Twisted (the asynchronous framework for Python) has an equivalent structure called a "Deferred": http://twistedmatrix.com/documents/current/core/howto/defe

Twisted (the asynchronous framework for Python) has an equivalent structure called a "Deferred": http://twistedmatrix.com/documents/current/core/howto/defer.html It's very similar to the "Promises/A" system described in the article, except that it has different method names (instead of .then(), it has the more prosaic .addCallback() and .addErrback()), and you generally have only one of them (instead of the callback-adding function returning a new, fresh, clean instance). Deferreds have the same ideal of composability, and the same mapping to 'try/except' statements in synchronous code. However, there's one particularly handy thing Deferreds do that I don't see described in the article: when "fulfilledHandler" gets called, it can return a value which gets passed to the next callback in the chain, _or it can return a Deferred_, in which case the callback chain pauses until the new Deferred fires and produces a value, at which point the original Deferred picks up and carries on. This comes in very handy for things like this: def getReservedBooksForUser(username): # Look up the auth token for the library service in the DB authTokenDeferred = db.runQuery(""" SELECT authToken FROM users WHERE username = ? """, (username,)) # We know the DB result can only have one row and one column. authTokenDeferred.addCallback(lambda resultset: resultset[0][0]) # When we have the auth token, use it when querying the library # service. authTokenDeferred.addCallback( lambda authToken: web.getPage( "http://library/books?authToken=%s&user=%s&format=JSON" % (authToken, request["username"]) ) ) # Extract the list of books from the returned document. authTokenDeferred.addCallback( lambda page: json.loads(page)['books'] ) return authTokenDeferred Note that the second callback (the web.getPage() one) is immediately called with the results of the first callback (the resultset-unpacking one), because the first callback returned an ordinary value. However, the third callback (the JSON-unpacking one) is not called with the Deferred returned by the second callback - instead, the callback chain is paused while we wait for the second callback's Deferred to fire, then that result is passed to the third callback. Is this kind of thing supported by JS Promises, too?