The method Collection.action is meant to allow access to the ArangoDB HTTP endpoints that do not have a dedicated method in Collection. However, the method always sends an empty request body. Here's the code:

def action(self, method, action, **params):
        """a generic fct for interacting everything that doesn't have an assigned fct"""
        fct = getattr(self.connection.session, method.lower())
        r = fct(self.getURL()   "/"   action, params = params)
        return r.json()

In the fourth line above, the session's HTTP method is called without a data argument. Everything aside from the URL is passed to the HTTP method through the params argument, which the method would treat as an additional keyword argument, leaving data with its default value (which is None). Consequently, none of the POST or PUT endpoints work using this method (I was trying to use the PUT /_api/collection/{collection-name}/properties endpoint when I discovered this error).

To fix the issue, params['data'] should be passed to the data argument, and the rest of params should be passed as kwargs.