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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def get_container_id(self, container_id=None):
        ''' a helper function shared between functions that will return a 
            container_id. First preference goes to a container_id provided by
            the user at runtime. Second preference goes to the container_id
            instantiated with the client.

            Parameters
            ==========
            container_id: image uri to parse (required)
        '''

        # The user must provide a container_id, or have one with the client
        if container_id == None and self.container_id == None:
            bot.exit('You must provide a container_id.')

        # Choose whichever is not None, with preference for function provided
        container_id = container_id or self.container_id
        return container_id