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Exception messages are translatable

Reasoning​

Sometimes something goes wrong and we want to show an error message to the user. Since Home Assistant is used by people all over the world, it is important that these error messages are translatable. This increases the usability of Home Assistant for people who do not use the application in English.

Home Assistant has builtin support for translating messages coming from the HomeAssistantError exception.

Example implementation​

In this example, we show a function registered as a Home Assistant service action. The integration domain and the key to the translation are passed along when raising the exception. The exception should inherit HomeAssistantError to support translations. The error message is then defined in the integration strings.json file.

async def async_set_schedule(call: ServiceCall) -> ServiceResponse:
    """Set the schedule for a day."""
    start_date = call.data[ATTR_START_DATE]
    end_date = call.data[ATTR_END_DATE]
    if end_date < start_date:
        raise ServiceValidationError(
            translation_domain=DOMAIN,
            translation_key="end_date_before_start_date",
        )
    try:
        await client.set_schedule(start_date, end_date)
    except MyConnectionError as err:
        raise HomeAssistantError(
            translation_domain=DOMAIN,
            translation_key="cannot_connect_to_schedule",
        ) from err

strings.json:

{
    "exceptions": {
        "end_date_before_start_date": "The end date cannot be before the start date.",
        "cannot_connect_to_schedule": "Cannot connect to the schedule."
    }
}

Additional resources​

For more info on raising exceptions, check the documentation.

Exceptions​

There are no exceptions to this rule.