Investigate ty type checker alongside mypy

.github/workflows/tests.yml

@@ -67,6 +67,10 @@ jobs:
       - name: Lint with mypy
         run: uv run mypy .
 
+      - name: Lint with ty
+        continue-on-error: true
+        run: uv run ty check .
+
       - name: Print python versions
         run: |
           python -V

justfile

@@ -114,6 +114,23 @@ watch-mypy:
         just _entr-warn
     fi
 
+# Run ty type checker
+[group: 'lint']
+ty:
+    uv run ty check src tests
+
+# Watch files and run ty on change
+[group: 'lint']
+watch-ty:
+    #!/usr/bin/env bash
+    set -euo pipefail
+    if command -v entr > /dev/null; then
+        {{ py_files }} | entr -c just ty
+    else
+        just ty
+        just _entr-warn
+    fi
+
 # Format markdown files with prettier
 [group: 'format']
 format-markdown:

pyproject.toml

@@ -72,6 +72,7 @@ dev = [
   # Lint
   "ruff",
   "mypy",
+  "ty",
 ]
 
 docs = [
@@ -97,6 +98,7 @@ lint = [
   "typing-extensions; python_version < '3.11'",
   "ruff",
   "mypy",
+  "ty",
 ]
 
 [project.entry-points.pytest11]
@@ -138,6 +140,73 @@ files = [
   "tests",
 ]
 
+[tool.ty.environment]
+python-version = "3.10"
+
+[tool.ty.src]
+include = ["src", "tests"]
+
+[tool.ty.rules]
+# Private _pytest APIs (e.g. RaisesContext) are not publicly exported;
+# both mypy and ty flag them. ty categorizes these as unresolved-import
+# rather than attr-defined.
+# https://github.com/astral-sh/ty/issues/1276
+unresolved-import = "ignore"
+# ty resolves cmd() as a union type including a (Any, Any, /) -> tmux_cmd
+# variant from CmdProtocol, causing false positives on *args methods.
+# ty does not yet fully support argument unpacking (*args/**kwargs).
+# https://github.com/astral-sh/ty/issues/404
+too-many-positional-arguments = "ignore"
+# Same root cause as too-many-positional-arguments: ty cannot verify
+# required params are present when calling with **kwargs unpacking.
+# https://github.com/astral-sh/ty/issues/785
+missing-argument = "ignore"
+# ty falls back to object.__init__ for union return types and
+# dataclass-transform decorators, flagging all kwargs as unknown.
+# https://github.com/astral-sh/ty/issues/2369
+unknown-argument = "ignore"
+# Tests use monkeypatch.setattr(libtmux.common, ...) without explicit
+# submodule import. The submodule is always available via __init__.py
+# re-exports but ty cannot detect implicit registration.
+# https://github.com/astral-sh/ty/issues/133
+possibly-missing-submodule = "ignore"
+# Vendored version.py 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.
+# https://github.com/astral-sh/ty/issues/1202
+unsupported-operator = "ignore"
+# ty cannot verify argument types through **kwargs unpacking and
+# narrows LiteralString to str differently than mypy. 20 false
+# positives, mostly from **call_kwargs and **filter_expr patterns.
+# https://github.com/astral-sh/ty/issues/785
+invalid-argument-type = "ignore"
+# ty doesn't narrow through dict value iteration (item.split() in
+# options.py) or union access patterns (Pane | None). 5 false
+# positives where mypy correctly narrows the type.
+unresolved-attribute = "ignore"
+# ty can't see through isinstance narrowing for TypeVars (subscript
+# assignment on _V in options.py) and IO[str] | None unions
+# (control_mode.py, already suppressed for mypy). 4 false positives.
+invalid-assignment = "ignore"
+# Pane, Session, Window inherit from Obj which defines defaulted fields;
+# subclasses add required server: Server. Python dataclass inheritance
+# handles this at runtime but ty's analysis doesn't account for it.
+dataclass-field-order = "ignore"
+# re.search(rhs, data) in query_list.py where isinstance narrows both
+# to str | bytes, but ty can't match the overload (str, str) | (bytes,
+# Buffer) against (str | bytes, str | bytes). 2 false positives.
+no-matching-overload = "ignore"
+# options.py returns dict subscript typed as object where str | int |
+# None expected; test_session.py returns MockTmuxCmd where tmux_cmd
+# expected (already suppressed for mypy). 2 false positives.
+invalid-return-type = "ignore"
+# query_list.py b[key] where b is typed as object from a narrowing
+# path ty can't follow (same context as unresolved-attribute).
+not-subscriptable = "ignore"
+# query_list.py filter_(k) where filter_ is a union including
+# T@QueryList & Top[(...) -> object] — ty's intersection type
+# resolution produces an uncallable Top type.
+call-top-callable = "ignore"
 
 [tool.coverage.run]
 branch = true