Adding a provider which can tell what accessor to use to go from the …

…parent to that child node (#807)

libcst/metadata/__init__.py

@@ -5,6 +5,7 @@
 
 
 from libcst._position import CodePosition, CodeRange
+from libcst.metadata.accessor_provider import AccessorProvider
 from libcst.metadata.base_provider import (
     BaseMetadataProvider,
     BatchableMetadataProvider,
@@ -86,6 +87,7 @@
     "Accesses",
     "TypeInferenceProvider",
     "FullRepoManager",
+    "AccessorProvider",
     # Experimental APIs:
     "ExperimentalReentrantCodegenProvider",
     "CodegenPartial",

libcst/metadata/accessor_provider.py

@@ -0,0 +1,19 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+
+import dataclasses
+
+import libcst as cst
+
+from libcst.metadata.base_provider import VisitorMetadataProvider
+
+
+class AccessorProvider(VisitorMetadataProvider[str]):
+    def on_visit(self, node: cst.CSTNode) -> bool:
+        for f in dataclasses.fields(node):
+            child = getattr(node, f.name)
+            self.set_metadata(child, f.name)
+        return True

libcst/metadata/tests/test_accessor_provider.py

@@ -0,0 +1,68 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+import dataclasses
+
+from textwrap import dedent
+
+import libcst as cst
+from libcst.metadata import AccessorProvider, MetadataWrapper
+from libcst.testing.utils import data_provider, UnitTest
+
+
+class DependentVisitor(cst.CSTVisitor):
+    METADATA_DEPENDENCIES = (AccessorProvider,)
+
+    def __init__(self, *, test: UnitTest) -> None:
+        self.test = test
+
+    def on_visit(self, node: cst.CSTNode) -> bool:
+        for f in dataclasses.fields(node):
+            child = getattr(node, f.name)
+            if type(child) is cst.CSTNode:
+                accessor = self.get_metadata(AccessorProvider, child)
+                self.test.assertEqual(accessor, f.name)
+
+        return True
+
+
+class AccessorProviderTest(UnitTest):
+    @data_provider(
+        (
+            (
+                """
+                foo = 'toplevel'
+                fn1(foo)
+                fn2(foo)
+                def fn_def():
+                    foo = 'shadow'
+                    fn3(foo)
+                """,
+            ),
+            (
+                """
+                global_var = None
+                @cls_attr
+                class Cls(cls_attr, kwarg=cls_attr):
+                    cls_attr = 5
+                    def f():
+                        pass
+                """,
+            ),
+            (
+                """
+                iterator = None
+                condition = None
+                [elt for target in iterator if condition]
+                {elt for target in iterator if condition}
+                {elt: target for target in iterator if condition}
+                (elt for target in iterator if condition)
+                """,
+            ),
+        )
+    )
+    def test_accessor_provier(self, code: str) -> None:
+        wrapper = MetadataWrapper(cst.parse_module(dedent(code)))
+        wrapper.visit(DependentVisitor(test=self))