test: skip af.NSS unit tests when optional nss dependency is absent

[Jammy2211]

Overview

13 tests under test_autofit/non_linear/search/nest/nss/ fail in any environment without the
optional nss / handley-lab/blackjax extra installed. They guard on JAX
(pytest.importorskip("jax")) but not on the nss package, so af.NSS.__init__ raises
ImportError at instantiation instead of the tests skipping. This is a test-hygiene gap, not a
code regression (confirmed reproducible with no relevant local changes). The fix adds a skip
guard mirroring the existing JAX one.

Plan

• Add an optional-dependency skip guard to the two nss test files, mirroring importorskip("jax").
• Prefer a surgical guard: only skip tests that instantiate af.NSS; keep the pure
  checkpoint-serialization round-trip tests running if they work without the nss extra.
• Verify test_autofit/ reports 0 failures (13 -> skipped) without the extra, and still passes with it.

Detailed implementation plan

Affected Repositories

• rhayes777/PyAutoFit (primary, test-only)

Work Classification

Library (test-only)

Branch Survey

Repository	Current Branch	Dirty?
./PyAutoFit (canonical)	feature/latent-nan-masking-fix	dirty (unregistered)
PyAutoFit (worktree)	feature/analysis-shared-state	held by analysis-shared-state

Suggested branch: feature/nss-test-optional-dep-skip
Worktree root: ~/Code/PyAutoLabs-wt/nss-test-optional-dep-skip/

Implementation Steps

1. test_autofit/.../nss/test_search.py — add pytest.importorskip for the nss package (or skipif(not _HAS_NSS)) at module top.
2. test_autofit/.../nss/test_checkpoint.py — same; but first check whether the _save_checkpoint/_load_checkpoint serialization tests pass without nss (they import fine) and keep those running if so — only guard the af.NSS-instantiating tests.
3. Run full test_autofit/ without the [nss] extra -> confirm 13 become skips, 0 failures.

Key Files

• test_autofit/non_linear/search/nest/nss/test_search.py
• test_autofit/non_linear/search/nest/nss/test_checkpoint.py
• autofit/non_linear/search/nest/nss/search.py (reference only — _HAS_NSS flag; do not change the guard)

Original Prompt

Click to expand starting prompt

Skip af.NSS unit tests when the optional nss dependency is absent

Primary repo: @PyAutoFit (test-only change).

Problem

13 tests under test_autofit/non_linear/search/nest/nss/ fail on a clean checkout of
main in any environment that does not have the optional nss extra installed:

test_autofit/non_linear/search/nest/nss/test_checkpoint.py  (4 failures)
test_autofit/non_linear/search/nest/nss/test_search.py      (9 failures)

All 13 share one root cause (confirmed by reproducing on clean main, no local changes):

ImportError: af.NSS requires the optional `nss` package and the matching
`handley-lab/blackjax` fork. Install via:
    pip install autofit[nss]

raised at autofit/non_linear/search/nest/nss/search.py:254 inside af.NSS.__init__
(guarded by the module's _HAS_NSS flag).

This is not a code regression — it is a test-hygiene gap. The tests guard on JAX being
present (jax = pytest.importorskip("jax") at the top of test_checkpoint.py) but do not
guard on the nss optional dependency. So on a machine with jax but without the [nss]
extra (the default dev/CI environment here), every test that constructs af.NSS hard-fails
at import/instantiation instead of skipping.

Fix (resolve in the issue — confirm the surgical vs blanket choice)

Add a skip guard for the optional nss dependency, mirroring the existing
pytest.importorskip("jax") pattern. Options:

1. Module-level skip on both files:

   from autofit.non_linear.search.nest.nss.search import _HAS_NSS
   pytestmark = pytest.mark.skipif(not _HAS_NSS, reason="requires optional `nss` extra")

   (or pytest.importorskip("nss") / the blackjax-fork module at module top).

2. Surgical: only guard the tests that actually instantiate af.NSS. Note that
   test_checkpoint.py imports _save_checkpoint / _load_checkpoint at module top and
   these import fine without nss — so the pure-serialization round-trip tests may be able
   to run without the extra (they use synthetic pytree states, per the file's docstring:
   "No real nss.ns.run_nested_sampling calls"). Check whether those serialization tests pass
   once the af.NSS-instantiating tests are skipped; if so, prefer keeping them running and
   only skip the instantiation/config tests.

Decide between (1) and (2) in the issue. Favour (2) if the serialization helpers genuinely
work without nss — it keeps real coverage on this machine; fall back to (1) if they don't.

Constraints / context

• Library policy: no JAX in library unit tests — these files already respect that
  (test_search.py docstring states it, test_checkpoint.py uses importorskip("jax")).
  The fix must not introduce a hard JAX/nss import at collection time.
• The heavy end-to-end nss runs live in
  autolens_workspace_developer/searches_minimal/nss_*.py (per the test docstrings) — out of
  scope here.
• Verify the fix against the full test_autofit/ run: after the change, test_autofit/
  should report 0 failures in an environment without the [nss] extra (the 13 become
  skips), and still pass when the extra IS installed.

Critical files

• test_autofit/non_linear/search/nest/nss/test_checkpoint.py
• test_autofit/non_linear/search/nest/nss/test_search.py
• autofit/non_linear/search/nest/nss/search.py (reference: _HAS_NSS flag + the
  __init__ ImportError guard — do not change the guard itself)

Out of scope

• Installing the nss / blackjax-fork extra in the default environment.
• Any change to af.NSS runtime behaviour or the _HAS_NSS import guard.