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
$ pip install promise
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+.
from promise import Promise promise = Promise( lambda resolve, reject: resolve('RESOLVED!') )
Before all examples, you will need:
from promise import Promise
This creates and returns a new promise.
resolver must be a function. The
resolver function is passed two arguments:
resolveshould 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).
rejectshould be called with a single argument. The returned promise will be rejected with that argument.
These methods are invoked by calling
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).
Returns a rejected promise with the given value.
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.
p = Promise.all([Promise.resolve('a'), 'b', Promise.resolve('c')]) \ .then(lambda res: res == ['a', 'b', 'c']) assert p.get() is True
This function wraps the
obj ect as a
Promise if possible.
Futures are supported, with a callback to
promise.done when resolved.
Has the same effects as
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.
This function checks if the
obj is a
Promise, or could be
This function wraps the result of calling
func in a
These methods are invoked on a promise instance by calling
This method follows the Promises/A+ spec. It explains things very clearly so I recommend you read it.
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.then(None, did_reject), to mirror
catch in synchronous code.
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)
After cloning this repo, ensure dependencies are installed by running:
pip install -e ".[test]"
After developing, the full test suite can be evaluated by running:
py.test tests --cov=promise --benchmark-skip # Use -v -s for verbose mode
You can also run the benchmarks with:
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:
pip install mypy mypy promise --ignore-missing-imports
pip install pyre-check pyre --source-directory promise check
This package is heavily insipired in aplus.