cpython/Lib/test/test_asyncio/test_futures.py

"""Tests for futures.py."""

import concurrent.futures
import gc
import re
import sys
import threading
import unittest
from unittest import mock

import asyncio
from asyncio import futures
from test.test_asyncio import utils as test_utils
from test import support


def tearDownModule():
    asyncio.set_event_loop_policy(None)


def _fakefunc(f):
    return f


def first_cb():
    pass


def last_cb():
    pass


class DuckFuture:
    # Class that does not inherit from Future but aims to be duck-type
    # compatible with it.

    _asyncio_future_blocking = False
    __cancelled = False
    __result = None
    __exception = None

    def cancel(self):
        if self.done():
            return False
        self.__cancelled = True
        return True

    def cancelled(self):
        return self.__cancelled

    def done(self):
        return (self.__cancelled
                or self.__result is not None
                or self.__exception is not None)

    def result(self):
        assert not self.cancelled()
        if self.__exception is not None:
            raise self.__exception
        return self.__result

    def exception(self):
        assert not self.cancelled()
        return self.__exception

    def set_result(self, result):
        assert not self.done()
        assert result is not None
        self.__result = result

    def set_exception(self, exception):
        assert not self.done()
        assert exception is not None
        self.__exception = exception

    def __iter__(self):
        if not self.done():
            self._asyncio_future_blocking = True
            yield self
        assert self.done()
        return self.result()


class DuckTests(test_utils.TestCase):

    def setUp(self):
        super().setUp()
        self.loop = self.new_test_loop()
        self.addCleanup(self.loop.close)

    def test_wrap_future(self):
        f = DuckFuture()
        g = asyncio.wrap_future(f)
        assert g is f

    def test_ensure_future(self):
        f = DuckFuture()
        g = asyncio.ensure_future(f)
        assert g is f


class BaseFutureTests:

    def _new_future(self,  *args, **kwargs):
        return self.cls(*args, **kwargs)

    def setUp(self):
        super().setUp()
        self.loop = self.new_test_loop()
        self.addCleanup(self.loop.close)

    def test_isfuture(self):
        class MyFuture:
            _asyncio_future_blocking = None

            def __init__(self):
                self._asyncio_future_blocking = False

        self.assertFalse(asyncio.isfuture(MyFuture))
        self.assertTrue(asyncio.isfuture(MyFuture()))
        self.assertFalse(asyncio.isfuture(1))

        # As `isinstance(Mock(), Future)` returns `False`
        self.assertFalse(asyncio.isfuture(mock.Mock()))

        f = self._new_future(loop=self.loop)
        self.assertTrue(asyncio.isfuture(f))
        self.assertFalse(asyncio.isfuture(type(f)))

        # As `isinstance(Mock(Future), Future)` returns `True`
        self.assertTrue(asyncio.isfuture(mock.Mock(type(f))))

        f.cancel()

    def test_initial_state(self):
        f = self._new_future(loop=self.loop)
        self.assertFalse(f.cancelled())
        self.assertFalse(f.done())
        f.cancel()
        self.assertTrue(f.cancelled())

    def test_constructor_without_loop(self):
        with self.assertWarns(DeprecationWarning) as cm:
            with self.assertRaisesRegex(RuntimeError, 'There is no current event loop'):
                self._new_future()
        self.assertEqual(cm.warnings[0].filename, __file__)

    def test_constructor_use_running_loop(self):
        async def test():
            return self._new_future()
        f = self.loop.run_until_complete(test())
        self.assertIs(f._loop, self.loop)
        self.assertIs(f.get_loop(), self.loop)

    def test_constructor_use_global_loop(self):
        # Deprecated in 3.10
        asyncio.set_event_loop(self.loop)
        self.addCleanup(asyncio.set_event_loop, None)
        with self.assertWarns(DeprecationWarning) as cm:
            f = self._new_future()
        self.assertEqual(cm.warnings[0].filename, __file__)
        self.assertIs(f._loop, self.loop)
        self.assertIs(f.get_loop(), self.loop)

    def test_constructor_positional(self):
        # Make sure Future doesn't accept a positional argument
        self.assertRaises(TypeError, self._new_future, 42)

    def test_uninitialized(self):
        # Test that C Future doesn't crash when Future.__init__()
        # call was skipped.

        fut = self.cls.__new__(self.cls, loop=self.loop)
        self.assertRaises(asyncio.InvalidStateError, fut.result)

        fut = self.cls.__new__(self.cls, loop=self.loop)
        self.assertRaises(asyncio.InvalidStateError, fut.exception)

        fut = self.cls.__new__(self.cls, loop=self.loop)
        with self.assertRaises((RuntimeError, AttributeError)):
            fut.set_result(None)

        fut = self.cls.__new__(self.cls, loop=self.loop)
        with self.assertRaises((RuntimeError, AttributeError)):
            fut.set_exception(Exception)

        fut = self.cls.__new__(self.cls, loop=self.loop)
        with self.assertRaises((RuntimeError, AttributeError)):
            fut.cancel()

        fut = self.cls.__new__(self.cls, loop=self.loop)
        with self.assertRaises((RuntimeError, AttributeError)):
            fut.add_done_callback(lambda f: None)

        fut = self.cls.__new__(self.cls, loop=self.loop)
        with self.assertRaises((RuntimeError, AttributeError)):
            fut.remove_done_callback(lambda f: None)

        fut = self.cls.__new__(self.cls, loop=self.loop)
        try:
            repr(fut)
        except (RuntimeError, AttributeError):
            pass

        fut = self.cls.__new__(self.cls, loop=self.loop)
        try:
            fut.__await__()
        except RuntimeError:
            pass

        fut = self.cls.__new__(self.cls, loop=self.loop)
        try:
            iter(fut)
        except RuntimeError:
            pass

        fut = self.cls.__new__(self.cls, loop=self.loop)
        self.assertFalse(fut.cancelled())
        self.assertFalse(fut.done())

    def test_future_cancel_message_getter(self):
        f = self._new_future(loop=self.loop)
        self.assertTrue(hasattr(f, '_cancel_message'))
        self.assertEqual(f._cancel_message, None)

        f.cancel('my message')
        with self.assertRaises(asyncio.CancelledError):
            self.loop.run_until_complete(f)
        self.assertEqual(f._cancel_message, 'my message')

    def test_future_cancel_message_setter(self):
        f = self._new_future(loop=self.loop)
        f.cancel('my message')
        f._cancel_message = 'my new message'
        self.assertEqual(f._cancel_message, 'my new message')

        # Also check that the value is used for cancel().
        with self.assertRaises(asyncio.CancelledError):
            self.loop.run_until_complete(f)
        self.assertEqual(f._cancel_message, 'my new message')

    def test_cancel(self):
        f = self._new_future(loop=self.loop)
        self.assertTrue(f.cancel())
        self.assertTrue(f.cancelled())
        self.assertTrue(f.done())
        self.assertRaises(asyncio.CancelledError, f.result)
        self.assertRaises(asyncio.CancelledError, f.exception)
        self.assertRaises(asyncio.InvalidStateError, f.set_result, None)
        self.assertRaises(asyncio.InvalidStateError, f.set_exception, None)
        self.assertFalse(f.cancel())

    def test_result(self):
        f = self._new_future(loop=self.loop)
        self.assertRaises(asyncio.InvalidStateError, f.result)

        f.set_result(42)
        self.assertFalse(f.cancelled())
        self.assertTrue(f.done())
        self.assertEqual(f.result(), 42)
        self.assertEqual(f.exception(), None)
        self.assertRaises(asyncio.InvalidStateError, f.set_result, None)
        self.assertRaises(asyncio.InvalidStateError, f.set_exception, None)
        self.assertFalse(f.cancel())

    def test_exception(self):
        exc = RuntimeError()
        f = self._new_future(loop=self.loop)
        self.assertRaises(asyncio.InvalidStateError, f.exception)

        # StopIteration cannot be raised into a Future - CPython issue26221
        self.assertRaisesRegex(TypeError, "StopIteration .* cannot be raised",
                               f.set_exception, StopIteration)

        f.set_exception(exc)
        self.assertFalse(f.cancelled())
        self.assertTrue(f.done())
        self.assertRaises(RuntimeError, f.result)
        self.assertEqual(f.exception(), exc)
        self.assertRaises(asyncio.InvalidStateError, f.set_result, None)
        self.assertRaises(asyncio.InvalidStateError, f.set_exception, None)
        self.assertFalse(f.cancel())

    def test_exception_class(self):
        f = self._new_future(loop=self.loop)
        f.set_exception(RuntimeError)
        self.assertIsInstance(f.exception(), RuntimeError)

    def test_yield_from_twice(self):
        f = self._new_future(loop=self.loop)

        def fixture():
            yield 'A'
            x = yield from f
            yield 'B', x
            y = yield from f
            yield 'C', y

        g = fixture()
        self.assertEqual(next(g), 'A')  # yield 'A'.
        self.assertEqual(next(g), f)  # First yield from f.
        f.set_result(42)
        self.assertEqual(next(g), ('B', 42))  # yield 'B', x.
        # The second "yield from f" does not yield f.
        self.assertEqual(next(g), ('C', 42))  # yield 'C', y.

    def test_future_repr(self):
        self.loop.set_debug(True)
        f_pending_debug = self._new_future(loop=self.loop)
        frame = f_pending_debug._source_traceback[-1]
        self.assertEqual(
            repr(f_pending_debug),
            f'<{self.cls.__name__} pending created at {frame[0]}:{frame[1]}>')
        f_pending_debug.cancel()

        self.loop.set_debug(False)
        f_pending = self._new_future(loop=self.loop)
        self.assertEqual(repr(f_pending), f'<{self.cls.__name__} pending>')
        f_pending.cancel()

        f_cancelled = self._new_future(loop=self.loop)
        f_cancelled.cancel()
        self.assertEqual(repr(f_cancelled), f'<{self.cls.__name__} cancelled>')

        f_result = self._new_future(loop=self.loop)
        f_result.set_result(4)
        self.assertEqual(
            repr(f_result), f'<{self.cls.__name__} finished result=4>')
        self.assertEqual(f_result.result(), 4)

        exc = RuntimeError()
        f_exception = self._new_future(loop=self.loop)
        f_exception.set_exception(exc)
        self.assertEqual(
            repr(f_exception),
            f'<{self.cls.__name__} finished exception=RuntimeError()>')
        self.assertIs(f_exception.exception(), exc)

        def func_repr(func):
            filename, lineno = test_utils.get_function_source(func)
            text = '%s() at %s:%s' % (func.__qualname__, filename, lineno)
            return re.escape(text)

        f_one_callbacks = self._new_future(loop=self.loop)
        f_one_callbacks.add_done_callback(_fakefunc)
        fake_repr = func_repr(_fakefunc)
        self.assertRegex(
            repr(f_one_callbacks),
            r'<' + self.cls.__name__ + r' pending cb=\[%s\]>' % fake_repr)
        f_one_callbacks.cancel()
        self.assertEqual(repr(f_one_callbacks),
                         f'<{self.cls.__name__} cancelled>')

        f_two_callbacks = self._new_future(loop=self.loop)
        f_two_callbacks.add_done_callback(first_cb)
        f_two_callbacks.add_done_callback(last_cb)
        first_repr = func_repr(first_cb)
        last_repr = func_repr(last_cb)
        self.assertRegex(repr(f_two_callbacks),
                         r'<' + self.cls.__name__ + r' pending cb=\[%s, %s\]>'
                         % (first_repr, last_repr))

        f_many_callbacks = self._new_future(loop=self.loop)
        f_many_callbacks.add_done_callback(first_cb)
        for i in range(8):
            f_many_callbacks.add_done_callback(_fakefunc)
        f_many_callbacks.add_done_callback(last_cb)
        cb_regex = r'%s, <8 more>, %s' % (first_repr, last_repr)
        self.assertRegex(
            repr(f_many_callbacks),
            r'<' + self.cls.__name__ + r' pending cb=\[%s\]>' % cb_regex)
        f_many_callbacks.cancel()
        self.assertEqual(repr(f_many_callbacks),
                         f'<{self.cls.__name__} cancelled>')

    def test_copy_state(self):
        from asyncio.futures import _copy_future_state

        f = self._new_future(loop=self.loop)
        f.set_result(10)

        newf = self._new_future(loop=self.loop)
        _copy_future_state(f, newf)
        self.assertTrue(newf.done())
        self.assertEqual(newf.result(), 10)

        f_exception = self._new_future(loop=self.loop)
        f_exception.set_exception(RuntimeError())

        newf_exception = self._new_future(loop=self.loop)
        _copy_future_state(f_exception, newf_exception)
        self.assertTrue(newf_exception.done())
        self.assertRaises(RuntimeError, newf_exception.result)

        f_cancelled = self._new_future(loop=self.loop)
        f_cancelled.cancel()

        newf_cancelled = self._new_future(loop=self.loop)
        _copy_future_state(f_cancelled, newf_cancelled)
        self.assertTrue(newf_cancelled.cancelled())

    def test_iter(self):
        fut = self._new_future(loop=self.loop)

        def coro():
            yield from fut

        def test():
            arg1, arg2 = coro()

        with self.assertRaisesRegex(RuntimeError, "await wasn't used"):
            test()
        fut.cancel()

    def test_log_traceback(self):
        fut = self._new_future(loop=self.loop)
        with self.assertRaisesRegex(ValueError, 'can only be set to False'):
            fut._log_traceback = True

    @mock.patch('asyncio.base_events.logger')
    def test_tb_logger_abandoned(self, m_log):
        fut = self._new_future(loop=self.loop)
        del fut
        self.assertFalse(m_log.error.called)

    @mock.patch('asyncio.base_events.logger')
    def test_tb_logger_not_called_after_cancel(self, m_log):
        fut = self._new_future(loop=self.loop)
        fut.set_exception(Exception())
        fut.cancel()
        del fut
        self.assertFalse(m_log.error.called)

    @mock.patch('asyncio.base_events.logger')
    def test_tb_logger_result_unretrieved(self, m_log):
        fut = self._new_future(loop=self.loop)
        fut.set_result(42)
        del fut
        self.assertFalse(m_log.error.called)

    @mock.patch('asyncio.base_events.logger')
    def test_tb_logger_result_retrieved(self, m_log):
        fut = self._new_future(loop=self.loop)
        fut.set_result(42)
        fut.result()
        del fut
        self.assertFalse(m_log.error.called)

    @mock.patch('asyncio.base_events.logger')
    def test_tb_logger_exception_unretrieved(self, m_log):
        fut = self._new_future(loop=self.loop)
        fut.set_exception(RuntimeError('boom'))
        del fut
        test_utils.run_briefly(self.loop)
        support.gc_collect()
        self.assertTrue(m_log.error.called)

    @mock.patch('asyncio.base_events.logger')
    def test_tb_logger_exception_retrieved(self, m_log):
        fut = self._new_future(loop=self.loop)
        fut.set_exception(RuntimeError('boom'))
        fut.exception()
        del fut
        self.assertFalse(m_log.error.called)

    @mock.patch('asyncio.base_events.logger')
    def test_tb_logger_exception_result_retrieved(self, m_log):
        fut = self._new_future(loop=self.loop)
        fut.set_exception(RuntimeError('boom'))
        self.assertRaises(RuntimeError, fut.result)
        del fut
        self.assertFalse(m_log.error.called)

    def test_wrap_future(self):

        def run(arg):
            return (arg, threading.get_ident())
        ex = concurrent.futures.ThreadPoolExecutor(1)
        f1 = ex.submit(run, 'oi')
        f2 = asyncio.wrap_future(f1, loop=self.loop)
        res, ident = self.loop.run_until_complete(f2)
        self.assertTrue(asyncio.isfuture(f2))
        self.assertEqual(res, 'oi')
        self.assertNotEqual(ident, threading.get_ident())
        ex.shutdown(wait=True)

    def test_wrap_future_future(self):
        f1 = self._new_future(loop=self.loop)
        f2 = asyncio.wrap_future(f1)
        self.assertIs(f1, f2)

    def test_wrap_future_without_loop(self):
        def run(arg):
            return (arg, threading.get_ident())
        ex = concurrent.futures.ThreadPoolExecutor(1)
        f1 = ex.submit(run, 'oi')
        with self.assertWarns(DeprecationWarning) as cm:
            with self.assertRaises(RuntimeError):
                asyncio.wrap_future(f1)
        self.assertEqual(cm.warnings[0].filename, __file__)
        ex.shutdown(wait=True)

    def test_wrap_future_use_running_loop(self):
        def run(arg):
            return (arg, threading.get_ident())
        ex = concurrent.futures.ThreadPoolExecutor(1)
        f1 = ex.submit(run, 'oi')
        async def test():
            return asyncio.wrap_future(f1)
        f2 = self.loop.run_until_complete(test())
        self.assertIs(self.loop, f2._loop)
        ex.shutdown(wait=True)

    def test_wrap_future_use_global_loop(self):
        # Deprecated in 3.10
        asyncio.set_event_loop(self.loop)
        self.addCleanup(asyncio.set_event_loop, None)
        def run(arg):
            return (arg, threading.get_ident())
        ex = concurrent.futures.ThreadPoolExecutor(1)
        f1 = ex.submit(run, 'oi')
        with self.assertWarns(DeprecationWarning) as cm:
            f2 = asyncio.wrap_future(f1)
        self.assertEqual(cm.warnings[0].filename, __file__)
        self.assertIs(self.loop, f2._loop)
        ex.shutdown(wait=True)

    def test_wrap_future_cancel(self):
        f1 = concurrent.futures.Future()
        f2 = asyncio.wrap_future(f1, loop=self.loop)
        f2.cancel()
        test_utils.run_briefly(self.loop)
        self.assertTrue(f1.cancelled())
        self.assertTrue(f2.cancelled())

    def test_wrap_future_cancel2(self):
        f1 = concurrent.futures.Future()
        f2 = asyncio.wrap_future(f1, loop=self.loop)
        f1.set_result(42)
        f2.cancel()
        test_utils.run_briefly(self.loop)
        self.assertFalse(f1.cancelled())
        self.assertEqual(f1.result(), 42)
        self.assertTrue(f2.cancelled())

    def test_future_source_traceback(self):
        self.loop.set_debug(True)

        future = self._new_future(loop=self.loop)
        lineno = sys._getframe().f_lineno - 1
        self.assertIsInstance(future._source_traceback, list)
        self.assertEqual(future._source_traceback[-2][:3],
                         (__file__,
                          lineno,
                          'test_future_source_traceback'))

    @mock.patch('asyncio.base_events.logger')
    def check_future_exception_never_retrieved(self, debug, m_log):
        self.loop.set_debug(debug)

        def memory_error():
            try:
                raise MemoryError()
            except BaseException as exc:
                return exc
        exc = memory_error()

        future = self._new_future(loop=self.loop)
        future.set_exception(exc)
        future = None
        test_utils.run_briefly(self.loop)
        support.gc_collect()

        if sys.version_info >= (3, 4):
            regex = f'^{self.cls.__name__} exception was never retrieved\n'
            exc_info = (type(exc), exc, exc.__traceback__)
            m_log.error.assert_called_once_with(mock.ANY, exc_info=exc_info)
        else:
            regex = r'^Future/Task exception was never retrieved\n'
            m_log.error.assert_called_once_with(mock.ANY, exc_info=False)
        message = m_log.error.call_args[0][0]
        self.assertRegex(message, re.compile(regex, re.DOTALL))

    def test_future_exception_never_retrieved(self):
        self.check_future_exception_never_retrieved(False)

    def test_future_exception_never_retrieved_debug(self):
        self.check_future_exception_never_retrieved(True)

    def test_set_result_unless_cancelled(self):
        fut = self._new_future(loop=self.loop)
        fut.cancel()
        futures._set_result_unless_cancelled(fut, 2)
        self.assertTrue(fut.cancelled())

    def test_future_stop_iteration_args(self):
        fut = self._new_future(loop=self.loop)
        fut.set_result((1, 2))
        fi = fut.__iter__()
        result = None
        try:
            fi.send(None)
        except StopIteration as ex:
            result = ex.args[0]
        else:
            self.fail('StopIteration was expected')
        self.assertEqual(result, (1, 2))

    def test_future_iter_throw(self):
        fut = self._new_future(loop=self.loop)
        fi = iter(fut)
        self.assertRaises(TypeError, fi.throw,
                          Exception, Exception("elephant"), 32)
        self.assertRaises(TypeError, fi.throw,
                          Exception("elephant"), Exception("elephant"))
        self.assertRaises(TypeError, fi.throw, list)

    def test_future_del_collect(self):
        class Evil:
            def __del__(self):
                gc.collect()

        for i in range(100):
            fut = self._new_future(loop=self.loop)
            fut.set_result(Evil())


@unittest.skipUnless(hasattr(futures, '_CFuture'),
                     'requires the C _asyncio module')
class CFutureTests(BaseFutureTests, test_utils.TestCase):
    try:
        cls = futures._CFuture
    except AttributeError:
        cls = None

    def test_future_del_segfault(self):
        fut = self._new_future(loop=self.loop)
        with self.assertRaises(AttributeError):
            del fut._asyncio_future_blocking
        with self.assertRaises(AttributeError):
            del fut._log_traceback


@unittest.skipUnless(hasattr(futures, '_CFuture'),
                     'requires the C _asyncio module')
class CSubFutureTests(BaseFutureTests, test_utils.TestCase):
    try:
        class CSubFuture(futures._CFuture):
            pass

        cls = CSubFuture
    except AttributeError:
        cls = None


class PyFutureTests(BaseFutureTests, test_utils.TestCase):
    cls = futures._PyFuture


class BaseFutureDoneCallbackTests():

    def setUp(self):
        super().setUp()
        self.loop = self.new_test_loop()

    def run_briefly(self):
        test_utils.run_briefly(self.loop)

    def _make_callback(self, bag, thing):
        # Create a callback function that appends thing to bag.
        def bag_appender(future):
            bag.append(thing)
        return bag_appender

    def _new_future(self):
        raise NotImplementedError

    def test_callbacks_remove_first_callback(self):
        bag = []
        f = self._new_future()

        cb1 = self._make_callback(bag, 42)
        cb2 = self._make_callback(bag, 17)
        cb3 = self._make_callback(bag, 100)

        f.add_done_callback(cb1)
        f.add_done_callback(cb2)
        f.add_done_callback(cb3)

        f.remove_done_callback(cb1)
        f.remove_done_callback(cb1)

        self.assertEqual(bag, [])
        f.set_result('foo')

        self.run_briefly()

        self.assertEqual(bag, [17, 100])
        self.assertEqual(f.result(), 'foo')

    def test_callbacks_remove_first_and_second_callback(self):
        bag = []
        f = self._new_future()

        cb1 = self._make_callback(bag, 42)
        cb2 = self._make_callback(bag, 17)
        cb3 = self._make_callback(bag, 100)

        f.add_done_callback(cb1)
        f.add_done_callback(cb2)
        f.add_done_callback(cb3)

        f.remove_done_callback(cb1)
        f.remove_done_callback(cb2)
        f.remove_done_callback(cb1)

        self.assertEqual(bag, [])
        f.set_result('foo')

        self.run_briefly()

        self.assertEqual(bag, [100])
        self.assertEqual(f.result(), 'foo')

    def test_callbacks_remove_third_callback(self):
        bag = []
        f = self._new_future()

        cb1 = self._make_callback(bag, 42)
        cb2 = self._make_callback(bag, 17)
        cb3 = self._make_callback(bag, 100)

        f.add_done_callback(cb1)
        f.add_done_callback(cb2)
        f.add_done_callback(cb3)

        f.remove_done_callback(cb3)
        f.remove_done_callback(cb3)

        self.assertEqual(bag, [])
        f.set_result('foo')

        self.run_briefly()

        self.assertEqual(bag, [42, 17])
        self.assertEqual(f.result(), 'foo')

    def test_callbacks_invoked_on_set_result(self):
        bag = []
        f = self._new_future()
        f.add_done_callback(self._make_callback(bag, 42))
        f.add_done_callback(self._make_callback(bag, 17))

        self.assertEqual(bag, [])
        f.set_result('foo')

        self.run_briefly()

        self.assertEqual(bag, [42, 17])
        self.assertEqual(f.result(), 'foo')

    def test_callbacks_invoked_on_set_exception(self):
        bag = []
        f = self._new_future()
        f.add_done_callback(self._make_callback(bag, 100))

        self.assertEqual(bag, [])
        exc = RuntimeError()
        f.set_exception(exc)

        self.run_briefly()

        self.assertEqual(bag, [100])
        self.assertEqual(f.exception(), exc)

    def test_remove_done_callback(self):
        bag = []
        f = self._new_future()
        cb1 = self._make_callback(bag, 1)
        cb2 = self._make_callback(bag, 2)
        cb3 = self._make_callback(bag, 3)

        # Add one cb1 and one cb2.
        f.add_done_callback(cb1)
        f.add_done_callback(cb2)

        # One instance of cb2 removed. Now there's only one cb1.
        self.assertEqual(f.remove_done_callback(cb2), 1)

        # Never had any cb3 in there.
        self.assertEqual(f.remove_done_callback(cb3), 0)

        # After this there will be 6 instances of cb1 and one of cb2.
        f.add_done_callback(cb2)
        for i in range(5):
            f.add_done_callback(cb1)

        # Remove all instances of cb1. One cb2 remains.
        self.assertEqual(f.remove_done_callback(cb1), 6)

        self.assertEqual(bag, [])
        f.set_result('foo')

        self.run_briefly()

        self.assertEqual(bag, [2])
        self.assertEqual(f.result(), 'foo')

    def test_remove_done_callbacks_list_mutation(self):
        # see http://bugs.python.org/issue28963 for details

        fut = self._new_future()
        fut.add_done_callback(str)

        for _ in range(63):
            fut.add_done_callback(id)

        class evil:
            def __eq__(self, other):
                fut.remove_done_callback(id)
                return False

        fut.remove_done_callback(evil())

    def test_schedule_callbacks_list_mutation_1(self):
        # see http://bugs.python.org/issue28963 for details

        def mut(f):
            f.remove_done_callback(str)

        fut = self._new_future()
        fut.add_done_callback(mut)
        fut.add_done_callback(str)
        fut.add_done_callback(str)
        fut.set_result(1)
        test_utils.run_briefly(self.loop)

    def test_schedule_callbacks_list_mutation_2(self):
        # see http://bugs.python.org/issue30828 for details

        fut = self._new_future()
        fut.add_done_callback(str)

        for _ in range(63):
            fut.add_done_callback(id)

        max_extra_cbs = 100
        extra_cbs = 0

        class evil:
            def __eq__(self, other):
                nonlocal extra_cbs
                extra_cbs += 1
                if extra_cbs < max_extra_cbs:
                    fut.add_done_callback(id)
                return False

        fut.remove_done_callback(evil())


@unittest.skipUnless(hasattr(futures, '_CFuture'),
                     'requires the C _asyncio module')
class CFutureDoneCallbackTests(BaseFutureDoneCallbackTests,
                               test_utils.TestCase):

    def _new_future(self):
        return futures._CFuture(loop=self.loop)


@unittest.skipUnless(hasattr(futures, '_CFuture'),
                     'requires the C _asyncio module')
class CSubFutureDoneCallbackTests(BaseFutureDoneCallbackTests,
                                  test_utils.TestCase):

    def _new_future(self):
        class CSubFuture(futures._CFuture):
            pass
        return CSubFuture(loop=self.loop)


class PyFutureDoneCallbackTests(BaseFutureDoneCallbackTests,
                                test_utils.TestCase):

    def _new_future(self):
        return futures._PyFuture(loop=self.loop)


class BaseFutureInheritanceTests:

    def _get_future_cls(self):
        raise NotImplementedError

    def setUp(self):
        super().setUp()
        self.loop = self.new_test_loop()
        self.addCleanup(self.loop.close)

    def test_inherit_without_calling_super_init(self):
        # See https://bugs.python.org/issue38785 for the context
        cls = self._get_future_cls()

        class MyFut(cls):
            def __init__(self, *args, **kwargs):
                # don't call super().__init__()
                pass

        fut = MyFut(loop=self.loop)
        with self.assertRaisesRegex(
            RuntimeError,
            "Future object is not initialized."
        ):
            fut.get_loop()


class PyFutureInheritanceTests(BaseFutureInheritanceTests,
                               test_utils.TestCase):
    def _get_future_cls(self):
        return futures._PyFuture


class CFutureInheritanceTests(BaseFutureInheritanceTests,
                              test_utils.TestCase):
    def _get_future_cls(self):
        return futures._CFuture


if __name__ == '__main__':
    unittest.main()