Enforce Child dispatch exhaustiveness with assert_never, drop the mock test

assert_never on the Child dataclass itself does not typecheck: mypy cannot
prove exhaustiveness from attribute patterns on a class with three optional
fields, so the fall-through arm keeps the type "Child" instead of "Never".
Matching on `child.background or child.rule or child.scenario` gives mypy a
proper union (Background | Rule | Scenario | None) to exhaust; removing any
arm is now a mypy error.

Both defensive arms (empty Child, assert_never) are unreachable with the
current gherkin version, so they are excluded from coverage instead of being
exercised through a monkeypatched document, and the mock-based test is
removed. "pragma: no cover" is re-added to coverage's exclude_lines: setting
exclude_lines in the config replaces coverage.py's defaults, so the pragma
had silently stopped working.

Author: Pierre-Sassoulas

pyproject.toml

@@ -96,6 +96,7 @@ lint.isort.required-imports = [
 
 [tool.coverage.report]
 exclude_lines = [
+    "pragma: no cover",
     "if TYPE_CHECKING:",
     "if typing\\.TYPE_CHECKING:",
 ]

src/pytest_bdd/parser.py

@@ -8,9 +8,11 @@
 from collections.abc import Generator, Iterable, Mapping, Sequence
 from dataclasses import dataclass, field
 
+from typing_extensions import assert_never
+
 from .exceptions import StepError
 from .gherkin_parser import Background as GherkinBackground
-from .gherkin_parser import Child, DataTable, GherkinDocument, get_gherkin_document
+from .gherkin_parser import DataTable, GherkinDocument, get_gherkin_document
 from .gherkin_parser import Feature as GherkinFeature
 from .gherkin_parser import Rule as GherkinRule
 from .gherkin_parser import Scenario as GherkinScenario
@@ -492,13 +494,19 @@ def parse(self) -> Feature:
         )
 
         for child in feature_data.children:
-            match child:
-                case Child(background=GherkinBackground() as background):
+            match child.background or child.rule or child.scenario:
+                case GherkinBackground() as background:
                     feature.background = self.parse_background(background)
-                case Child(rule=GherkinRule() as rule):
+                case GherkinRule() as rule:
                     self._parse_and_add_rule(rule, feature)
-                case Child(scenario=GherkinScenario() as scenario):
+                case GherkinScenario() as scenario:
                     self._parse_and_add_scenario(scenario, feature)
+                case None:  # pragma: no cover
+                    # An empty Child (a child kind newer than this gherkin version);
+                    # skip it, like unknown children were skipped before the match.
+                    pass
+                case unreachable:  # pragma: no cover
+                    assert_never(unreachable)
 
         return feature
 

tests/parser/test_parser.py

@@ -1,9 +1,7 @@
 from __future__ import annotations
 
-import textwrap
 from pathlib import Path
 
-from src.pytest_bdd import parser as parser_module
 from src.pytest_bdd.gherkin_parser import (
     Background,
     Cell,
@@ -853,35 +851,3 @@ def test_parser():
     )
 
     assert gherkin_doc == expected_document
-
-
-def test_feature_parser_skips_unknown_child_kinds(tmp_path, monkeypatch):
-    """A Child with none of background/rule/scenario set is skipped.
-
-    The gherkin AST reserves the right to grow new child kinds; ``Child.from_dict``
-    would map such a child to an all-``None`` dataclass, and ``FeatureParser.parse``
-    must ignore it instead of crashing.
-    """
-    (tmp_path / "minimal.feature").write_text(
-        textwrap.dedent(
-            """\
-            Feature: Minimal
-              Scenario: A scenario
-                Given a step
-            """
-        )
-    )
-
-    real_get_gherkin_document = parser_module.get_gherkin_document
-
-    def get_gherkin_document_with_unknown_child(abs_filename: str, encoding: str = "utf-8") -> GherkinDocument:
-        document = real_get_gherkin_document(abs_filename, encoding)
-        document.feature.children.append(Child())
-        return document
-
-    monkeypatch.setattr(parser_module, "get_gherkin_document", get_gherkin_document_with_unknown_child)
-
-    feature = parser_module.FeatureParser(str(tmp_path), "minimal.feature").parse()
-
-    assert list(feature.scenarios) == ["A scenario"]
-    assert feature.background is None