Fix AsyncRunner failure on Python 3.14 due to asyncio.get_event_loop() removal

In Python 3.14, asyncio.get_event_loop() raises RuntimeError when no event
loop is running. Replace with a version-agnostic approach that:
1. Tries asyncio.get_running_loop() first
2. Falls back to creating a new event loop if none exists

This works on Python 3.7+ and handles all cases correctly.

Fixes #489

Author: basnijholt

adaptive/runner.py

@@ -626,7 +626,18 @@ def __init__(
             raise_if_retries_exceeded=raise_if_retries_exceeded,
             allow_running_forever=True,
         )
-        self.ioloop = ioloop or asyncio.get_event_loop()
+        if ioloop is not None:
+            self.ioloop = ioloop
+        else:
+            try:
+                self.ioloop = asyncio.get_running_loop()
+            except RuntimeError:
+                # No running event loop exists (e.g., running outside of async context).
+                # Create a new event loop. This is needed for Python 3.10+ where
+                # asyncio.get_event_loop() is deprecated when no loop is running,
+                # and Python 3.14+ where it raises RuntimeError.
+                self.ioloop = asyncio.new_event_loop()
+                asyncio.set_event_loop(self.ioloop)
 
         # When the learned function is 'async def', we run it
         # directly on the event loop, and not in the executor.

adaptive/tests/test_runner.py

@@ -1,3 +1,4 @@
+import asyncio
 import platform
 import sys
 import time
@@ -263,3 +264,35 @@ def counting_ask(self, n, tell_pending=True):
     finally:
         # Restore original method
         Learner1D.ask = original_ask
+
+
+def test_async_runner_without_event_loop():
+    """Test that AsyncRunner works when no event loop exists.
+
+    In Python 3.10+, asyncio.get_event_loop() was deprecated when no running
+    event loop exists. In Python 3.12+ it emits a DeprecationWarning, and in
+    Python 3.14+ it raises a RuntimeError.
+
+    This test ensures AsyncRunner properly handles the case when no event
+    loop exists by creating one.
+
+    Regression test for: https://github.com/python-adaptive/adaptive/issues/489
+    """
+
+    def run_in_thread():
+        """Run AsyncRunner in a thread with no event loop."""
+        # Ensure no event loop exists in this thread
+        with pytest.raises(RuntimeError, match="no.*event loop"):
+            asyncio.get_running_loop()
+
+        # AsyncRunner should still work - it should create its own event loop
+        learner = Learner1D(linear, (-1, 1))
+        runner = AsyncRunner(learner, npoints_goal=10, executor=SequentialExecutor())
+        runner.block_until_done()
+        assert learner.npoints >= 10
+
+    import threading
+
+    thread = threading.Thread(target=run_in_thread)
+    thread.start()
+    thread.join()