Scenario > Event

In Scenario, the Event object corresponds to the homonym Juju data structure – a Juju event.

See more: Event (in Juju)

Like a Juju event, the scenario.Event data structure encapsulates some environment variables that the charm is being executed with and roughly represents the “reason why” the charm is being executed.

The fundamental piece of information carried by an Event is therefore the name of the ‘hook’ that the Juju controller has decided should be executed by this charm.

Basic usage

In many situations, the name of the event is enough for Scenario to work.

Look at the code snippet below:

from scenario import Context, State
from charm import MyCharm

ctx = Context(MyCharm)
ctx.run(event="update-status", state=State())

Context.run will first cast the string 'update-status' to an Event data structure. The following code is therefore equivalent:

from scenario import Event
...
ctx.run(event=Event("update-status"), state=State())

Event metadata: generic usage

Depending on the hook, certain additional environment variables need to be set for ops to function, just like the Juju unit agent guarantees them to be present when the charm is run for real.

In those situations, you cannot simply use strings to refer to events, but you need to provide an Event object to which you can pass some additional required metadata. Scenario’s runtime will use that information to populate the envvars that ops expects to be set.

For example, if you want to simulate relation events in Scenario, you need to:

from scenario import Event, Relation
...
relation = Relation('foo')
ctx.run(
    event=Event("foo-relation-changed", relation=relation), 
    state=State(relations=[relation])
)

The same holds for workload events and secret events.

Syntactic sugar

As a way to simplify the user experience, all event types that are associated to some operator data structure can be obtained from the data structure itself.

So instead of doing

relation = Relation('foo')
relation_changed_evt = Event('foo_relation_changed', relation=relation)

you can simply:

relation_changed_evt = Relation('foo').changed_event

And similarly for Container and Secret.