Move variable initialization in AsyncToSync from __init__ to __call__

asgiref/sync.py

@@ -153,30 +153,30 @@ def __init__(
             self.__self__ = self.awaitable.__self__  # type: ignore[union-attr]
         except AttributeError:
             pass
-        if force_new_loop:
-            # They have asked that we always run in a new sub-loop.
-            self.main_event_loop = None
-        else:
-            try:
-                self.main_event_loop = asyncio.get_running_loop()
-            except RuntimeError:
-                # There's no event loop in this thread. Look for the threadlocal if
-                # we're inside SyncToAsync
-                main_event_loop_pid = getattr(
-                    SyncToAsync.threadlocal, "main_event_loop_pid", None
-                )
-                # We make sure the parent loop is from the same process - if
-                # they've forked, this is not going to be valid any more (#194)
-                if main_event_loop_pid and main_event_loop_pid == os.getpid():
-                    self.main_event_loop = getattr(
-                        SyncToAsync.threadlocal, "main_event_loop", None
-                    )
-                else:
-                    self.main_event_loop = None
+        self.force_new_loop = force_new_loop
+        self.main_event_loop = None
+        try:
+            self.main_event_loop = asyncio.get_running_loop()
+        except RuntimeError:
+            # There's no event loop in this thread.
+            pass
 
     def __call__(self, *args: _P.args, **kwargs: _P.kwargs) -> _R:
         __traceback_hide__ = True  # noqa: F841
 
+        if not self.force_new_loop and not self.main_event_loop:
+            # There's no event loop in this thread. Look for the threadlocal if
+            # we're inside SyncToAsync
+            main_event_loop_pid = getattr(
+                SyncToAsync.threadlocal, "main_event_loop_pid", None
+            )
+            # We make sure the parent loop is from the same process - if
+            # they've forked, this is not going to be valid any more (#194)
+            if main_event_loop_pid and main_event_loop_pid == os.getpid():
+                self.main_event_loop = getattr(
+                    SyncToAsync.threadlocal, "main_event_loop", None
+                )
+
         # You can't call AsyncToSync from a thread with a running event loop
         try:
             event_loop = asyncio.get_running_loop()

tests/test_sync.py

@@ -223,6 +223,56 @@ def sync_function():
     assert result["thread"] == threading.current_thread()
 
 
+@pytest.mark.asyncio
+async def test_async_to_sync_to_async_decorator():
+    """
+    Test async_to_sync as a function decorator uses the outer thread
+    when used inside sync_to_async.
+    """
+    result = {}
+
+    # Define async function
+    @async_to_sync
+    async def inner_async_function():
+        result["worked"] = True
+        result["thread"] = threading.current_thread()
+        return 42
+
+    # Define sync function
+    @sync_to_async
+    def sync_function():
+        return inner_async_function()
+
+    # Check it works right
+    number = await sync_function()
+    assert number == 42
+    assert result["worked"]
+    # Make sure that it didn't needlessly make a new async loop
+    assert result["thread"] == threading.current_thread()
+
+@pytest.mark.asyncio
+async def test_async_to_sync_to_thread_decorator():
+    """
+    Test async_to_sync as a function decorator uses the outer thread
+    when used inside another sync thread.
+    """
+    result = {}
+
+    # Define async function
+    @async_to_sync
+    async def inner_async_function():
+        result["worked"] = True
+        result["thread"] = threading.current_thread()
+        return 42
+
+    # Check it works right
+    number = await asyncio.to_thread(inner_async_function)
+    assert number == 42
+    assert result["worked"]
+    # Make sure that it didn't needlessly make a new async loop
+    assert result["thread"] == threading.current_thread()
+
+
 def test_async_to_sync_fail_non_function():
     """
     async_to_sync raises a TypeError when applied to a non-function.