fix(core): make cuda.core.system actually fall back when NVML is unimportable - #2574

Problem

cuda_core/cuda/core/system/_system.pyx opens with the contract it is meant to satisfy:

# This file needs to either use NVML exclusively, or when `cuda.bindings.nvml`
# isn't available, fall back to non-NVML-based methods for backward
# compatibility.

The code that is supposed to implement that does neither:

if CUDA_BINDINGS_NVML_IS_COMPATIBLE:
    try:
        from cuda.bindings import nvml
    except ImportError:
        CUDA_BINDINGS_NVML_IS_COMPATIBLE = False

    from cuda.core.system._nvml_context import initialize
else:
    from cuda.core._utils.cuda_utils import driver, handle_return, runtime

The except clears CUDA_BINDINGS_NVML_IS_COMPATIBLE, but the else hangs off the outer if, which has already been evaluated. So on the one path that clears the flag, the non-NVML names are never bound — while every consumer in the file keys off the now-False flag and reaches for exactly those names:

Both would raise NameError. In practice the module never reaches them: from cuda.core.system._nvml_context import initialize on the next line runs unconditionally, and _nvml_context.pyx:7 is itself from cuda.bindings import nvml — so the very ImportError the try/except exists to absorb is re-raised one statement later and import cuda.core.system fails outright.

cuda.core.system.__init__ is built around the flag being usable in this state (elif CUDA_BINDINGS_NVML_IS_COMPATIBLE: gates the NVML-only submodules), and cuda.core._device reads it at _device.pyx:1053, so the degraded mode is a supported configuration — it just cannot be reached.

Fix

Move the _nvml_context import into the same try (it depends on nvml, so it belongs there), and replace the else with a second if not CUDA_BINDINGS_NVML_IS_COMPATIBLE: so a flag cleared by the failed import selects the fallback.

Tests

The branch is import-time, so it can only be exercised in a fresh interpreter. test_system_falls_back_when_nvml_is_unimportable runs a subprocess that installs a sys.meta_path finder raising ImportError for cuda.bindings.nvml, then imports cuda.core.system and asserts:

• the import succeeds (it used to raise),
• CUDA_BINDINGS_NVML_IS_COMPATIBLE is False,
• driver, handle_return, runtime are bound on cuda.core.system._system.

It does not call into the driver, so it needs no GPU.

What I ran

Environment: macOS, no CUDA driver and no CUDA toolkit, so cuda.core cannot be built or imported here.

• Did not run: the new test, or anything else under cuda_core/tests/ — they need a built cuda.core.
• Ran: a reduction of the import block with two stand-in modules (repro_nvml, unimportable; repro_nvml_context, which imports it), keeping the control flow verbatim:

before: import of the module FAILED -> ImportError: libnvidia-ml not present in this build
after:  CUDA_BINDINGS_NVML_IS_COMPATIBLE=False, non-NVML fallback bound=True

• Ran: the sys.meta_path blocker used by the new test, standalone against a stdlib submodule, to confirm it turns both from X import Y and import X.Y into ImportError.
• Ran: python -m py_compile on the changed test file and compile() on the embedded subprocess script (both parse), plus ruff check / ruff format --check. ruff check reports the same single pre-existing I001 on this file as it does on main; no new findings.
• Checked: every reader of CUDA_BINDINGS_NVML_IS_COMPATIBLE in the tree (_system.pyx, system/__init__.py, _device.pyx:1053, and four test modules) — none of them depend on the else form.