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>`__

.. |travis| image:: https://img.shields.io/travis/syrusakbary/promise.svg?style=flat
   :target: https://travis-ci.org/syrusakbary/promise
.. |pypi| image:: https://img.shields.io/pypi/v/promise.svg?style=flat
   :target: https://pypi.python.org/pypi/promise
.. |coveralls| image:: https://coveralls.io/repos/syrusakbary/promise/badge.svg?branch=master&service=github
   :target: https://coveralls.io/github/syrusakbary/promise?branch=master
