Attributes lost after subsequent after_update rule – help merging sensor attributes

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Hi everyone,

I’m developing a custom integration for Modbus sensors that postprocess CSV values. The goal is to update sensor state and also store the timestamp of the last update as an attribute. I have an after_update rule that sets an attribute (for example, "A1B2C3D4E5F6_last_hourly": "2025021715"), and the log confirms that it’s set correctly. However, when a subsequent rule (such as a calculation rule) runs, the attributes from the previous update seem to be lost, leaving only the attributes from the latest update.

Here’s a snippet of my helper function for updating the target sensor state and attributes:

def _set_target_attribute(self, attribute: str, value: Any) -> None:
    """Update the target sensor's state and set an attribute value."""
    try:
        self._target_sensor = self._hass.states.get(self._target)
        # Make a copy of the current attributes
        new_attributes = dict(self._target_sensor.attributes)
        # Update with the new attribute
        new_attributes[attribute] = value
        # Set the state with the merged attributes
        self._hass.states.set(self._target, self._target_sensor.state, new_attributes)
        _LOGGER.debug("AUR: Set attribute '%s' to '%s' on target '%s'.", attribute, value, self._target)
    except Exception as exc:
        _LOGGER.error("AUR: Error setting attribute '%s' for target '%s': %s", attribute, self._target, exc)

And here’s part of the log output:

2025-02-17 15:20:41.684 DEBUG ... New attributes: {
    'icon': 'mdi:calendar-today',
    'friendly_name': 'burner_pellets_counter_hour',
    'unit_of_measurement': 'rotations/hour',
    'precision': 2,
    'previous_value': None,
    'A1B2C3D4E5F6_last_hourly': '2025021715'
}
2025-02-17 15:20:41.713 DEBUG ... New attributes: {
    'icon': 'mdi:calendar-today',
    'friendly_name': 'burner_pellets_counter_hour',
    'unit_of_measurement': 'rotations/hour',
    'precision': 2,
    'previous_value': None,
    'A1B2C3D4E5F6_last_calculation': '20250217152041696097'
}

As you can see, after the hourly update the attribute "A1B2C3D4E5F6_last_hourly" is set, but then when the calculation rule runs, only "A1B2C3D4E5F6_last_calculation" is in the attribute dictionary. It appears that the new update is overwriting the entire attributes dictionary, rather than merging with the previous values.

Question:
Is there a way to update the sensor’s attributes so that subsequent updates merge with (and preserve) existing attributes? I want each after_update rule to add or update its own attribute without losing previously stored attributes.

Any help or suggestions would be greatly appreciated!

Thanks in advance.

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