ops-scenario

ops-scenario, the unit testing framework for ops charms

ops-scenario is a Python library that provides state-transition testing for Ops charms. These tests are higher level than typical unit tests, but run at similar speeds and are the recommended approach for testing charms within requiring a full Juju installation.

Test are written in the arrange/act/assert pattern, arranging an object representing the current Juju state, acting by emulating an event from Juju, and then asserting on the (simulated) output Juju state.

Writing tests

Here's a test that verifies that a unit is active after the start event, with a very minimal initial state:

from ops import testing

# 'src/charm.py' typically contains the charm class.
from charm import MyCharm

def test_start():
    ctx = testing.Context(MyCharm)
    state_in = testing.State()
    state_out = ctx.run(ctx.on.start(), state_in)
    assert state_out.unit_status == testing.ActiveStatus()

More comprehensive tests will include relations, containers, secrets, and other components in the input state, and assertions against both the output state and the context. The 'act' stage remains a simple single call, although additional arguments may be required for the event, such as the relation or container that triggered it. For example:

import pytest
from ops import testing

from charm import MyCharm

@pytest.mark.parametrize(
    'leader',
    [pytest.param(True, id='leader'), pytest.param(False, id='non-leader')],
)
def test_(leader: bool):
    # Arrange:
    ctx = testing.Context(MyCharm)
    relation = testing.Relation('db', local_app_data={'hostname': 'example.com'})
    peer_relation = testing.PeerRelation('peer')
    container = testing.Container('workload', can_connect=True)
    relation_secret = testing.Secret({'certificate': 'xxxxxxxx'})
    user_secret = testing.Secret({'username': 'admin', 'password': 'xxxxxxxx'})
    config = {'port': 8443, 'admin-credentials': 'secret:1234'}
    state_in = testing.State(
        leader=leader,
        config=config,
        relations={relation, peer_relation},
        containers={container},
        secrets={relation_secret, user_secret},
        unit_status=testing.BlockedStatus(),
        workload_version='1.0.1',
    )

    # Act:
    state_out = ctx.run(ctx.on.relation_changed(relation), state_in)

    # Assert:
    assert testing.JujuLogLine(level='INFO', message='Distributing secret.') in ctx.juju_log
    peer_relation_out = state_out.get_relation(peer_relation.id)
    assert peer_relation_out.peers_data[0] == {'secret_id': relation_secret.id}

You don't have to use pytest for your charm tests, but it's what we recommend. pytest's assert-based approach is a straightforward way to write tests, and its fixtures are helpful for structuring setup and teardown.

Installation

For charm tests, install the testing framework by adding the testing extra of ops in your unit testing environment. For example, in pyproject.toml:

[dependency-groups]
test = ['ops[testing]<4.0']

Ops checks if ops-scenario is installed, and, if so, makes the classes (such as Context, State, and Relation) available in the ops.testing namespace. Use from ops import testing rather than importing the scenario package.

ops-scenario supports the same platforms and Python versions as ops itself.

Documentation

• To get started, work through our 'Write your first Kubernetes charm' tutorial, following the instructions for adding unit tests at the end of each chapter.
• When you need to write a test that involves specific ops functionality, refer to our how-to guides which all conclude with examples of tests of the ops functionality.
• Use our extensive reference documentation when you need to know how each testing object works. These docs are also available via the standard Python help() functionality and in your IDE.

Read the full documentation

Community

ops-scenario is a member of the Charming family. It's an open source project that warmly welcomes community contributions, suggestions, fixes and constructive feedback.

• Read our code of conduct: As a community we adhere to the Ubuntu code of conduct.
• Get support: Discourse is the go-to forum for all Ops-related discussions, including around testing.
• Join our online chat: Meet us in the #charmhub-charmdev channel on Matrix.
• Report bugs: We want to know about the problems so we can fix them.
• Contribute docs: Get started on GitHub.

Contributing and developing

Anyone can contribute to ops and ops-scenario. It's best to start by opening an issue with a clear description of the problem or feature request, but you can also open a pull request directly.

Read our guide for more details on how to work on and contribute to ops-scenario.

Currently, releases of ops-scenario are done in lockstep with releases of ops itself, with matching minor and bugfix release numbers. The ops documentation outlines how to create a new release.