successresultof.py

import pytest
from twisted.internet import defer


def successResultOf(d):
    completed = []

    d.addCallback(completed.append)

    assert completed, "{!r} not yet fired".format(d)
    return completed[0]


def test_successResultOf():
    value = "OK"
    assert successResultOf(defer.succeed(value)) == value
    with pytest.raises(AssertionError) as exc:
        successResultOf(defer.fail(ValueError()))

    assert "Deferred" in str(exc.value)

    with pytest.raises(AssertionError):
        successResultOf(defer.Deferred())

    assert "Deferred" in str(exc.value)