charmhelpers.core.hookenv is a servicable organically grown library/framework for doing charming. Using it and charming in general, I've desired to improve it in a few ways:

• create a single entry point to interacting with live hook execution environment.
• Have a syntonic environment I can explore and discover more about the possibilities of what I can do in charm.
• Have an easy way to write tests without unexpected sideeffects.
• Have a charming system that is naturally easy to extend to fit different usecases

Charms currently require 3 forms of interaction with their environment: filesystem, environmental variable, and jujud based tools. Charmer may add more subproccess or system calls atop these basic dependencies, but charm.base aims to make it easy to avoid sideeffects for testing and development outside of the context of a charm.

The entry point to the charm is the execution context.

Here's how to get a basic one to play with:

>>> from charmbase import execution_context
>>> from charmbase.model.tests import here
>>> fake_env = dict(
...     JUJU_AGENT_SOCKET="@/var/lib/juju/agents/unit-test-0/agent.socket",
...     JUJU_API_ADDRESSES="172.31.1.40:17070",
...     JUJU_AVAILABILITY_ZONE="",
...     JUJU_CHARM_DIR='ignore',
...     JUJU_CONTEXT_ID="test/0-install-1002841046484389285",
...     JUJU_DEBUG="/tmp/tmp.2esNGFMfdX",
...     JUJU_ENV_NAME="ec2-va-1",
...     JUJU_ENV_UUID="b8ba8824-16f2-43b0-8b10-5b1dfa063906",
...     JUJU_HOOK_NAME="install",
...     JUJU_MACHINE_ID="1",
...     JUJU_METER_INFO="",
...     JUJU_METER_STATUS="NOT SET",
...     JUJU_UNIT_NAME="test/0")
>>> xc = execution_context(environment=fake_env, charmdir=here / 'basecharm')

The execution context provides direct access to any JUJU prefixed environmental variables:

>>> xc.hook_name 'install'

>>> xc.machine_id '1'

As well as conveniences derived from these vars:

>>> xc.service_name 'test'

The creation of the exection context allows for us to pass in a charmdir and rootdir (or it will use a temporary directory for both as a convenience):

>>> xc.charmdir.name
Path(u'basecharm')

>>> xc.rootdir.startswith('/tmp')
True

This allows for non-destructive testing of filesystem operations such as adding and remove files in /etc.

Both of these are path objects, giving easy access to a variety of operations common to charming :

>>> [string(x.name) for x in xc.charmdir.files()]

>>> xc.charmdir.metadata.text()

The execution context is a root node of a simple tree data structure. While a number of attributes are exposed as a convenience to the root node, the root also serves as a reference to other namespaces as a way of logical organizing charming tasks.

Namespaces are registered roughly around files concerns or tools: config, metadata, juju (log and reboot), relation, unit, leader, storage, status. Tool namespace may register specific functions that wrap jujud calls, and if they implement the function signature correctly, they may be automatically mocked.

Let's demonstrate a simple example. We'll add leader_yolo to our leader namespace:

>>> from charmbase.model.tools import Leader
>>> @Leader.register_tool("leader_yolo")
>>> def is_leader(command="is-leader", runner=subprocess.check_output):
...     return runner([command])

Now, we will load our execution environment with an argument add_includes. This argument loads a special callable that will replace all of our tools runners (ie subprocess calls) with mock objects. :

>>> xc = execution_context(environment=fake_env, charmdir=here / 'basecharm', add_includes="charmbase.model.test.runner_mocks")

We can now manipulate the output of the new tool:

>>> xc.runner_mocks.leader.lead_yolo.return_value = "YOLO" >>> xc.leader.lead_yolo() "YOLO"

We can now manipulate the output of an implicitly added tool:

>>> xc.runner_mocks.config.get_state.return_value = "'hey'" >>> xc.config.get_state(key="wat") 'hey'

We can clean up our class tree registrations so our tests are isolated:

>>> xc.runner_mock_cleanup(ns)

So in one example, we've seen:

• how to simply manage "tool" subprocess mocking (avoid stacks of mock.patch decorator or sideeffect whackamole)
• arbitrary alteration of the object structure at initialization time
• simple extension of namespaces by decorator (to a sideeffecting function)

The current api provides decorators for register child namespaces to existing namespaces (available to any subclass of charmbase.model.namespace), and a decorator for registering tools (available to any subclass of charmbase.model.namespace.ToolNamespace). charmbase.model.context.ExecutionContext is a tool namespace and may register both tools and child namespaces.