test: add test_api_contracts with Editor/Document method coverage check

tests/test_api_contracts.py

@@ -0,0 +1,50 @@
+"""Structural API contract tests.
+
+These tests assert invariants about the library's surface — method coverage,
+class relationships, etc. — rather than testing runtime behaviour.
+"""
+
+from __future__ import annotations
+
+import inspect
+
+from yamltrip.document import Document
+from yamltrip.editor import Editor
+
+# Document methods that Editor intentionally does not expose:
+#   dump / dumps   — Editor owns the write path; callers don't call these directly.
+#   has_anchors    — read-only introspection, not a mutation.
+#   query_pretty   — read-only introspection, not a mutation.
+_DOCUMENT_ONLY = frozenset({"dump", "dumps", "has_anchors", "query_pretty"})
+
+
+def _public_methods(cls: type) -> set[str]:
+    return {
+        name
+        for name, _ in inspect.getmembers(cls, predicate=callable)
+        if not name.startswith("_")
+    }
+
+
+class TestEditorDocumentMethodCoverage:
+    """Every public Document method must have a corresponding Editor wrapper,
+    except for those explicitly listed in _DOCUMENT_ONLY."""
+
+    def test_editor_exposes_all_document_mutation_methods(self):
+        doc_methods = _public_methods(Document)
+        ed_methods = _public_methods(Editor)
+        missing = (doc_methods - _DOCUMENT_ONLY) - ed_methods
+        assert not missing, (
+            f"Editor is missing wrappers for Document methods: {sorted(missing)}\n"
+            "Add the wrapper or add the method name to _DOCUMENT_ONLY if intentional."
+        )
+
+    def test_document_only_set_is_accurate(self):
+        """Fail if _DOCUMENT_ONLY names a method that Editor actually does expose,
+        so the exclusion list doesn't silently become stale."""
+        ed_methods = _public_methods(Editor)
+        stale = _DOCUMENT_ONLY & ed_methods
+        assert not stale, (
+            f"_DOCUMENT_ONLY lists methods that Editor already exposes: {sorted(stale)}\n"
+            "Remove them from _DOCUMENT_ONLY."
+        )