refactor: Switch method to module function

I should have done this ages ago.  Instead of `_validate_stage_name`
being a static method in the `StagedScript` class, make it a function in
the `staged_script` module.  We then no longer need to ignore the MyPy
error about `__class__` being undefined, and Ruff's SLF linter won't
complain about private member access.

Author: jmgate

pyproject.toml

@@ -116,7 +116,7 @@ extend-select = [
     "RUF",
     "S",
     "SIM",
-    # "SLF",
+    "SLF",
     "SLOT",
     "T10",
     "T20",

staged_script/staged_script.py

@@ -43,6 +43,28 @@
 rich.traceback.install()
 
 
+def validate_stage_name(stage_name: str) -> None:
+    """
+    Validate a stage name.
+
+    Ensure a stage name consists of only lowercase letters.  This is
+    both to simplify implementation details within the
+    :class:`StagedScript` class, and to provide the best user experience
+    for users of your :class:`StagedScript` subclasses.
+
+    Args:
+        stage_name:  The name of the stage.
+
+    Raises:
+        ValueError:  If the stage name is invalid.
+    """
+    if not re.match("^[a-z]+$", stage_name):
+        message = (
+            f"Stage name {stage_name!r} must contain only lowercase letters."
+        )
+        raise ValueError(message)
+
+
 class StagedScript:
     """
     The base class for all staged scripts.
@@ -147,7 +169,7 @@ def __init__(
             and optionally pass in additional arguments.
         """
         for stage in stages:
-            self._validate_stage_name(stage)
+            validate_stage_name(stage)
         self.args = Namespace()
         self.commands_executed: list[str] = []
         self.console = Console(
@@ -165,29 +187,6 @@ def __init__(
         self.stages_to_run: set[str] = set()
         self.start_time = datetime.now(tz=timezone.utc)
 
-    @staticmethod
-    def _validate_stage_name(stage_name: str) -> None:
-        """
-        Validate the stage name.
-
-        Ensure the stage name consists of only lowercase letters.  This
-        is both to simplify implementation details within the class, and
-        to provide the best user experience for users of your
-        :class:`StagedScript` subclasses.
-
-        Args:
-            stage_name:  The name of the stage.
-
-        Raises:
-            ValueError:  If the stage name is invalid.
-        """
-        if not re.match("^[a-z]+$", stage_name):
-            message = (
-                f"Stage name {stage_name!r} must contain only lowercase "
-                "letters."
-            )
-            raise ValueError(message)
-
     #
     # The `stage` decorator.
     #
@@ -245,9 +244,7 @@ def stage(stage_name: str, heading: str) -> Callable:
             heading:  A heading message to print indicating what will
                 happen in the stage.
         """
-        __class__._validate_stage_name(  # type: ignore[name-defined]
-            stage_name
-        )
+        validate_stage_name(stage_name)
 
         def decorator(func: Callable) -> Callable:
             def get_phase_method(  # noqa: D417

test/test_staged_script.py

@@ -18,6 +18,7 @@
 from rich.console import Console
 
 from staged_script import StagedScript, StageDuration
+from staged_script.staged_script import validate_stage_name
 
 
 @pytest.fixture
@@ -39,21 +40,21 @@ def test_print_dry_run_message(
     assert expected in captured.out
 
 
-def test__validate_stage_name() -> None:
-    """Test the :func:`validate_stage_name` method."""
-    StagedScript._validate_stage_name("valid")
+def test_validate_stage_name() -> None:
+    """Test the :func:`validate_stage_name` function."""
+    validate_stage_name("valid")
 
 
 @pytest.mark.parametrize(
     "stage_name", ["Uppercase", "spa ces", "hyphen-ated", "under_scores"]
 )
-def test__validate_stage_name_raises(stage_name: str) -> None:
+def test_validate_stage_name_raises(stage_name: str) -> None:
     """Ensure :func:`validate_stage_name` raises an exception when needed."""
     with pytest.raises(
         ValueError,
         match=f"'{stage_name}' must contain only lowercase letters",
     ):
-        StagedScript._validate_stage_name(stage_name)
+        validate_stage_name(stage_name)
 
 
 def test__begin_stage(