Promise

This is a implementation of Promises in Python. It is a super set of Promises/A+ designed to have readable, performant code and to provide just the extensions that are absolutely necessary for using promises in Python.

Its fully compatible with the Promises/A+ spec <http://promises-aplus.github.io/promises-spec/>__

|travis| |pypi| |coveralls|

Installation

::

$ pip install promise

Usage

The example below shows how you can load the promise library. It then demonstrates creating a promise from scratch. You simply call Promise(fn). There is a complete specification for what is returned by this method in Promises/A+ <http://promises-aplus.github.com/promises-spec/>__.

.. code:: python

from promise import Promise

promise = Promise(
    lambda resolve, reject: resolve('RESOLVED!')
)

API

Before all examples, you will need:

.. code:: python

from promise import Promise

Promise(resolver)

This creates and returns a new promise. ``resolver`` must be a function.
The ``resolver`` function is passed two arguments:

1. ``resolve`` should be called with a single argument. If it is called
   with a non-promise value then the promise is fulfilled with that
   value. If it is called with a promise (A) then the returned promise
   takes on the state of that new promise (A).
2. ``reject`` should be called with a single argument. The returned
   promise will be rejected with that argument.

Class Methods
~~~~~~~~~~~~~

These methods are invoked by calling ``Promise.methodName``.

Promise.resolve(value)
^^^^^^^^^^^^^^^^^^^^^^

Converts values and foreign promises into Promises/A+ promises. If you
pass it a value then it returns a Promise for that value. If you pass it
something that is close to a promise (such as a jQuery attempt at a
promise) it returns a Promise that takes on the state of ``value``
(rejected or fulfilled).

Promise.reject(value)
^^^^^^^^^^^^^^^^^^^^^

Returns a rejected promise with the given value.

Promise.all(list)
^^^^^^^^^^^^^^^^^

Returns a promise for a list. If it is called with a single argument
then this returns a promise for a copy of that list with any promises
replaced by their fulfilled values. e.g.

.. code:: python

    p = Promise.all([Promise.resolve('a'), 'b', Promise.resolve('c')]) \
           .then(lambda res: res == ['a', 'b', 'c'])

    assert p.get() is True

Promise.cast(obj)
^^^^^^^^^^^^^^^^^

This function wraps the ``obj`` act as a ``Promise`` if possible. Python
``Future``\ s are supported, with a callback to ``promise.done`` when
resolved. Have the same effects as ``Promise.resolve(obj)``.

Promise.for\_dict(d)
^^^^^^^^^^^^^^^^^^^^

A special function that takes a dictionary of promises and turns them
into a promise for a dictionary of values. In other words, this turns an
dictionary of promises for values into a promise for a dictionary of
values.

Promise.is\_thenable(obj)
^^^^^^^^^^^^^^^^^^^^^^^^^

This function checks if the ``obj`` is a ``Promise``, or could be
``cast``\ ed.

Promise.promisify(func)
^^^^^^^^^^^^^^^^^^^^^^^

This function wraps the result of calling ``func`` in a ``Promise``
instance.

Instance Methods
~~~~~~~~~~~~~~~~

These methods are invoked on a promise instance by calling
``myPromise.methodName``

promise.then(did\_fulfill, did\_reject)

This method follows the Promises/A+ spec <http://promises-aplus.github.io/promises-spec/>__. It explains things very clearly so I recommend you read it.

Either did_fulfill or did_reject will be called and they will not be called more than once. They will be passed a single argument and will always be called asynchronously (in the next turn of the event loop).

If the promise is fulfilled then did_fulfill is called. If the promise is rejected then did_reject is called.

The call to .then also returns a promise. If the handler that is called returns a promise, the promise returned by .then takes on the state of that returned promise. If the handler that is called returns a value that is not a promise, the promise returned by .then will be fulfilled with that value. If the handler that is called throws an exception then the promise returned by .then is rejected with that exception.

promise.catch(did_reject) ^^^^^^^^^^^^^^^^^^^^^^^^^^

Sugar for promise.then(None, did_reject), to mirror catch in synchronous code.

promise.done(did_fulfill, did_reject) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The same semantics as .then except that it does not return a promise and any exceptions are re-thrown so that they can be logged (crashing the application in non-browser environments)

Contributing

After cloning this repo, ensure dependencies are installed by running:

.. code:: sh

pip install -e ".[test]"

After developing, the full test suite can be evaluated by running:

.. code:: sh

py.test tests --cov=promise --benchmark-skip # Use -v -s for verbose mode

You can also run the benchmarks with:

.. code:: sh

py.test tests --benchmark-only

Static type checking

Python type annotations are very useful for making sure we use the libary the way is intended.

You can run mypy static type checker:

.. code:: sh

pip install mypy
mypy promise  --ignore-missing-imports

Or pyre:

.. code:: sh

pip install pyre-check
pyre --source-directory promise check

Notes

This package is heavily insipired in aplus <https://github.com/xogeny/aplus>__.

License

MIT License <https://github.com/syrusakbary/promise/blob/master/LICENSE>__

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