Backend fastpath

doc/whats-new.rst

@@ -35,6 +35,13 @@ Internal Changes
 ~~~~~~~~~~~~~~~~
 
 
+Performance
+~~~~~~~~~~~
+
+- Add a fastpath to the backend plugin system for standard engines (:issue:`10178`, :pull:`10937`).
+  By `Sam Levang <https://github.com/slevang>`_.
+
+
 .. _whats-new.2025.12.0:
 
 v2025.12.0 (Dec 5, 2025)

xarray/backends/plugins.py

@@ -215,15 +215,21 @@ def guess_engine(
 def get_backend(engine: str | type[BackendEntrypoint]) -> BackendEntrypoint:
     """Select open_dataset method based on current engine."""
     if isinstance(engine, str):
-        engines = list_engines()
-        if engine not in engines:
-            raise ValueError(
-                f"unrecognized engine '{engine}' must be one of your download engines: {list(engines)}. "
-                "To install additional dependencies, see:\n"
-                "https://docs.xarray.dev/en/stable/user-guide/io.html \n"
-                "https://docs.xarray.dev/en/stable/getting-started-guide/installing.html"
-            )
-        backend = engines[engine]
+        if engine in BACKEND_ENTRYPOINTS:
+            # fast path for built-in engines
+            backend_cls = BACKEND_ENTRYPOINTS[engine][1]
+            set_missing_parameters({engine: backend_cls})
+            backend = backend_cls()
+        else:
+            engines = list_engines()
+            if engine not in engines:
+                raise ValueError(
+                    f"unrecognized engine '{engine}' must be one of your download engines: {list(engines)}. "
+                    "To install additional dependencies, see:\n"
+                    "https://docs.xarray.dev/en/stable/user-guide/io.html \n"
+                    "https://docs.xarray.dev/en/stable/getting-started-guide/installing.html"
+                )
+            backend = engines[engine]
     elif issubclass(engine, BackendEntrypoint):
         backend = engine()
     else:

xarray/tests/test_plugins.py

@@ -208,6 +208,15 @@ def test_engines_not_installed() -> None:
         plugins.guess_engine("foo.nc")
 
 
+@pytest.mark.parametrize("engine", common.BACKEND_ENTRYPOINTS.keys())
+def test_get_backend_fastpath_skips_list_engines(engine: str) -> None:
+    """Test that built-in engines skip list_engines (fastpath)."""
+    plugins.list_engines.cache_clear()
+    initial_misses = plugins.list_engines.cache_info().misses
+    plugins.get_backend(engine)
+    assert plugins.list_engines.cache_info().misses == initial_misses
+
+
 def test_lazy_import() -> None:
     """Test that some modules are imported in a lazy manner.