Investigate ty type checker alongside mypy#682
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why: Evaluate Astral's Rust-based ty type checker (beta) as a companion to mypy. ty is 10-100x faster, checks unannotated function bodies by default, and supports intersection types and reachability analysis. libtmux is well-positioned: no mypy plugins, minimal type: ignore comments, and already uses Astral's ruff and uv tooling. what: - Add ty to dev and lint dependency groups - Add [tool.ty] configuration (python-version 3.10, src/tests scope) - Add justfile targets: ty, watch-ty - Add non-blocking ty CI step (continue-on-error: true)
why: All 3 unresolved-import errors target _pytest.python_api.RaisesContext, a private pytest API that is not publicly exported. mypy flags these as attr-defined (already suppressed with type: ignore comments); ty categorizes them differently as unresolved-import. This is a known ty limitation with private/internal module members. what: - Add unresolved-import = "ignore" to [tool.ty.rules] - Reference: astral-sh/ty#1276
why: 48 false positives. ty resolves the cmd() method as a union type that includes a (Any, Any, /) -> tmux_cmd variant from CmdProtocol, limiting it to 2 positional args. The real signature accepts *args via cmd(cmd: str, *args: Any, *, target=None). ty does not yet fully support argument unpacking for *args methods (astral-sh/ty#404). what: - Add too-many-positional-arguments = "ignore" to [tool.ty.rules] - Reference: astral-sh/ty#404
why: 26 false positives from the same *args/**kwargs unpacking limitation. When calling with **kwargs, ty cannot statically verify that required parameters are present and reports them as missing. mypy and pyright handle **kwargs unpacking correctly. what: - Add missing-argument = "ignore" to [tool.ty.rules] - Reference: astral-sh/ty#785
why: 12 false positives. ty falls back to object.__init__ when resolving calls through union return types or dataclass-transform decorators, then flags all keyword arguments as unknown. This affects the cmd() method pattern used throughout libtmux. what: - Add unknown-argument = "ignore" to [tool.ty.rules] - Reference: astral-sh/ty#2369
why: 14 warnings in tests using monkeypatch.setattr(libtmux.common, ...) without an explicit import of libtmux.common. The submodule is always available because libtmux.__init__.py re-exports it, but ty cannot detect implicit submodule registration via __init__.py imports. Neither mypy nor pyright flag this pattern. what: - Add possibly-missing-submodule = "ignore" to [tool.ty.rules] - Reference: astral-sh/ty#133
why: 6 false positives, primarily in vendored _vendor/version.py which uses tuple comparison with mixed-type elements (int, str, InfinityType) for PEP 440 version ordering. ty cannot resolve element-wise comparison operators across union-typed tuples. Also affects `in` operator checks in tests. what: - Add unsupported-operator = "ignore" to [tool.ty.rules] - Reference: astral-sh/ty#1202
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #682 +/- ##
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Coverage 51.29% 51.29%
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Files 25 25
Lines 3488 3488
Branches 686 686
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Hits 1789 1789
Misses 1404 1404
Partials 295 295 ☔ View full report in Codecov by Sentry. 🚀 New features to boost your workflow:
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why: 20 false positives. ty cannot verify argument types through **kwargs unpacking (11 from **call_kwargs in test_pane, 4 from **filter_expr in test_query_list) and narrows LiteralString to str differently than mypy (2 in test_options). Same root cause as the missing-argument and unknown-argument suppressions. what: - Add invalid-argument-type = "ignore" to [tool.ty.rules] - References: astral-sh/ty#785, astral-sh/ty#404
why: 5 false positives. ty doesn't narrow through dict value iteration (item.split() in options.py where item is typed as object from dict iteration) or union access patterns (pane_id on Pane | None in test_client.py, already suppressed for mypy with type: ignore). what: - Add unresolved-attribute = "ignore" to [tool.ty.rules]
why: 4 false positives. ty cannot narrow TypeVars through isinstance guards (subscript assignment on _V@convert_values in options.py after isinstance(value, dict)), and flags IO[str] | None -> IO[str] in control_mode.py (already suppressed for mypy with type: ignore). what: - Add invalid-assignment = "ignore" to [tool.ty.rules]
why: 3 false positives in Pane, Session, and Window classes. These inherit from Obj which defines fields with defaults; the subclasses add a required server: Server field. Python's dataclass inheritance handles this correctly at runtime, but ty's dataclass analysis doesn't account for the inheritance init pattern. what: - Add dataclass-field-order = "ignore" to [tool.ty.rules]
why: 2 false positives in query_list.py lookup_regex/lookup_iregex. isinstance narrows data and rhs to str | bytes, but ty cannot resolve re.search overloads for this union — it expects either (str, str) or (bytes, Buffer), not (str | bytes, str | bytes). what: - Add no-matching-overload = "ignore" to [tool.ty.rules]
why: 2 false positives. options.py:1235 returns a dict subscript on a complex nested type that ty resolves as object instead of the declared str | int | None. test_session.py:567 returns MockTmuxCmd where tmux_cmd is expected (already suppressed for mypy with type: ignore). what: - Add invalid-return-type = "ignore" to [tool.ty.rules]
why: 1 false positive in query_list.py:505. b[key] where b is typed as object from a type narrowing path ty cannot follow — the same context as the unresolved-attribute false positive on b.keys() nearby. what: - Add not-subscriptable = "ignore" to [tool.ty.rules]
why: 1 false positive in query_list.py:553. filter_(k) where filter_ is a union of callable types including T@QueryList & Top[(...) -> object]. ty's intersection type resolution produces an uncallable Top type from the complex QueryList generic parameter. what: - Add call-top-callable = "ignore" to [tool.ty.rules]
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Summary
Closes #681.
Adds ty (Astral's Rust-based Python type checker, beta v0.0.38) as a companion to mypy for evaluation. ty is 10-100x faster than mypy, checks unannotated function bodies by default, and supports intersection types and reachability analysis.
libtmux is well-positioned for ty adoption:
type: ignorecomments across the codebaseWhat's included
devandlintdependency groups[tool.ty]configuration inpyproject.toml(python-version 3.10, src/tests scope)just tyandjust watch-tytargets in justfilecontinue-on-error: true)Rule suppressions
Each rule is suppressed in its own commit with a documented rationale. All are false positives from ty's beta-stage limitations.
unresolved-import_pytest.python_api.RaisesContextnot publicly exportedtoo-many-positional-argumentscmd()union-type resolution with*args— incomplete argument unpackingmissing-argument*args/**kwargsunpacking limitationunknown-argumentobject.__init__for union return typespossibly-missing-submodulemonkeypatch.setattr(libtmux.common, ...)— implicit submodule via__init__.pyunsupported-operatorinvalid-argument-type**kwargsunpacking +LiteralStringvsstrnarrowingunresolved-attributeobject, union narrowing gapsinvalid-assignmentIO[str] | Noneunionsdataclass-field-orderno-matching-overloadre.searchoverload withstr | bytesunion argsinvalid-return-typeobject, mock return typesnot-subscriptablecall-top-callableToptypeTotal: 147 diagnostics → 0 after all suppressions.
Speed comparison
ty completes in under 2 seconds on the full codebase — mypy takes ~10-15s.
Test plan
uv run ty check src tests— 0 diagnostics (all checks passed)uv run mypy .— 69 files, no issuesuv run ruff check . && uv run ruff format . --check— all checks passeduv run pytest --reruns 0— 1258 passed, 2 skippedjust build-docs— builds successfully