`ops.Relation.load()` on unit data results in `JSONDecodeError: Extra data`

[dsloanm]

Description

When ops.Relation.load() is used to load from unit data into a data class, a json.decoder.JSONDecodeError: Extra data is raised.

I believe this is due to the default json.loads() decoder failing to interpret the standard values in the unit data, e.g. {'egress-subnets': '10.200.245.189/32', 'ingress-address': '10.200.245.189', 'private-address': '10.200.245.189'}.

To reproduce

The error can be reproduced by this charm code, which save()s then load()s unit data in a peer relation:

class PeerData(pydantic.BaseModel):
    myflag: bool = False

class ReloadIssueCharm(ops.CharmBase):
    """Charm the application."""

    def __init__(self, framework: ops.Framework):
        super().__init__(framework)
        framework.observe(self.on.start, self._on_start)

    def _on_start(self, _: ops.StartEvent):
        if peers := self.model.get_relation("peers"):
            save_data = PeerData(myflag=True)
            peers.save(save_data, self.unit)
            logger.info("Saved peer data: %s", save_data)

            load_data = peers.load(PeerData, self.unit)
            logger.info("Loaded peer data: %s", load_data)

The _on_start handler fails at peers.load(PeerData, self.unit):

`json.decoder.JSONDecodeError: Extra data` traceback

2026-07-01 09:58:38 INFO unit.reload-issue-charm/0.juju-log server.go:405 Saved peer data: myflag=True
2026-07-01 09:58:38 ERROR unit.reload-issue-charm/0.juju-log server.go:405 Uncaught exception while in charm code:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/src/charm.py", line 39, in <module>
    ops.main(ReloadIssueCharm)
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/__init__.py", line 360, in __call__
    return _main.main(charm_class=charm_class, use_juju_for_storage=use_juju_for_storage)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/_main.py", line 507, in main
    manager.run()
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/_main.py", line 491, in run
    self._emit()
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/_main.py", line 426, in _emit
    self._emit_charm_event(self.dispatcher.event_name)
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/_main.py", line 470, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/framework.py", line 337, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/framework.py", line 910, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/framework.py", line 1028, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/src/charm.py", line 32, in _on_start
    load_data = peers.load(PeerData, self.unit)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/juju/agents/unit-reload-issue-charm-0/charm/venv/lib/python3.12/site-packages/ops/model.py", line 1829, in load
    value = decoder(value)
            ^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/json/decoder.py", line 340, in decode
    raise JSONDecodeError("Extra data", s, end)
json.decoder.JSONDecodeError: Extra data: line 1 column 7 (char 6)

Or, more immediately:

>>> json.loads('10.200.245.189')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.12/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/json/decoder.py", line 340, in decode
    raise JSONDecodeError("Extra data", s, end)
json.decoder.JSONDecodeError: Extra data: line 1 column 7 (char 6)

Workaround

We are working around this with a custom decoder that interprets the values as strings:

def _unit_data_decoder(value: str) -> str:
    if not (value.startswith('"') and value.endswith('"')):
        value = f'"{value}"'
    return json.loads(value)

E.g. equivalent to json.loads('"10.200.245.189"').

Then load_data = peers.load(PeerData, self.unit, decoder=_unit_data_decoder) runs as expected:

2026-07-01 10:01:29 INFO unit.reload-issue-charm-workaround/0.juju-log server.go:405 Saved peer data: myflag=True
2026-07-01 10:01:29 INFO unit.reload-issue-charm-workaround/0.juju-log server.go:405 Loaded peer data: myflag=True

Expectation

ops.Relation.load() should be able to read unit data without without raising a JSONDecodeError and without need for a custom decoder.

[joelnguemeta]

I can reproduce this. The root cause is that Relation.load() decodes values before filtering by class fields, so json.loads runs on the raw (non-JSON) keys Juju sets on unit databags (ingress-address, etc.).

I'd like to work on this. PR with fix + regression test coming.