jupyter-run: avoid traceback for NoSuchKernel

NoSuchKernel used to raise during KernelManager instantiation,
but it is now delayed.

Access kernel_spec to ensure it's raised where it will be caught.

Also removes a redundant warning log immediately before raising,
which prevents complete handling of NoSuchError and produces unavoidable duplicate logs.

jupyter_client/consoleapp.py

@@ -306,8 +306,11 @@ def init_kernel_manager(self) -> None:
                 parent=self,
                 data_dir=self.data_dir,
             )
+            # access kernel_spec to ensure the NoSuchKernel error is raised
+            # if it's going to be
+            kernel_spec = self.kernel_manager.kernel_spec  # noqa: F841
         except NoSuchKernel:
-            self.log.critical("Could not find kernel %s", self.kernel_name)
+            self.log.critical("Could not find kernel %r", self.kernel_name)
             self.exit(1)  # type:ignore[attr-defined]
 
         self.kernel_manager = t.cast(KernelManager, self.kernel_manager)

jupyter_client/kernelspec.py

@@ -281,7 +281,6 @@ def get_kernel_spec(self, kernel_name: str) -> KernelSpec:
 
         resource_dir = self._find_spec_directory(kernel_name.lower())
         if resource_dir is None:
-            self.log.warning("Kernelspec name %s cannot be found!", kernel_name)
             raise NoSuchKernel(kernel_name)
 
         return self._get_kernel_spec_by_name(kernel_name, resource_dir)

tests/test_runapp.py

@@ -0,0 +1,34 @@
+import sys
+from subprocess import run
+
+
+def test_runapp(tmp_path):
+    test_py = tmp_path / "test.py"
+    with test_py.open("w") as f:
+        f.write("print('hello')")
+
+    p = run(
+        [sys.executable, "-m", "jupyter_client.runapp", str(test_py)],
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+    assert p.stdout.strip() == "hello"
+
+
+def test_no_such_kernel(tmp_path):
+    test_py = tmp_path / "test.py"
+    with test_py.open("w") as f:
+        f.write("print('hello')")
+    kernel_name = "nosuchkernel"
+    p = run(
+        [sys.executable, "-m", "jupyter_client.runapp", "--kernel", kernel_name, str(test_py)],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert p.returncode
+    assert "Could not find kernel" in p.stderr
+    assert kernel_name in p.stderr
+    # shouldn't show a traceback
+    assert "Traceback" not in p.stderr