Add ClockEventLoop class with fixture and test (close #95)

pytest_asyncio/plugin.py

@@ -193,6 +193,7 @@ def pytest_runtest_setup(item):
 # to marked test functions
 _markers_2_fixtures = {
     'asyncio': 'event_loop',
+    'asyncio_clock': 'clock_event_loop',
 }
 
 
@@ -204,6 +205,56 @@ def event_loop(request):
     loop.close()
 
 
+def _clock_event_loop_class():
+    """
+    Create a new class for ClockEventLoop based on the current
+    class-type produced by `asyncio.new_event_loop()`.  This is important
+    for instances in which the enent-loop-policy has been changed.
+    """
+    class ClockEventLoop(asyncio.new_event_loop().__class__):
+        """
+        A custom event loop that explicitly advances time when requested. Otherwise,
+        this event loop advances time as expected.
+        """
+        def __init__(self, *args, **kwargs):
+            super().__init__(*args, **kwargs)
+            self._offset = 0
+
+        def time(self):
+            """
+            Return the time according the event loop's clock.
+
+            This time is adjusted by the stored offset that allows for advancement
+            with `advance_time`.
+            """
+            return super().time() + self._offset
+
+        def advance_time(self, seconds):
+            '''
+            Advance time by a given offset in seconds. Returns an awaitable
+            that will complete after all tasks scheduled for after advancement
+            of time are proceeding.
+            '''
+            if seconds > 0:
+                # advance the clock by the given offset
+                self._offset += seconds
+
+            # Once the clock is adjusted, new tasks may have just been
+            # scheduled for running in the next pass through the event loop
+            return self.create_task(asyncio.sleep(0))
+
+    return ClockEventLoop
+
+
+@pytest.yield_fixture
+def clock_event_loop(request):
+    """Create an instance of the default event loop for each test case."""
+    loop = _clock_event_loop_class()()
+    asyncio.get_event_loop_policy().set_event_loop(loop)
+    yield loop
+    loop.close()
+
+
 def _unused_tcp_port():
     """Find an unused localhost TCP port from 1024-65535 and return it."""
     with contextlib.closing(socket.socket()) as sock:

tests/test_simple_35.py

@@ -86,3 +86,41 @@ async def test_asyncio_marker_method(self, event_loop):
 def test_async_close_loop(event_loop):
     event_loop.close()
     return 'ok'
+
+
+@pytest.mark.asyncio_clock
+async def test_mark_asyncio_clock():
+    """
+    Test that coroutines marked with asyncio_clock are run with a ClockEventLoop
+    """
+    assert hasattr(asyncio.get_event_loop(), 'advance_time')
+
+
+def test_clock_loop_loop_fixture(clock_event_loop):
+    """
+    Test that the clock_event_loop fixture returns a proper instance of the loop
+    """
+    assert hasattr(asyncio.get_event_loop(), 'advance_time')
+    clock_event_loop.close()
+    return 'ok'
+
+
+@pytest.mark.asyncio_clock
+async def test_clock_loop_advance_time(clock_event_loop):
+    """
+    Test the sliding time event loop fixture
+    """
+    # A task is created that will sleep some number of seconds
+    SLEEP_TIME = 10
+
+    # create the task
+    task = clock_event_loop.create_task(asyncio.sleep(SLEEP_TIME))
+    assert not task.done()
+
+    # start the task
+    await clock_event_loop.advance_time(0)
+    assert not task.done()
+
+    # process the timeout
+    await clock_event_loop.advance_time(SLEEP_TIME)
+    assert task.done()