Respect NUMEXPR_MAX_THREADS when setting numexpr thread count

Avoid calling numexpr.set_num_threads() with a value exceeding
NUMEXPR_MAX_THREADS to prevent numexpr from printing a warning
to stderr during blosc2 import.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: smendez-hrt

src/blosc2/__init__.py

@@ -9,6 +9,8 @@
 # ruff: noqa: E402 - Module level import not at top of file
 # ruff: noqa: F401 - `var` imported but unused
 
+import contextlib
+import os
 import platform
 from enum import Enum
 
@@ -402,7 +404,14 @@ def _raise(exc):
     nthreads -= nthreads // 8
 if not IS_WASM:
     # WASM does not support threading
-    numexpr.set_num_threads(nthreads)
+    # Only call set_num_threads if within NUMEXPR_MAX_THREADS limit to avoid warning
+    numexpr_max_env = os.environ.get("NUMEXPR_MAX_THREADS")
+    numexpr_max: int | None = None
+    if numexpr_max_env is not None:
+        with contextlib.suppress(ValueError):
+            numexpr_max = int(numexpr_max_env)
+    if numexpr_max is None or nthreads <= numexpr_max:
+        numexpr.set_num_threads(nthreads)
 
 # This import must be before ndarray and schunk
 from .storage import (  # noqa: I001

tests/test_numexpr_threads.py

@@ -0,0 +1,38 @@
+#######################################################################
+# Copyright (c) 2019-present, Blosc Development Team <blosc@blosc.org>
+# All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#######################################################################
+
+import os
+import subprocess
+import sys
+
+
+def test_numexpr_max_threads_no_warning():
+    """Test that importing blosc2 with NUMEXPR_MAX_THREADS set does not produce a warning.
+
+    When NUMEXPR_MAX_THREADS is set to a value lower than the number of threads
+    blosc2 would use, we should NOT call numexpr.set_num_threads() to avoid
+    the numexpr warning being printed to stderr.
+    """
+    # Inherit the current environment but set NUMEXPR_MAX_THREADS to a low value
+    env = os.environ.copy()
+    env["NUMEXPR_MAX_THREADS"] = "1"
+
+    result = subprocess.run(
+        [sys.executable, "-c", "import blosc2; print(blosc2.__version__)"],
+        capture_output=True,
+        text=True,
+        env=env,
+        check=True,
+    )
+
+    # Check that no warning about NUMEXPR_MAX_THREADS was printed
+    assert "NUMEXPR_MAX_THREADS" not in result.stderr, (
+        f"Unexpected numexpr warning in stderr: {result.stderr}"
+    )
+    assert "nthreads cannot be larger" not in result.stderr, (
+        f"Unexpected numexpr warning in stderr: {result.stderr}"
+    )