To a certain degree, I share your confusion - ordinarily you wouldn't expect the result of a database query to be stored within an object, however this may be the author's attempt at memoization whereby the results of a previous query are stored for efficiency reasons.

That being said, the code snippet gives no clues as to the rationale behind the existence of the method e.g. why is the result of a get_sites() invocation not implemented within the constructor since it always returns a list of all sites i.e. takes no arg(s) that might constrain the result set in some way - unless it's available purely and simply to refresh the result set.

Of course, as has been said elsewhere, the memoization directly raises it's own problems e.g. caching, and so is not a really sensible way of working; The method should, IMO, merely return the result set from the query, letting the caller determine to what use it should be put - by doing so it would provide the necessary degree of abstraction between the structure of the query on the backend DB and the data returned from the query - a la the GoF Facade Pattern.