Promises acting synchronously

Greetings!

When it comes to promises, I hail from the land of AngularJS, which includes a few nice wrappers. I’ve only had to deal with pure promises a few times, so this issue is most likely me not understanding how to set this up (and there is a lack of basic examples in the README, instead directing readers to the spec)

This is the current code I have:

from promise import Promise
import urllib.request

url = 'https://jsonplaceholder.typicode.com/photos'

def success(data):
    print("promise success!")
    # print(data)

def reject(data):
    print("rejected!")
    print(data)

def getData(url):
    def func(resolve, reject):
        
        req = urllib.request.urlopen(url)
        if req.getcode() == 200:
            resolve(req.read())
        else:
            reject(Exception(req.getcode()))

    return Promise(func)

print("getting resource")
p = getData(url)
p.then(success, reject)
print("Waiting on promise")

This seems to work synchronously: once getData() is called, it returns a promise, but it also blocks execution. This is what the output looks like:

getting resource
promise success!
Waiting on promise

The order I was expecting:

getting resource
Waiting on promise
promise success!

I’ve also tested this with a simple time.sleep(5) and resolving immediately after that. Again, once getData() is run, it simply sleeps for 5 seconds without hitting the “Waiting” print, then continues onword.

Sorry if this is a bit of an obtuse question, I acknowledge I’m probably not working with it correctly, but I can’t seem to figure it out.

[pkopac]

Hi! I’ve just tried a variant of your code example with time measuring and multiple promises locally and… it really doesn’t work (python 3.6):

Bug 1: Promise(func) executes immediately and blocks main thread, whatever the scheduler.

Bug 2: Promise.promisify(func) returns a function, not a promise.

Executing promises
0
Time elapsed (hh:mm:ss.ms) 0:00:02.121857
1
Time elapsed (hh:mm:ss.ms) 0:00:06.121378
2
Time elapsed (hh:mm:ss.ms) 0:00:11.926724
[<Promise at 0x7f00fa7180b8 pending>, <Promise at 0x7f00fa701a58 pending>, <Promise at 0x7f00fa7289e8 pending>]
Waiting on all promises
promise success!
promise success!
promise success!
Time elapsed (hh:mm:ss.ms) 0:00:11.928643

With ThreadScheduler:

Executing promises
0
promise success!
Time elapsed (hh:mm:ss.ms) 0:00:01.692070
1
promise success!
Time elapsed (hh:mm:ss.ms) 0:00:03.782801
2
promise success!
Time elapsed (hh:mm:ss.ms) 0:00:05.826248
[<Promise at 0x7fd12df93a90 fulfilled with None>, <Promise at 0x7fd12de42cc0 fulfilled with None>, <Promise at 0x7fd12de5a320 fulfilled with None>]
Waiting on all promises
Time elapsed (hh:mm:ss.ms) 0:00:05.827753

My full code:

from promise import Promise, set_default_scheduler, get_default_scheduler
from promise.schedulers.asyncio import AsyncioScheduler
from promise.schedulers.gevent import GeventScheduler
from promise.schedulers.thread import ThreadScheduler
from datetime import datetime
import urllib.request

# set_default_scheduler(AsyncioScheduler())
set_default_scheduler(ThreadScheduler())
# set_default_scheduler(GeventScheduler())

url = 'https://jsonplaceholder.typicode.com/photos'
def getData(url):
    def func(resolve, reject):
        req = urllib.request.urlopen(url)
        if req.getcode() == 200:
            resolve(req.read())
        else:
            reject(Exception(req.getcode()))

    return Promise(func)

start_time = datetime.now()

def success(data):
    print("promise success!")
    # print(data)

def reject(data):
    print("rejected!")
    print(data)

def print_elapsed_time():
    time_elapsed = datetime.now() - start_time
    print('Time elapsed (hh:mm:ss.ms) {}'.format(time_elapsed))

print("Executing promises")
l = []
for x in range(0, 3):
    print(x)
    l.append(getData(url).then(success, reject))
    print_elapsed_time()

print(l)
print("Waiting on all promises")
Promise.all(l).get()
print_elapsed_time()

I was able to get it work as async promises with a really ugly syntax:

def getDataHack(url):
    def func(value):
        req = urllib.request.urlopen(url)
        if req.getcode() == 200:
            Promise.resolve(req.read())
        else:
            Promise.reject(Exception(req.getcode()))

    return Promise.resolve("foo").then(func, None)

Executing promises
0
Time elapsed (hh:mm:ss.ms) 0:00:00.001408
1
Time elapsed (hh:mm:ss.ms) 0:00:00.001448
2
Time elapsed (hh:mm:ss.ms) 0:00:00.001474
[<Promise at 0x7f8e75321eb8 pending>, <Promise at 0x7f8e75321fd0 pending>, <Promise at 0x7f8e7533a0f0 pending>]
Waiting on all promises
promise success!
promise success!
promise success!
Time elapsed (hh:mm:ss.ms) 0:00:05.912553

[pollaro]

I’m having trouble with my DataLoader queueing things. It seems to load the data immediately when I call load() instead of waiting.