test(extract): cover Python decorator reference edges (#2154)

Nine cases: the issue's imported-decorator repro, same-file resolution to
the local definition, called and attribute decorators, stacked
decorators, class-qualified method owners, a decorated class, the #1050
@property class-qualification regression guard, and an absence control.

tests/test_python_decorators.py

@@ -0,0 +1,168 @@
+"""Regression tests: Python decorator references (#2154).
+
+Applying a Python decorator emitted no edge to the decorator symbol, so
+`affected <decorator>` answered "No affected nodes found" for every function it
+wraps — a silent false negative on reverse-impact queries.
+
+TS/JS already emitted these edges (`_ts_emit_decorator_edges`); the Python
+`decorated_definition` branch walked its children only to propagate the parent
+class id (#1050) and never looked at the `decorator` children. Python now emits
+the same shape: `references` edges with context="decorator" from the decorated
+function/class to the decorator symbol, resolved through the same
+sourceless-stub path as type references so an imported decorator collapses onto
+its real definition.
+"""
+from pathlib import Path
+
+from graphify.extract import _file_stem, _make_id, extract
+
+
+def _write(path: Path, text: str) -> Path:
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(text, encoding="utf-8")
+    return path
+
+
+def _stem(file: str) -> str:
+    return _file_stem(Path(file))
+
+
+def _func_nid(file: str, func: str) -> str:
+    return _make_id(_stem(file), func)
+
+
+def _class_nid(file: str, cls: str) -> str:
+    return _make_id(_stem(file), cls)
+
+
+def _method_nid(file: str, cls: str, method: str) -> str:
+    return _make_id(_class_nid(file, cls), method)
+
+
+def _deco_edges(result: dict, owner_nid: str) -> set[str]:
+    """Decorator-reference edge targets emitted from owner_nid."""
+    return {
+        e["target"]
+        for e in result["edges"]
+        if e["source"] == owner_nid
+        and e["relation"] == "references"
+        and e.get("context") == "decorator"
+    }
+
+
+def test_module_level_function_decorator(tmp_path):
+    # The issue repro: decorator imported from another module, applied to a
+    # module-level function. Target is the sourceless stub the rewire collapses.
+    f = _write(tmp_path / "pkg" / "consumer.py",
+               "from deco import my_decorator\n"
+               "\n"
+               "@my_decorator\n"
+               "def business_logic():\n"
+               "    pass\n")
+    r = extract([f], cache_root=tmp_path)
+    assert _make_id("my_decorator") in _deco_edges(
+        r, _func_nid("pkg/consumer.py", "business_logic"))
+
+
+def test_same_file_decorator_resolves_to_local_definition(tmp_path):
+    # Decorator defined above its use in the same file: the edge must point at
+    # the real local node, not a stub.
+    f = _write(tmp_path / "pkg" / "local.py",
+               "def my_decorator(fn):\n"
+               "    return fn\n"
+               "\n"
+               "@my_decorator\n"
+               "def business_logic():\n"
+               "    pass\n")
+    r = extract([f], cache_root=tmp_path)
+    assert _func_nid("pkg/local.py", "my_decorator") in _deco_edges(
+        r, _func_nid("pkg/local.py", "business_logic"))
+
+
+def test_decorator_with_arguments(tmp_path):
+    # `@deco(arg)` is a `call` node; the head symbol is its `function` field.
+    f = _write(tmp_path / "pkg" / "args.py",
+               "from deco import retry\n"
+               "\n"
+               "@retry(times=3)\n"
+               "def flaky():\n"
+               "    pass\n")
+    r = extract([f], cache_root=tmp_path)
+    assert _make_id("retry") in _deco_edges(r, _func_nid("pkg/args.py", "flaky"))
+
+
+def test_attribute_decorator_targets_the_symbol_not_the_module(tmp_path):
+    # `@app.route("/")` is an `attribute` under a `call`; the target is `route`,
+    # matching _ts_decorator_name's member_expression handling.
+    f = _write(tmp_path / "pkg" / "web.py",
+               "import app\n"
+               "\n"
+               "@app.route(\"/\")\n"
+               "def index():\n"
+               "    pass\n")
+    r = extract([f], cache_root=tmp_path)
+    targets = _deco_edges(r, _func_nid("pkg/web.py", "index"))
+    assert _make_id("route") in targets
+    assert _make_id("app") not in targets
+
+
+def test_stacked_decorators_all_emit(tmp_path):
+    f = _write(tmp_path / "pkg" / "stack.py",
+               "from deco import a, b, c\n"
+               "\n"
+               "@a\n"
+               "@b\n"
+               "@c\n"
+               "def target():\n"
+               "    pass\n")
+    r = extract([f], cache_root=tmp_path)
+    targets = _deco_edges(r, _func_nid("pkg/stack.py", "target"))
+    assert {_make_id("a"), _make_id("b"), _make_id("c")} <= targets
+
+
+def test_decorated_method_owner_is_class_qualified(tmp_path):
+    # Guards the #1050 interaction: the owner must be the class-qualified method
+    # id, not a bare module-level id.
+    f = _write(tmp_path / "pkg" / "svc.py",
+               "from deco import traced\n"
+               "\n"
+               "class Service:\n"
+               "    @traced\n"
+               "    def handle(self):\n"
+               "        pass\n")
+    r = extract([f], cache_root=tmp_path)
+    assert _make_id("traced") in _deco_edges(
+        r, _method_nid("pkg/svc.py", "Service", "handle"))
+
+
+def test_property_still_class_qualified(tmp_path):
+    # #1050 regression guard: @property/@staticmethod must not change the
+    # method node's class-qualified id.
+    f = _write(tmp_path / "pkg" / "prop.py",
+               "class Config:\n"
+               "    @property\n"
+               "    def name(self):\n"
+               "        return 1\n")
+    r = extract([f], cache_root=tmp_path)
+    assert any(n["id"] == _method_nid("pkg/prop.py", "Config", "name")
+               for n in r["nodes"])
+
+
+def test_decorated_class(tmp_path):
+    f = _write(tmp_path / "pkg" / "model.py",
+               "from dataclasses import dataclass\n"
+               "\n"
+               "@dataclass\n"
+               "class Point:\n"
+               "    x: int\n")
+    r = extract([f], cache_root=tmp_path)
+    assert _make_id("dataclass") in _deco_edges(
+        r, _class_nid("pkg/model.py", "Point"))
+
+
+def test_undecorated_function_emits_no_decorator_edge(tmp_path):
+    f = _write(tmp_path / "pkg" / "plain.py",
+               "def plain():\n"
+               "    pass\n")
+    r = extract([f], cache_root=tmp_path)
+    assert _deco_edges(r, _func_nid("pkg/plain.py", "plain")) == set()