Privatize nurseries

trio/_core/_run.py

@@ -37,7 +37,7 @@
 )
 from .. import _core
 from .._deprecate import deprecated
-from .._util import Final
+from .._util import Final, NoPublicConstructor
 
 # At the bottom of this file there's also some "clever" code that generates
 # wrapper functions for runner and io manager methods, and adds them to
@@ -717,7 +717,7 @@ class NurseryManager:
     async def __aenter__(self):
         self._scope = CancelScope()
         self._scope.__enter__()
-        self._nursery = Nursery(current_task(), self._scope)
+        self._nursery = Nursery._create(current_task(), self._scope)
         return self._nursery
 
     @enable_ki_protection
@@ -761,7 +761,7 @@ def open_nursery():
     return NurseryManager()
 
 
-class Nursery:
+class Nursery(metaclass=NoPublicConstructor):
     def __init__(self, parent_task, cancel_scope):
         self._parent_task = parent_task
         parent_task._child_nurseries.append(self)

trio/_core/tests/test_run.py

@@ -2142,6 +2142,30 @@ async def inner():
     _core.run(inner)
 
 
+def test_Nursery_init():
+    check_Nursery_error = pytest.raises(
+        TypeError, match='no public constructor available'
+    )
+
+    with check_Nursery_error:
+        _core._run.Nursery(None, None)
+
+
+async def test_Nursery_private_init():
+    # context manager creation should not raise
+    async with _core.open_nursery() as nursery:
+        assert False == nursery._closed
+
+
+def test_Nursery_subclass():
+    with pytest.raises(
+        TypeError, match='`Nursery` does not support subclassing'
+    ):
+
+        class Subclass(_core._run.Nursery):
+            pass
+
+
 def test_Cancelled_init():
     check_Cancelled_error = pytest.raises(
         TypeError, match='no public constructor available'