call me mad, will they

ajson.py

from __future__ import print_function

import json

from twisted.internet.defer import Deferred

def adumps(o):
    awaiting = set()
    fulfilled = {}
    def proceed():
        ultimately = None
        def default(unknown):
            if isinstance(unknown, Deferred):
                if unknown in fulfilled:
                    return fulfilled[unknown]
                awaiting.add(unknown)
                @unknown.addCallback
                def ok(result):
                    awaiting.remove(unknown)
                    fulfilled[unknown] = result
                    if ultimately is not None and not awaiting:
                        ultimately.callback(None)
                return "<awaiting>"
            raise TypeError("nope", unknown)
        value = json.dumps(o, default=default)
        if not awaiting:
            return value
        ultimately = Deferred()
        return ultimately.addCallback(lambda ignored: proceed())
    return proceed()

if __name__ == '__main__':
    d = Deferred()
    e = Deferred()
    d.addCallback(lambda x: e.addCallback(lambda y: x + y))
    f = Deferred()
    r = adumps({"a": d, "b": f})
    r.addCallback(lambda result: print("dumped", result))
    d.callback(3)
    e.callback(4)
    new = Deferred()
    f.callback({"more": d, "yet_more": new})
    new.callback("complete")