An implementation of the Promises/A+ specification and test suite in Python
Latest commit ebe7d24 Jul 12, 2014 @mtiller mtiller Updated the readme
This acknowledges Adrian Kuendig and documents some of the improvements
he made.

What is This?

I've heard a lot about deferreds, futures and promises in working with Python, Scala and Javascript and I worked with deferreds in Python in conjunction with some work I did using Twisted. But then, I came across this talk (which is really great, you should watch it) about Promises and specifically Promises/A+ and I really wanted to have similar functionality in Python.


I can already hear Python users saying "But what about Deferred in Twisted?" or "But what about PEP 3148/PEP 3156 /concurrent.futures?"

Yes, I'm aware of those (and even the fact that futures have been backported to 2.x). But those implementations aren't really the same as Promises/A+. Not only are the APIs different, they also make certain assumptions about where the asyncronicity comes from which I thought was too specific. Also, because I'm doing stuff that involves both Python and Javascript, I thought it would be useful to have implementations of Promises on both sides that were conceptually the same. Finally, I liked the fact that Promises/A+ was a specification with a test suite so you can really support claims of conformance.

Getting Started

I'm not even going to begin to explain what I find so interesting about promises. As I said above, go watch this video and it will do a much better job than me of demonstrating the various use cases.

To use the aplus package, all you need to do is just type:

from aplus import Promise

From there, you can start creating promises (which will presumably be fulfilled or rejected in some asyncronous way). Note that this library doesn't constrain you at all as to how such promises are actually fulfilled or rejected. They could happen in threads, in responses to messages in a message queue, when an HTTP request gets a response, etc. The important part is that you can chain together these promises to notify you when they are completed or to chain together further computations.

Note, when a promise is rejected, it is necessary that the value passed to the rejection is a subclass of Exception.


I've added just a few extras beyond the Promises/A+ spec.


The first is that I added wait and get methods. This way, if you want to block (although I'm not sure why you would, apart from testing), you can. These methods also includea timeout argument so you can limit the amount of time you block.

Python Datatypes

I've also added to useful utility functions called listPromise and dictPromise. The listPromise function takes arguments that are Promise objects and returns to you a Promise for a list of values. In other words, it takes a list of promises for values and returns a promise for a list of values. Got that? :-)

Not surprisingly, the dictPromise method works in a similar way. It takes a dictionary of promises (as values, not keys) and returns a promise for a dictionary of values.

One could try and apply a monadic approach to such cases, but I just focused on these two collection types. If you feel that a monadic approach is required, I look forward to your pull request. :-)


It is possible to add callbacks to promises for pure notification purposes. To do this, use the addCallback(...) or addErrback(...) methods. These methods expect a function that takes a single argument (the fulfilled value in the case of callbacks and the reason for rejection in the case of errbacks).


Test Suite and Coverage

I attempted to create a test suite based on the Promises/A+ spec. I couldn't really reuse the existing Javascript test suite, so I pretty much made my own from scratch. It's possible that it is not a completely accurate reflection of the specification semantics, but I tried.

The good news is that the library passes the entire test suite (plus some extra tests for extended functionality). Furthermore, the test suite provides 100% coverage of the package source code.

One more thing...

The Promises/A+ specification includes a section numbered 3.2.4. This section is problematic when it comes to Python. It basically says that the then method should return a promise but that any callbacks triggered by the relationships that are being defined when calling the then method shouldn't fire until after the then method returns. This is possible in Javascript because there is a built-in event-loop/deferral mechanism in the language. This is not the case for Python. As such, I could not build a generic Python package that could conform to that section. The next best thing would be to use something like twisted which provides something like that. But I didn't want to chain people down to one particular asyncronous framework to utilize an otherwise general purpose capability.


Special thanks to Adrian Kuendig for putting in considerable effort to ensure that this library functions properly in the face of concurrency.