PyPromise i open source (New BSD License) Python Promise implementation.

Simplifies working with asynchronous code and unifies way asynchronous and synchronous code is called.

Download the latest release from http://github.com/michalbachowski/pypromise

tar xvzf pypromise-$VERSION.tar.gz
cd pypromise-$VERSION
python setup.py build
sudo python setup.py install

The PyPromise source code is hosted on GitHub: http://github.com/michalbachowski/pypromise

It is also possible to simply add the PyPromise directory to your PYTHONPATH instead of building with setup.py, since it uses only standard parts of Python library.

1. Instantinate promise.Deferred
2. attach as many callbacks (both done or fail) as you want
3. wait for resolution/rejection (eg. from asynchronous action)

import promise
# chaining madness in action :D
defer = promise.Deferred().done(my_callback).fail(on_error_cb).then(on_success, other_on_error_cb)
some_async_action(callback=defer.resolve)

When "some_async_action" will finish and return result to "defer.resolve" all "done" callbacks ("my_callback" and "on_success") will be trigerred. In this example error callbacks will not be called - in order to do that one should call "defer.reject" manually.

Deferred object can accept both callable and non-callable input argument. When callable is given it will be called with "deferred" keyword argument pointing to Deferred`s "self". When non-callable is given it is considered as resolution for Deferred instance.

promise.Deferred(123).resolved # True
promise.Deferred(lambda deferred: 1).resolved # False

Promise is just proxy that makes Deferreds read-only.

import promise
promise.Deferred().promise().resolve() # RuntimeException

Using "promise.when" simplifies managing multiple asynchronous actions. This function returns Promise instance.

import promise

def success(resp_action1, resp_action2):
    pass

def failure(*args, **kwargs):
    pass

promise.when(Deferred(async_action1), Deferred(async_action2)).then(success, failure)

"success" callback will receive response from both action in one call (as positional arguments passed in the same order as Deferred objects to 'when' helper) while "failure" callback will receive only one failure data (will be called just once). One can pass either Deferred instance and await resolution or non-Deferred object which will be considered as immediate resolution for itself. You can mix Deferred and non-Deferred objects in one call.

import promise
promise.when(Deferred(async_action), 'foo').resolved # False
promise.when('foo', 123).resolved # True