bedekelly / promises

A modified, simple implementation of promises in Python.

Promises

A Promise is a way of giving functions callbacks to run, depending on whether they succeed or fail.

This implementation modifies this slightly -- instead of a single Success or Failure return type, it provides an "on" method to specify what the return value should be.

@promise
def add_one(n):
    return n + 1

def success(n):
    print("This function will be called with n: {}".format(n))

def failure(n):
    assert False, "This function won't be called!"

(
    add_one(12)
        .on(13, success, failure)
        .go()
)

Promises run asynchronously by default: the .go() method doesn't block.

Later on, if you need to retrieve the result of the success() or failure() method called last, you can run .wait() on the promise.

my_promise = add_one(5).go()
print("5+1 is being calculated in the background")
result = my_promise.wait()
assert result == 6

(You can also run .wait() straight away, and the .go() method will be called implicitly.)

result = (
    add_one(5)
        .on(6, add_one, failure)
        .wait()
)

assert result == 7

If you've decorated some function with the @promise decorator, but don't want to use it as a promise, you can pass the no_promise keyword argument in like so:

result = add_one(5, no_promise=True)
assert result == 6

Some more example code can be found in example.py