(Cls, **kw)	—	—	—

• Session = one container per worker, lazy first-request start, shared across all tests. The right default.
• Function-scoped = fresh container per test. Almost always wrong; see the scope ladder below.
• Clean-session = session container + per-test fresh DB / keyspace. What most users who think they need a function-scoped fixture actually want.
• make_container(Cls, ...) is the escape hatch for services we don't ship a fixture for (Kafka, Localstack, your bespoke image).

Image overrides — write a custom fixture in one line

You don't override defaults in some config file — you write your own fixture using the maker:

import pytest
from pytest_testcontainers import make_postgres

# Postgres 18.
@pytest.fixture(scope="session")
def pg_18():
    with make_postgres(image="postgres:18") as pg:
        yield pg

# Specific minor version (pinning for reproducibility).
@pytest.fixture(scope="session")
def pg_pinned():
    with make_postgres(image="postgres:16.13") as pg:
        yield pg

# Custom user-built image (e.g. one with extensions baked in).
@pytest.fixture(scope="session")
def my_psql():
    with make_postgres(image="acme/pg-with-postgis:16") as pg:
        yield pg

# Custom image + extra env (image-specific tuning knobs).
@pytest.fixture(scope="session")
def my_psql_fast():
    with make_postgres(
        image="acme/pg-with-postgis:16",
        env={
            "POSTGRES_INITDB_ARGS": "--encoding=UTF8",
            "POSTGRES_HOST_AUTH_METHOD": "trust",
        },
    ) as pg:
        yield pg

# Custom credentials (must match what the app expects).
@pytest.fixture(scope="session")
def my_psql_creds():
    with make_postgres(
        image="acme/pg:16",
        username="appuser",
        password="appsecret",
        database="appdb",
    ) as pg:
        yield pg

Same pattern for make_redis, make_mysql, make_mongo, make_rabbitmq. Swapping the image is a one-line change; the rest of your test code is unchanged because the maker returns a vanilla testcontainers-python instance with the same API.

Override the built-in tc_psql directly

If you want one customized PG everywhere instead of writing a new fixture name, redefine tc_psql in your own conftest.py — pytest's nearest-conftest resolution takes the user's version and silently shadows the plugin's:

# conftest.py
import pytest
from pytest_testcontainers import make_postgres

@pytest.fixture(scope="session")
def tc_psql():
    with make_postgres(image="acme/pg:16", username="bpp", password="pw", database="bpp") as pg:
        yield pg

Container scope: when to use what

This is the educational core. Read this before reaching for tc_psql_func.

Default: session fixture (tc_psql)

One container per worker, shared across all tests. Cheapest, fastest. The right default.

Use for:

• Read-heavy tests.
• Tests that wrap their work in transactions and roll back (pytest-django's db fixture; SQLAlchemy savepoints).
• Tests that delete their own rows in teardown.

Don't use for:

• DDL-heavy tests (CREATE TABLE / ALTER TABLE / CREATE EXTENSION) unless rolled back inside a transaction.
• Tests that mutate global server state (ALTER SYSTEM SET …, replication slots, prepared statements that persist).

Clean-session: session container + fresh DB (tc_psql_db)

Same container as above, but each test gets a brand-new database created and dropped around it.

Use for:

• DDL-heavy suites (Django migration tests; schema-change tests).
• Tests that vacuum, reset sequences with autocommit, or do anything else that fights with transactional isolation.
• Replication slot / pg_stat_* tests where you want a clean view.

Cost: ~50–200ms per test for CREATE DATABASE / DROP DATABASE. Two orders of magnitude faster than per-test container start.

This is what most users who think they need a function-scoped container actually want.

Function-scoped container (tc_psql_func) — avoid

Fresh container per test. Available, strongly discouraged.

The math, with concrete numbers:

Container start = 3–5 seconds for Postgres. 100 tests × 5s = 8 minutes wasted on container startup. The clean-session pattern gives the same isolation ~480× faster (50ms per fresh DB vs 5s per fresh container).

Use ONLY when:

1. Test mutates global server config that can't be reset (ALTER SYSTEM SET … followed by a restart requirement).
2. Test corrupts container-level state (broken cluster, FS corruption, killed postmaster, etc.).
3. You're testing the testcontainer machinery itself.

If your reasoning is "I want isolation between tests," that's the clean-session pattern, not this.

Why we don't ship "shared DB + transaction rollback"

(Sometimes called "Variant B".) Doesn't work for:

• DDL tests (migrations, schema changes) — DDL implicit-commits in many SQL dialects, breaking the rollback-around-each-test model.
• VACUUM, sequence operations with autocommit.
• Replication state.
• Anything cross-connection.

If you genuinely want this pattern for your read-heavy suite, write your own fixture against tc_psql — but this plugin doesn't try to ship a one-size-fits-all transactional layer.

Surprising thing #1: xdist scope="session" is per-worker

If you only read one section, read this one.

Under pytest-xdist, each worker process runs its own pytest session. @pytest.fixture(scope="session") therefore means "one per worker", NOT "one for the whole pytest invocation". With -n 8, you get 8 containers, one per worker:

$ pytest -n 4 tests/                   # 4 workers
[gw0] container started: postgres on localhost:54321
[gw1] container started: postgres on localhost:54322
[gw2] container started: postgres on localhost:54323
[gw3] container started: postgres on localhost:54324

Many users assume the opposite (because "session" sounds like "global"). It isn't.

If you need one container shared across all workers, you need to start it before pytest forks workers — that's eager-start machinery that lives in pytest-testcontainers-django (if you're a Django user) or in a wrapper script:

# wrapper.sh — start once, pass coordinates to all workers
docker run -d --rm --name shared-pg \
    -e POSTGRES_PASSWORD=test \
    -p 54321:5432 postgres:16
export SHARED_PG_HOST=localhost
export SHARED_PG_PORT=54321
trap "docker rm -f shared-pg" EXIT
PYTEST_TESTCONTAINERS=0 pytest -n 8 tests/

# conftest.py — each worker reads env, no container started by us
import os, pytest

@pytest.fixture(scope="session")
def shared_pg():
    return {
        "host": os.environ["SHARED_PG_HOST"],
        "port": int(os.environ["SHARED_PG_PORT"]),
        "username": "postgres",
        "password": "test",
    }

PYTEST_TESTCONTAINERS=0 (or --no-testcontainers) disables our tc_* fixtures so they don't try to start their own per-worker container alongside yours.

Clean-session pattern walkthrough — tc_psql_db

tc_psql_db yields a DbConnInfo (host/port/user/pass/db) for a brand-new Postgres database created on the session-scoped tc_psql container. The DB is dropped after the test:

from pytest_testcontainers import DbConnInfo

def test_isolated_schema(tc_psql_db: DbConnInfo):
    # tc_psql_db.database is e.g. "test_a3f9b2c1d4e5"
    url = tc_psql_db.url()  # "postgresql://test:test@localhost:5xxxx/test_a3f9b2c1d4e5"

    import psycopg
    with psycopg.connect(**dataclasses.asdict(tc_psql_db)) as conn:
        conn.execute("CREATE TABLE widgets (id serial PRIMARY KEY)")
        # …
    # On teardown, DROP DATABASE … WITH (FORCE) wipes everything.

The DB name is test_<12-hex> (secrets.token_hex(6) — ~6e13 unique values per session, more than enough for any conceivable test count). Same shape for tc_mysql_db, tc_mongo_db. tc_redis_clean FLUSHALL-s the session Redis container after each test instead.

How admin commands run

We need to issue six single-shot commands behind the clean-session fixtures (CREATE DATABASE, DROP DATABASE, dropDatabase(), FLUSHALL). Two paths:

1. Python client fast path (~5ms/call) — used automatically when psycopg / pymysql / pymongo is importable.
2. docker exec fallback (~50–100ms/call) — used otherwise. Zero host-side client deps. Works out of the box.

Both raise the same CleanSessionFixtureError so user code catches one exception type. To get the fast path everywhere in one install:

pip install pytest-testcontainers[clients]

Or per-service: pytest-testcontainers[clients-postgres], [clients-mysql], [clients-mongo]. Redis is exec-only by design — for a single FLUSHALL, opening a TCP connection costs more than just exec-ing the in-container CLI.

The fallback emits a one-shot stderr advisory the first time it runs in a session; suppress with PYTEST_TESTCONTAINERS_QUIET=1.

Building custom services with make_container

For services we don't ship a maker for, use the generic escape hatch:

import pytest
from testcontainers.kafka import KafkaContainer
from pytest_testcontainers import make_container

@pytest.fixture(scope="session")
def tc_kafka():
    with make_container(KafkaContainer, image="confluentinc/cp-kafka:7.5.0") as k:
        yield k

Every plumbing concern — daemon ping, reuse name, atexit cleanup, Ryuk-disable-when-reuse — applies. args/kwargs go to the upstream constructor verbatim.

Reuse mode

For iterative dev loops where you don't want to pay container-start on every pytest run:

PYTEST_TESTCONTAINERS_REUSE=1 pytest tests/
# or
pytest --testcontainers-reuse tests/

What changes:

• Each container gets a stable name <project>-tc-<service>-<worker> (e.g. myproject-tc-psql-master). The project name comes from pyproject.toml [project].name.
• Ryuk (testcontainers' reaper) is disabled so the named containers survive between runs.
• On the next run we look up by name — found-and-running gets bound immediately; found-stopped gets restarted; not-found falls through to fresh start.

To clean up:

pytest --testcontainers-clean

This stops and removes all <project>-tc-* containers and exits with code 0 — a one-liner equivalent of the docker-py containers.list + remove(force=True) loop.

Two pytest invocations running concurrently: each must use its own PYTEST_TESTCONTAINERS_PROJECT to avoid name collisions. The plugin doesn't auto-namespace by PID — that would defeat the "reuse across runs" point.

Configuration

No TOML config table. The handful of toggles read at maker-call time:

Environment variables

Variable	Effect
PYTEST_TESTCONTAINERS=0	Disable plugin fixtures (raise UsageError).
PYTEST_TESTCONTAINERS_REUSE=1	Reuse named containers across runs.
PYTEST_TESTCONTAINERS_PROJECT=<name>	Override the <project> part of reuse names.
PYTEST_TESTCONTAINERS_NO_DAEMON_CHECK=1	Skip Docker daemon ping (rare).
PYTEST_TESTCONTAINERS_QUIET=1	Suppress one-shot informational advisories.

CLI flags

Flag	Same as
--no-testcontainers	PYTEST_TESTCONTAINERS=0
--testcontainers-reuse	PYTEST_TESTCONTAINERS_REUSE=1
--testcontainers-no-reuse	force fresh-each-run mode
--testcontainers-project=NAME	PYTEST_TESTCONTAINERS_PROJECT=NAME
--testcontainers-clean	prune <project>-tc-* and exit 0

CLI > env > defaults.

Error scenarios

When Docker is not running you get a normalized pytest.UsageError, not a wall of docker-py traceback:

[pytest-testcontainers] Docker daemon is not reachable.
Is Docker Desktop / colima / Rancher Desktop running?

Options:
  - start it and re-run pytest, OR
  - disable the plugin: --no-testcontainers (or PYTEST_TESTCONTAINERS=0), OR
  - point at remote Docker via DOCKER_HOST.

Underlying error: ...

When a stopped reused container can't be brought back (typically because the previously-mapped port is now held by something else):

[pytest-testcontainers] Cannot start existing container 'myproj-tc-psql-master': ...

Most common cause: another process now holds the port this
container was previously bound to. To start fresh:
    docker rm -f myproj-tc-psql-master
Or pass --testcontainers-no-reuse for this run.

When a clean-session admin command fails (CREATE DATABASE etc.), you get CleanSessionFixtureError with command, stderr, and (for the docker-exec path) exit_code — surfaced like any other in-test exception.

Comparison

Tool	Backend	Lifecycle	Notes
testcontainers-python	docker-py	manual	The library this plugin sits on. No fixtures.
pytest-testcontainers	testcontainers	session (lazy) / func	This. Fixtures, makers, clean-session fixtures.
pytest-docker-compose	compose	per-test (default)	Different abstraction. Complementary, not a dup.
pytest-docker	compose	per-test	Compose-driven; not testcontainers-driven.
pytest-container	testinfra	image-under-test	For testing image content, not dependency setup.

If you need intra-network communication between several testcontainers, a compose-based plugin is the better choice. Each service we start is reachable on its mapped host port — that's enough for >95% of test setups.

Supported versions

Python

Python	3.10	3.11	3.12	3.13
	✓	✓	✓	✓

CI runs the full test matrix on every supported version. 3.13 is the default we develop against.

pytest

pytest	7.x	8.x
	✓	✓

Tested separately in CI; floor is pytest>=7.4.

Other runtime requirements

• testcontainers-python ≥ 4.7 (modern wait_strategies API)
• docker-py ≥ 6.1 (we use docker.errors.NotFound, containers.exec_run(demux=True))
• A working Docker daemon: Docker Desktop / colima / Rancher Desktop / Podman with the Docker socket / remote Docker via DOCKER_HOST.

License

MIT — see LICENSE. Matches testcontainers-python's license.