How to write a Custom Action in mistral

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- [ ] This is a doc addition request.
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example runner will raise this exception when using mistral run-action:

# mistral run-action example.runner '{"param": 1}'
{"result": "The action raised an exception [action_ex_id=None, action_cls='<class 'mistral.actions.action_factory.RunnerAction'>', attributes='{}', params='{'param': 1}']\n run() takes 1 positional argument but 2 were given"}

fix:
from mistral_lib import actions

class RunnerAction(actions.Action):
    def __init__(self, param):
        # store the incoming params
        super(RunnerAction, self).__init__()
        self.param = param

    def run(self, context):
        # return your results here
        return {'status': 0}

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-----------------------------------
Release: 8.1.0.dev129 on 2018-07-23 14:14:40
SHA: f75e7198c61da0547ab6ab7e96c797ac5c6657da
Source: https://opendev.org/openstack/mistral/src/doc/source/contributor/creating_custom_action.rst
URL: https://docs.openstack.org/mistral/latest/contributor/creating_custom_action.html