Backport PR #858 on branch 7.x (Defer creation of ready future) (#859)

Co-authored-by: Steven Silvester <[email protected]>

Author: meeseeksmachine

jupyter_client/manager.py

@@ -55,6 +55,16 @@ class _ShutdownStatus(Enum):
 F = t.TypeVar('F', bound=t.Callable[..., t.Any])
 
 
+def _get_future() -> t.Union[Future, CFuture]:
+    """Get an appropriate Future object"""
+    try:
+        asyncio.get_running_loop()
+        return Future()
+    except RuntimeError:
+        # No event loop running, use concurrent future
+        return CFuture()
+
+
 def in_pending_state(method: F) -> F:
     """Sets the kernel to a pending state by
     creating a fresh Future for the KernelManager's `ready`
@@ -65,12 +75,8 @@ def in_pending_state(method: F) -> F:
     @functools.wraps(method)
     async def wrapper(self, *args, **kwargs):
         # Create a future for the decorated method
-        if self._attempted_start:
-            try:
-                self._ready = Future()
-            except RuntimeError:
-                # No event loop running, use concurrent future
-                self._ready = CFuture()
+        if self._attempted_start or not self._ready:
+            self._ready = _get_future()
         try:
             # call wrapped method, await, and set the result or exception.
             out = await method(self, *args, **kwargs)
@@ -92,19 +98,13 @@ class KernelManager(ConnectionFileMixin):
     This version starts kernels with Popen.
     """
 
-    _ready: t.Union[Future, CFuture]
+    _ready: t.Optional[t.Union[Future, CFuture]]
 
     def __init__(self, *args, **kwargs):
         super().__init__(**kwargs)
         self._shutdown_status = _ShutdownStatus.Unset
         self._attempted_start = False
-        # Create a place holder future.
-        try:
-            asyncio.get_running_loop()
-            self._ready = Future()
-        except RuntimeError:
-            # No event loop running, use concurrent future
-            self._ready = CFuture()
+        self._ready = None
 
     _created_context: Bool = Bool(False)
 
@@ -189,6 +189,8 @@ def _default_cache_ports(self) -> bool:
     @property
     def ready(self) -> t.Union[CFuture, Future]:
         """A future that resolves when the kernel process has started for the first time"""
+        if not self._ready:
+            self._ready = _get_future()
         return self._ready
 
     @property

pyproject.toml

@@ -51,7 +51,7 @@ Homepage = "https://jupyter.org"
 test = [
     "codecov",
     "coverage",
-    "ipykernel>=6.5",
+    "ipykernel>=6.12",
     "ipython",
     "mypy",
     "pre-commit",