pytest-pubsub

In-memory mock of Google Cloud Pub/Sub topics and subscriptions, for use in tests. No network access, no credentials, no emulator process required. Compatible with Python 3.9+.

Install

pip install pytest-pubsub

How it works

Everything is backed by a single in-memory PubSubBroker that tracks topics, subscriptions, and their pending/in-flight messages. Publishing to a topic fans the message out to every subscription attached to it, matching real Pub/Sub delivery semantics. Nothing touches the network — the broker is just plain Python data structures.

Errors mirror google.api_core.exceptions: operating on a topic or subscription that doesn't exist raises NotFound; creating one that already exists raises AlreadyExists. Code written against the real client's exception handling works unchanged against the mock.

Publisher usage

MockPublisherClient implements the subset of google.cloud.pubsub_v1.PublisherClient used in practice — topic_path, create_topic/get_topic/delete_topic/list_topics, and publish. It's constructed with an explicit PubSubBroker and passed into your code via dependency injection — nothing is monkeypatched globally:

from pytest_pubsub.publisher import MockPublisherClient
from pytest_pubsub.registry import PubSubBroker

broker = PubSubBroker()
client = MockPublisherClient(broker)

topic_path = client.topic_path("my-project", "events")
client.create_topic(name=topic_path)

future = client.publish(topic_path, b"hello", source="test")
message_id = future.result()

Subscriber usage

MockSubscriberClient implements the subset of google.cloud.pubsub_v1.SubscriberClient used in practice — subscription_path, create_subscription/get_subscription/ delete_subscription, synchronous pull, acknowledge, modify_ack_deadline, and a simplified subscribe() for streaming-pull-style consumers:

from pytest_pubsub.registry import PubSubBroker
from pytest_pubsub.publisher import MockPublisherClient
from pytest_pubsub.subscriber import MockSubscriberClient

broker = PubSubBroker()
publisher = MockPublisherClient(broker)
subscriber = MockSubscriberClient(broker)

topic_path = publisher.topic_path("my-project", "events")
publisher.create_topic(name=topic_path)

sub_path = subscriber.subscription_path("my-project", "events-sub")
subscriber.create_subscription(name=sub_path, topic=topic_path)

publisher.publish(topic_path, b"hello")

response = subscriber.pull(subscription=sub_path, max_messages=1)
message = response.received_messages[0].message
assert message.data == b"hello"
message.ack()

Or with a streaming-pull-style callback:

def handle(message):
    print(message.data)
    message.ack()

future = subscriber.subscribe(sub_path, handle)
# ... later
future.cancel()
future.result(timeout=1)

pytest fixtures

Installing the package registers a pytest plugin (via the pytest11 entry point), so these fixtures are available in any test suite with no extra configuration:

No monkeypatching happens automatically. Your code under test should accept a client instance (constructor injection, a factory function, etc.) so the fixture can be passed in explicitly:

# app/publisher.py — code under test, accepts an injected client
def send_event(client, topic_path: str, payload: bytes):
    future = client.publish(topic_path, payload)
    return future.result()

# tests/test_publisher.py
def test_send_event(mock_publisher_client, mock_subscriber_client, pubsub_broker):
    topic_path = mock_publisher_client.topic_path("proj", "events")
    mock_publisher_client.create_topic(name=topic_path)

    sub_path = mock_subscriber_client.subscription_path("proj", "events-sub")
    mock_subscriber_client.create_subscription(name=sub_path, topic=topic_path)

    from app.publisher import send_event
    message_id = send_event(mock_publisher_client, topic_path, b"hello")
    assert message_id

    response = mock_subscriber_client.pull(subscription=sub_path, max_messages=1)
    assert response.received_messages[0].message.data == b"hello"
    mock_subscriber_client.acknowledge(
        subscription=sub_path,
        ack_ids=[response.received_messages[0].ack_id],
    )

Limitations / non-goals

• Client-level mock only. There is no fake gRPC/HTTP server — this does not emulate the Pub/Sub emulator wire protocol, so it's not a drop-in for tests that talk to the emulator over the network. Code under test must accept a client instance instead of constructing google.cloud.pubsub_v1.PublisherClient/SubscriberClient directly.
• No automatic ack-deadline expiry. pull/acknowledge are synchronous and in-process, so messages never redeliver on their own after a deadline; use message.nack() or modify_ack_deadline(..., ack_deadline_seconds=0) to force redelivery in a test.
• No IAM, retention policies, dead-lettering, or push subscriptions.

Running the tests

pip install -e ".[dev]"
pytest

Linting and type checking

pip install -e ".[dev]"
pre-commit install   # runs ruff and mypy automatically on every commit
pre-commit run --all-files

CI (.github/workflows/ci.yml) runs ruff check, ruff format --check, mypy, and the test suite across Python 3.9–3.13 on every pull request and on every push to main.

Contributing

Issues and pull requests are welcome. Please include tests for any behavior change — every source change in this repo ships with its own tests in the same commit.

License

MIT