How to manage resources

See also: Juju | Charm resource, Juju | Manage charm resources, Charmcraft | Manage resources

Implement the feature

In your charm’s src/charm.py file, use ops to fetch the path to the resource and then manipulate it as needed.

For example, suppose your charmcraft.yaml file contains this simple resource definition:

resources:
  my-resource:
    type: file
    filename: somefile.txt
    description: test resource

In your charm’s src/charm.py you can now use Model.resources.fetch(<resource_name>) to get the path to the resource, then manipulate it as needed. For example:

# ...
import logging
import ops
# ...
logger = logging.getLogger(__name__)

def _on_config_changed(self, event):
    # Get the path to the file resource named 'my-resource'
    try:
        resource_path = self.model.resources.fetch("my-resource")
    except ops.ModelError as e:
        self.unit.status = ops.BlockedStatus(
            "Something went wrong when claiming resource 'my-resource; "
            "run `juju debug-log` for more info'"
        )
       # might actually be worth it to just reraise this exception and let the charm error out;
       # depends on whether we can recover from this.
        logger.error(e)
        return
    except NameError as e:
        self.unit.status = ops.BlockedStatus(
            "Resource 'my-resource' not found; "
            "did you forget to declare it in charmcraft.yaml?"
        )
        logger.error(e)
        return

    # Open the file and read it
    with open(resource_path, "r") as f:
        content = f.read()
    # do something

The fetch() method will raise a NameError if the resource does not exist, and returns a Python Path object to the resource if it does.

Note: During development, it may be useful to specify the resource at deploy time to facilitate faster testing without the need to publish a new charm/resource in between minor fixes. In the below snippet, we create a simple file with some text content, and pass it to the Juju controller to use in place of any published my-resource resource:

echo "TEST" > /tmp/somefile.txt
charmcraft pack
juju deploy ./my-charm.charm --resource my-resource=/tmp/somefile.txt

Test the feature

See first: How to write unit tests for a charm

If your charm requires access to resources, you can make them available to it through State.resources. For example, to make a foo resource that is a path to an OCI image available:

import pathlib

from ops import testing

ctx = testing.Context(MyCharm, meta={'name': 'julie', 'resources': {'foo': {'type': 'oci-image'}}})
resource = testing.Resource(name='foo', path='/path/to/resource.tar')
with ctx(ctx.on.start(), testing.State(resources={resource})) as mgr:
    path = mgr.charm.model.resources.fetch('foo')
    assert path == pathlib.Path('/path/to/resource.tar')