fix: unclosed event loop on Python 3.12+

Author: Chaoses-Ib

README.md

@@ -44,7 +44,11 @@ generally can't be patched.
 
 ## Comparison with `nest_asyncio`
 `nest-asyncio2` is a fork of the unmaintained [`nest_asyncio`](https://github.com/erdewit/nest_asyncio), with the following changes:
-- Python 3.12 `loop_factory` parameter support
+- Python 3.12 support
+  - `loop_factory` parameter support
+  - Fix `ResourceWarning: unclosed event loop` at exit
+
+    Note that if you call `asyncio.get_event_loop()` on the main thread without setting the loop before, `ResourceWarning` is expected on Python 3.12~3.13, not caused by `nest-asyncio2`.
 - Python 3.14 support
   - Fix broken `asyncio.current_task()` and others
   - Fix `DeprecationWarning: 'asyncio.get_event_loop_policy' is deprecated and slated for removal in Python 3.16`

nest_asyncio2.py

@@ -15,8 +15,29 @@ def apply(loop=None):
     _patch_policy()
     _patch_tornado()
 
-    loop = loop or asyncio.get_event_loop()
-    _patch_loop(loop)
+    loop = loop or _get_event_loop()
+    if loop is not None:
+        _patch_loop(loop)
+
+if sys.version_info < (3, 12, 0):
+    def _get_event_loop():
+        return asyncio.get_event_loop()
+elif sys.version_info < (3, 14, 0):
+    def _get_event_loop():
+        # Python 3.12~3.13:
+        # Calling get_event_loop() will result in ResourceWarning: unclosed event loop
+        loop = events._get_running_loop()
+        if loop is None:
+            policy = events.get_event_loop_policy()
+            loop = policy._local._loop
+        return loop
+else:
+    def _get_event_loop():
+        # Python 3.14: Raises a RuntimeError if there is no current event loop.
+        try:
+            return asyncio.get_event_loop()
+        except RuntimeError:
+            return None
 
 if sys.version_info < (3, 12, 0):    
     def run(main, *, debug=False):
@@ -32,11 +53,11 @@ def run(main, *, debug=False):
                     loop.run_until_complete(task)
 else:
     def run(main, *, debug=False, loop_factory=None):
+        new_event_loop = False
+        set_event_loop = None
         try:
             loop = asyncio.get_running_loop()
         except RuntimeError:
-            if loop_factory is None:
-                loop_factory = asyncio.new_event_loop
             # if sys.version_info < (3, 16, 0):
             #     policy = asyncio.events._get_event_loop_policy()
             #     try:
@@ -45,7 +66,14 @@ def run(main, *, debug=False, loop_factory=None):
             #         loop = loop_factory()
             # else:
             #     loop = loop_factory()
-            loop = loop_factory()
+            if loop_factory is None:
+                loop = asyncio.new_event_loop()
+                # Not running
+                set_event_loop = _get_event_loop()
+                asyncio.set_event_loop(loop)
+            else:
+                loop = loop_factory()
+            new_event_loop = True
 
         loop.set_debug(debug)
         task = asyncio.ensure_future(main, loop=loop)
@@ -56,6 +84,12 @@ def run(main, *, debug=False, loop_factory=None):
                 task.cancel()
                 with suppress(asyncio.CancelledError):
                     loop.run_until_complete(task)
+            if set_event_loop:
+                # asyncio.Runner just set_event_loop(None) but we are nested
+                asyncio.set_event_loop(set_event_loop)
+            if new_event_loop:
+                # Avoid ResourceWarning: unclosed event loop
+                loop.close()
 
 def _patch_asyncio():
     """Patch asyncio module to use pure Python tasks and futures."""
@@ -94,9 +128,13 @@ def _get_event_loop(stacklevel=3):
 def _patch_policy():
     """Patch the policy to always return a patched loop."""
 
+    # Python 3.14:
+    # get_event_loop() raises a RuntimeError if there is no current event loop.
+    # So there is no need to _patch_loop() in it.
+    # Patching new_event_loop() may be better, but policy is going to be removed...
     # Removed in Python 3.16
     # https://github.com/python/cpython/issues/127949
-    if sys.version_info >= (3, 16, 0):
+    if sys.version_info >= (3, 14, 0):
         return
 
     def get_event_loop(self):

tests/314_task.py

@@ -1,5 +1,5 @@
 # /// script
-# requires-python = ">=3.14"
+# requires-python = ">=3.12"
 # dependencies = [
 #     "nest-asyncio2",
 # ]

tests/314_task_mix.py

@@ -1,5 +1,5 @@
 # /// script
-# requires-python = ">=3.14"
+# requires-python = ">=3.12"
 # dependencies = [
 #     "nest-asyncio2",
 # ]

tests/test.ps1

@@ -1,5 +1,23 @@
 pushd tests
 
+function Test {
+    param(
+        [string]$Py,
+        [string[]]$V
+    )
+
+    foreach ($v in $V) {
+        Write-Host Test: $Py on $v
+        # Catch ResourceWarning: unclosed event loop
+        $out = uv run --python $v $Py 2>&1
+        Write-Output $out
+        if (!$? -or $out -match "Warning") {
+            throw "$Py on $v : $out"
+        }
+        Write-Host
+    }
+}
+
 # uv run --python 3.5 nest_test.py
 uv run --python 3.8 nest_test.py
 if (!$?) {
@@ -39,13 +57,7 @@ if (!$?) {
     throw "314_loop_factory"
 }
 
-uv run --python 3.14 314_task.py
-if (!$?) {
-    throw "314_task"
-}
-uv run --python 3.14 314_task_mix.py
-if (!$?) {
-    throw "314_task_mix"
-}
+Test -V @("3.12", "3.13", "3.14") -Py 314_task.py
+Test -V @("3.12", "3.13", "3.14") -Py 314_task_mix.py
 
 popd