docs: def modify_process_property(self, key, value, pid=None):
          '''
          modify_process_property(self, key, value, pid=None)
  
          Modify process output property.
          Please note that the process property key provided must be declared as an output property in the relevant service specification.
  
          :Parameters:
          * *key* (`String`) -- key of property to modify
          * *key* (`value`) -- value of property to modify
          * *pid* (`string`) -- Identifier of an existing process
  
          :Example:
          .. code-block:: python
  
             pid = opereto_client.create_process(service='simple_shell_command', title='Test simple shell command service')
             opereto_client.modify_process_property("my_output_param", "1" , pid)
  
          '''
          pid = self._get_pid(pid)
          request_data={"key" : key, "value": value}
          return self._call_rest_api('post', '/processes/'+pid+'/output', data=request_data, error='Failed to modify output property [%s]'%key)