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# 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][travis-image]][travis-url]
[![pypi][pypi-image]][pypi-url]
[![coveralls][coveralls-image]][coveralls-url]
[travis-image]: https://img.shields.io/travis/syrusakbary/promise.svg?style=flat
[travis-url]: https://travis-ci.org/syrusakbary/promise
[pypi-image]: https://img.shields.io/pypi/v/promise.svg?style=flat
[pypi-url]: https://pypi.python.org/pypi/promise
[coveralls-image]: https://coveralls.io/repos/syrusakbary/promise/badge.svg?branch=master&service=github
[coveralls-url]: https://coveralls.io/github/syrusakbary/promise?branch=master
## 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/).
```python
from promise import Promise
promise = Promise(
lambda resolve, reject: resolve('RESOLVED!')
)
```
## API
Before all examples, you will need:
```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.
```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` ect as a `Promise` if possible.
Python `Future`s are supported, with a callback to `promise.done` when resolved.
Has 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
a 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:
```sh
pip install -e ".[test]"
```
After developing, the full test suite can be evaluated by running:
```sh
py.test tests --cov=promise --benchmark-skip # Use -v -s for verbose mode
```
You can also run the benchmarks with:
```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:
```sh
pip install mypy
mypy promise --ignore-missing-imports
```
Or `pyre`:
```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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