Adopt DF12 Python linting

.rules/python-pyproject.md

@@ -106,7 +106,7 @@ Published "extras" that an *end user* opts into to enable an optional feature
 of the package, requested with `package[extra]` syntax (for example,
 `pandas[excel]`). Reach for this only when the extra dependency powers
 user-facing functionality that not everyone needs — never for development
-tooling. Add them with `uv add --optional <extra>`:
+tooling. Add them with `uv add --optional <extra> <package>`:
 
 ```toml
 [project.optional-dependencies]
@@ -122,8 +122,9 @@ Tooling only contributors need: test frameworks, linters, type checkers,
 documentation builders, and property or mutation testers. These are
 **local-only** — PEP 735 dependency groups are *not* included in published
 package metadata (they are not part of the wheel), so they must live here rather
-than in `project.optional-dependencies`. Add them with `uv add --dev` (the
-`dev` group) or `uv add --group <name>`:
+than in `project.optional-dependencies`. Add them with
+`uv add --dev <package>` (the `dev` group) or
+`uv add --group <name> <package>`:
 
 ```toml
 [dependency-groups]
@@ -138,8 +139,9 @@ dev = [
 `uv sync` include the `dev` group with no extra flags, so a bare `uv sync`
 gives a contributor the full toolchain. Adjust this with:
 
-- `--no-dev` or `--no-default-groups` to exclude development dependencies (for
-  example, when building a wheel or a production install).
+- `--no-dev` to exclude only the `dev` group.
+- `--no-default-groups` to disable configured default groups while still
+  permitting explicit selection of other groups.
 - `--group <name>` or `--only-group <name>` to include or isolate a
   non-default group.
 - `[tool.uv].default-groups` to change which groups sync by default:
@@ -190,19 +192,19 @@ ______________________________________________________________________
 ## 5. Declaring a Build System
 
 PEP 517/518 require a `[build-system]` table to tell tools how to build and
-install your project. A "modern" convention is to specify `setuptools>=61.0`
+install your project. A "modern" convention is to specify `setuptools>=64.0`
 (for editable installs without `setup.py`) or a lighter alternative like
 `flit_core`. Below is the typical setup using setuptools:
 
 ```toml
 [build-system]
-requires = ["setuptools>=61.0", "wheel"]
+requires = ["setuptools>=64.0"]
 build-backend = "setuptools.build_meta"
 ```
 
 - **`requires`:** A list of packages needed at build time. For editable installs
-  in `uv`, you need at least `setuptools>=61.0` and `wheel`. (Python
-  Packaging[^4], Astral Docs[^7])
+  in `uv`, you need at least `setuptools>=64.0`. (Python Packaging[^4], Astral
+  Docs[^7])
 - **`build-backend`:** The entry point for your build backend.
   `setuptools.build_meta` is the PEP 517-compliant backend for setuptools.
   (Python Packaging[^4], Astral Docs[^7])
@@ -282,7 +284,7 @@ docs = [
 mycli = "my_project.cli:main"
 
 [build-system]
-requires = ["setuptools>=61.0", "wheel"]
+requires = ["setuptools>=64.0"]
 build-backend = "setuptools.build_meta"
 
 [tool.uv]
@@ -318,9 +320,8 @@ package = true
 
 4. **Build System:**
 
-   - `setuptools>=61.0` plus `wheel` ensures both legacy and editable installs
-     work. ✱ Newer versions of setuptools support PEP 660 editable installs
-     without a `setup.py` stub. (Python Packaging[^4], Astral Docs[^7])
+   - `setuptools>=64.0` supports PEP 660 editable installs without a
+     `setup.py` stub. (Python Packaging[^4], Astral Docs[^7])
    - `build-backend = "setuptools.build_meta"` tells `uv` how to compile your
      package. (Python Packaging[^4], Astral Docs[^7])
 
@@ -372,7 +373,7 @@ A "modern" `pyproject.toml` for an Astral `uv` project should:
   `[dependency-groups]` (the `dev` group installs by default).
 - Define any CLI or GUI entry points under `[project.scripts]` or
   `[project.gui-scripts]`.
-- Declare a PEP 517 `[build-system]` (e.g. `setuptools>=61.0`, `wheel`,
+- Declare a PEP 517 `[build-system]` (e.g. `setuptools>=64.0`,
   `setuptools.build_meta`) to support editable installs, or omit it and rely on
   `tool.uv.package = true`.
 - Include a `[tool.uv]` section, at minimum `package = true` if you want `uv` to

.rules/python-return.md

@@ -11,6 +11,7 @@ flow. Follow these rules:
 def func():
     return None
 
+
 # GOOD:
 def func():
     return
@@ -30,6 +31,7 @@ def func(x):
         return x
     # implicitly returns None (bad)
 
+
 # GOOD:
 def func(x):
     if x > 0:
@@ -41,14 +43,15 @@ Ensure all branches explicitly return a value if any branch does.
 
 ______________________________________________________________________
 
-## R503 — Add an Explicit Return at the End
+## R503 — Add an Explicit Return at the End When a Function May Return a Value
 
 ```python
 # BAD:
 def func(x):
     if x > 0:
         return x
-    # no return (bad)
+    # missing terminal return (bad)
+
 
 # GOOD:
 def func(x):
@@ -57,7 +60,17 @@ def func(x):
     return -1
 ```
 
-Don't rely on implicit `None`—always return something at the end.
+Don't rely on implicit `None` if the function may return a value elsewhere—always
+return something at the end.
+
+Functions whose only possible result is `None` do not need a final bare `return`:
+
+```python
+# GOOD:
+def func():
+    do_something()
+    # implicit None is fine here
+```
 
 ______________________________________________________________________
 
@@ -69,6 +82,7 @@ def func():
     result = compute()
     return result
 
+
 # GOOD:
 def func():
     return compute()

Makefile

@@ -14,6 +14,11 @@ PYLINT_PYPY_SHIM_REF ?= 726d09f968b4d729ee4b29c71fc732e744854f3b
 PYLINT_PYPY_SHIM = git+https://github.com/leynos/pylint-pypy-shim.git@$(PYLINT_PYPY_SHIM_REF)
 PYLINT_BASELINE_DISABLE = no-else-return,unnecessary-ellipsis,too-many-lines,too-many-arguments,too-many-positional-arguments,subprocess-run-check,use-implicit-booleaness-not-comparison-to-string,unnecessary-dunder-call,use-implicit-booleaness-not-comparison
 PYLINT = $(UV_ENV) $(UV) tool run --python $(PYLINT_PYTHON) --from '$(PYLINT_PYPY_SHIM)' pylint-pypy --disable=$(PYLINT_BASELINE_DISABLE)
+DF12_PYTHON ?= 3.14
+DF12_PYTHON_LINTS_REF ?= 9c835f35b0f1690597ade799c9c6a30bc5922959
+DF12_PYTHON_LINTS = git+https://github.com/leynos/df12-python-lints.git@$(DF12_PYTHON_LINTS_REF)
+DF12_PYLINT = $(UV_ENV) UV_PYTHON_PREFERENCE=only-managed $(UV) run --isolated --python $(DF12_PYTHON) --with '$(DF12_PYTHON_LINTS)' pylint
+AMBRLEAKS = $(UV_ENV) UV_PYTHON_PREFERENCE=only-managed $(UV) run --isolated --python $(DF12_PYTHON) --with '$(DF12_PYTHON_LINTS)' ambrleaks
 WINDOWS_SMOKE_ARGS = tests/test_windows_environment.py \
 	tests/test_windows_support_bdd.py \
 	--log-file=windows-ipc.log \
@@ -89,6 +94,8 @@ markdownlint-run: ## Run markdownlint-cli2 with pinned fallback
 lint: build ## Run linters
 	$(RUFF) check
 	$(PYLINT) $(PYLINT_TARGETS)
+	$(DF12_PYLINT) --rcfile=pylintrc-df12.toml $(PYLINT_TARGETS)
+	$(AMBRLEAKS) tests
 	+$(MAKE) spelling
 
 typecheck: build ## Run typechecking

cmd_mox/__init__.py

@@ -49,7 +49,30 @@ def __getattr__(name: str) -> _ModuleType: ...
 
 
 def __getattr__(name: str) -> _ModuleType | cabc.Callable[..., object]:
-    """Lazily import optional dependencies when requested."""
+    """Lazily import optional dependencies when requested.
+
+    Parameters
+    ----------
+    name : str
+        Module attribute to resolve.
+
+    Returns
+    -------
+    types.ModuleType or collections.abc.Callable[..., object]
+        Imported submodule, or the ``cmd_mox`` pytest fixture callable when
+        ``name`` is ``"cmd_mox_fixture"``.
+
+    Raises
+    ------
+    AttributeError
+        If ``name`` does not identify a submodule in :mod:`cmd_mox`.
+    RuntimeError
+        If resolving ``"cmd_mox_fixture"`` fails because an optional
+        dependency of the pytest plugin is unavailable.
+    ModuleNotFoundError
+        If importing a requested submodule fails because a dependency other
+        than the requested module is unavailable.
+    """
     if name == "cmd_mox_fixture":
         try:
             from .pytest_plugin import cmd_mox as _cmd_mox_fixture

cmd_mox/_path_utils.py

@@ -9,7 +9,13 @@
 
 
 def normalize_path_string(path: str) -> str:
-    """Return a normalized string path using platform rules."""
+    """Return a normalized string path using platform rules.
+
+    Returns
+    -------
+    str
+        The normalized path, using case-folding on Windows.
+    """
     module = ntpath if IS_WINDOWS else os.path
     normalized = module.normpath(path)
     if IS_WINDOWS:
@@ -18,5 +24,11 @@ def normalize_path_string(path: str) -> str:
 
 
 def normalize_path(path: os.PathLike[str] | str) -> str:
-    """Normalize *path* regardless of whether it is a string or Path."""
+    """Normalize *path* regardless of whether it is a string or Path.
+
+    Returns
+    -------
+    str
+        The normalized path string.
+    """
     return normalize_path_string(os.fspath(path))

cmd_mox/_shim_bootstrap.py

@@ -19,7 +19,13 @@
 
 
 def _try_get_stdlib_path() -> str | None:
-    """Return the configured stdlib path, or ``None`` if config lookup fails."""
+    """Return the configured stdlib path, or ``None`` if lookup fails.
+
+    Returns
+    -------
+    str or None
+        The configured standard-library directory when available.
+    """
     try:
         return sysconfig.get_path("stdlib")
     except (AttributeError, ImportError, KeyError, OSError, ValueError, TypeError):
@@ -43,7 +49,13 @@ def _temporary_sys_path(entries: tuple[str, ...]) -> cabc.Iterator[None]:
 
 
 def _get_stdlib_path() -> str | None:
-    """Return the stdlib path, guarding against missing or invalid configs."""
+    """Return the stdlib path, guarding against invalid configuration.
+
+    Returns
+    -------
+    str or None
+        The standard-library directory, if it can be resolved safely.
+    """
     stdlib_path = _try_get_stdlib_path()
     if stdlib_path is not None:
         return stdlib_path
@@ -53,7 +65,13 @@ def _get_stdlib_path() -> str | None:
 
 
 def _create_module_from_file(module_name: str, file_path: Path) -> ModuleType | None:
-    """Load and return a module from *file_path*, or ``None`` on failure."""
+    """Load a module from *file_path*, or return ``None`` on failure.
+
+    Returns
+    -------
+    ModuleType or None
+        The loaded module when the file has a usable import specification.
+    """
     try:
         if not file_path.is_file():
             return None
@@ -72,7 +90,13 @@ def _create_module_from_file(module_name: str, file_path: Path) -> ModuleType |
 
 
 def _load_stdlib_platform() -> ModuleType:
-    """Return the stdlib platform module, falling back to regular import."""
+    """Return the stdlib platform module, with a regular-import fallback.
+
+    Returns
+    -------
+    ModuleType
+        The platform module loaded from the standard library.
+    """
     importlib.invalidate_caches()
     stdlib_path = _get_stdlib_path()