Fix Local behaviour with asyncio Task (#478)

Redid CVar storage as a dict of contextvars

Author: spanezz

asgiref/local.py

@@ -2,37 +2,38 @@
 import contextlib
 import contextvars
 import threading
-from typing import Any, Dict, Union
+from typing import Any, Union
 
 
 class _CVar:
     """Storage utility for Local."""
 
     def __init__(self) -> None:
-        self._data: "contextvars.ContextVar[Dict[str, Any]]" = contextvars.ContextVar(
-            "asgiref.local"
-        )
+        self._data: dict[str, contextvars.ContextVar[Any]] = {}
 
-    def __getattr__(self, key):
-        storage_object = self._data.get({})
+    def __getattr__(self, key: str) -> Any:
         try:
-            return storage_object[key]
+            var = self._data[key]
         except KeyError:
             raise AttributeError(f"{self!r} object has no attribute {key!r}")
 
+        try:
+            return var.get()
+        except LookupError:
+            raise AttributeError(f"{self!r} object has no attribute {key!r}")
+
     def __setattr__(self, key: str, value: Any) -> None:
         if key == "_data":
             return super().__setattr__(key, value)
 
-        storage_object = self._data.get({})
-        storage_object[key] = value
-        self._data.set(storage_object)
+        var = self._data.get(key)
+        if var is None:
+            self._data[key] = var = contextvars.ContextVar(key)
+        var.set(value)
 
     def __delattr__(self, key: str) -> None:
-        storage_object = self._data.get({})
-        if key in storage_object:
-            del storage_object[key]
-            self._data.set(storage_object)
+        if key in self._data:
+            del self._data[key]
         else:
             raise AttributeError(f"{self!r} object has no attribute {key!r}")
 

tests/test_local.py

@@ -1,6 +1,7 @@
 import asyncio
 import gc
 import threading
+from threading import Thread
 
 import pytest
 
@@ -338,3 +339,39 @@ async def async_function():
     # inner value was set inside a new async context, meaning that
     # we do not see it, as context vars don't propagate up the stack
     assert not hasattr(test_local_not_tc, "test_value")
+
+
+def test_visibility_thread_asgiref() -> None:
+    """Check visibility with subthreads."""
+    test_local = Local()
+    test_local.value = 0
+
+    def _test() -> None:
+        # Local() is cleared when changing thread
+        assert not hasattr(test_local, "value")
+        setattr(test_local, "value", 1)
+        assert test_local.value == 1
+
+    thread = Thread(target=_test)
+    thread.start()
+    thread.join()
+
+    assert test_local.value == 0
+
+
+@pytest.mark.asyncio
+async def test_visibility_task() -> None:
+    """Check visibility with asyncio tasks."""
+    test_local = Local()
+    test_local.value = 0
+
+    async def _test() -> None:
+        # Local is inherited when changing task
+        assert test_local.value == 0
+        test_local.value = 1
+        assert test_local.value == 1
+
+    await asyncio.create_task(_test())
+
+    # Changes should not leak to the caller
+    assert test_local.value == 0