Airflow UI link in SlackAPIPostOperator?

You can build the url to the UI with the config value base_url under the [webserver] section and then use Slack's message format <http://example.com|stuff> for links.

from airflow import configuration

def slack_failed_task(context):
    link = '<{base_url}/admin/airflow/log?dag_id={dag_id}&task_id={task_id}&execution_date={execution_date}|logs>'.format(
        base_url=configuration.get('webserver', 'BASE_URL'),
        dag_id=context['dag'].dag_id,
        task_id=context['task_instance'].task_id,
        execution_date=context['ts']))  # equal to context['execution_date'].isoformat())

    failed_alert = SlackAPIPostOperator(
        task_id='slack_failed',
        channel="#mychannel",
        token="...",
        text=':red_circle: Failure on: ' + 
             str(context['dag']) +
             '\nRun ID: ' + str(context['run_id']) +
             '\nTask: ' + str(context['task_instance']) +
             '\nSee ' + link + ' to debug')
    return failed_alert.execute(context=context)