warn users when they are using older versions.

kernels/src/kernels/_versions.py

@@ -1,9 +1,12 @@
+import logging
 import warnings
 
 from huggingface_hub.hf_api import GitRefInfo
 from packaging.specifiers import SpecifierSet
 from packaging.version import InvalidVersion, Version
 
+logger = logging.getLogger(__name__)
+
 
 def _get_available_versions(repo_id: str) -> dict[int, GitRefInfo]:
     """Get kernel versions that are available in the repository."""
@@ -55,6 +58,16 @@ def resolve_version_spec_as_ref(repo_id: str, version_spec: int | str) -> GitRef
             raise ValueError(
                 f"Version {version_spec} not found, available versions: {', '.join(sorted(str(v) for v in versions.keys()))}"
             )
+
+        latest_version = max(versions.keys())
+        if version_spec < latest_version:
+            logger.warning(
+                "You are using version %d of '%s', but version %d is available.",
+                version_spec,
+                repo_id,
+                latest_version,
+            )
+
         return ref
     else:
         warnings.warn(

kernels/tests/test_basic.py

@@ -1,3 +1,5 @@
+import logging
+
 import pytest
 import torch
 import torch.nn.functional as F
@@ -161,6 +163,22 @@ def test_version():
         kernel = get_kernel("kernels-test/versions", version=0)
 
 
+def test_version_outdated_warning(caplog):
+    with caplog.at_level(logging.WARNING, logger="kernels._versions"):
+        kernel = get_kernel("kernels-test/versions", version=1)
+    assert kernel.version() == "1"
+    assert (
+        "You are using version 1 of 'kernels-test/versions', but version 2 is available."
+        in caplog.text
+    )
+
+    caplog.clear()
+    with caplog.at_level(logging.WARNING, logger="kernels._versions"):
+        kernel = get_kernel("kernels-test/versions", version=2)
+    assert kernel.version() == "2"
+    assert "but version" not in caplog.text
+
+
 @pytest.mark.cuda_only
 def test_universal_kernel(universal_kernel):
     torch.manual_seed(0)