Countdown of letters with reactor

July 8, 2022

I am learning twisted and have developed a simple python script that should count down each letter of the alphabet and stop the reactor when it gets to z. However, I am getting the following error:

assert builtins.callable(callable), f"{callable} is not callable"
builtins.AssertionError: a is not callable

Here’s my script:

class Countletters(object):
    letters = list( map(chr, range(97, 123)) )

    def countletters(self):
        for l in self.letters:
            if l  == 'z':
                rector.stop()
            else:
                print(l)
                reactor.callLater(1, l)
from twisted.internet import reactor
reactor.callWhenRunning(Countletters().countletters)
print('Start countdown')
reactor.run()
print('Stop countdown')

>Solution :

You are almost there – the problem is the callLater part.

callLater expects a function, something "Callable", but you give it l; a string (the current letter).

Try this for an example:
I pass functools.partial(print, l) to callLater which basically creates a callable object, which will simply do print(l) when called.

import functools

from twisted.internet import reactor

class Countletters(object):
    letters = list(map(chr, range(97, 123)))

    def countletters(self):
        for l in self.letters:
            if l == 'z':
                reactor.stop()
            else:
                print(l)
                reactor.callLater(1, functools.partial(print, l))


reactor.callWhenRunning(Countletters().countletters)
print('Start countdown')
reactor.run()
print('Stop countdown')