fix(app): avoid KeyError when no filter OP provides tunable stats

[Linxiushen]

Fixes #872 (assigned to @HYLcool and @Dludora since January without a linked branch — if a fix is already planned, feel free to close this).

Problem

app.py::Visualize.filter_dataset builds one boolean mask per filter OP whose stats keys are listed in op_stats_dict. That dict covers 13 of the 58 registered filters — none of the image/video/audio filters, nor newer text filters such as general_field_filter. With a recipe whose filters are all uncovered (e.g. only image_shape_filter, the reporter's case), conds stays empty, and with "Process by op order" unchecked (the default):

all_conds = np.all([list(cond.values())[0] for cond in conds], axis=0)   # np.all([], axis=0) -> np.True_
...
examples = dataframe.loc[cond]                                             # label lookup, not a mask
KeyError: True

Reproduced by executing the real app.py (Streamlit stubbed) with a recipe containing only image_shape_filter; the pandas frames in the traceback match #872 line for line (pandas 2.2.3, the locked version; also fails on 3.0). The bundled default recipe (demos/process_simple/process.yaml, language_id_score_filter) is covered, so this needs a user recipe — but any multimodal recipe is.

Change

Move the mask combination into a module-level combine_conds(conds, num_samples): the non-empty branch keeps the exact np.all([...], axis=0) expression; the empty case returns np.ones(num_samples, dtype=bool) ("nothing to filter on ⇒ keep everything"), mirroring the range(len(stats)) shape the ordered branch already uses. The ordered=True branch and np.invert(all_conds) for the discarded view are unchanged.

Tests

tests/tools/test_app.py loads app.py by path (as tests/tools/test_build_op_doc.py does for the root-level hook script) and checks: empty conds yields an all-True mask that dataframe.loc accepts; non-empty conds still combine element-wise; the inverted mask selects nothing. Fails on main (KeyError: True), passes here.

black / isort / flake8 with the versions pinned in .pre-commit-config.yaml pass on app.py (the tests directory is excluded from pre-commit). Note for CI: partial mode maps app.py to a root-level test_app.py that does not exist, so this PR will fall back to the full test run.

Written with Claude Code (AI-assisted) and submitted under the account owner's authorization.

[cmgzn]

LGTM, Thanks for your work