gh-121210: handle nodes with missing attributes/fields in `ast.compare` (#121211)

1 files changed, 15 insertions, 4 deletions

diff --git a/Lib/ast.py b/Lib/ast.py
index fb4d21b87d8..a954d4a97d3 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -422,6 +422,8 @@ def compare(
     might differ in whitespace or similar details.
     """
 
+    sentinel = object()  # handle the possibility of a missing attribute/field
+
     def _compare(a, b):
         # Compare two fields on an AST object, which may themselves be
         # AST objects, lists of AST objects, or primitive ASDL types
@@ -449,8 +451,14 @@ def compare(
         if a._fields != b._fields:
             return False
         for field in a._fields:
-            a_field = getattr(a, field)
-            b_field = getattr(b, field)
+            a_field = getattr(a, field, sentinel)
+            b_field = getattr(b, field, sentinel)
+            if a_field is sentinel and b_field is sentinel:
+                # both nodes are missing a field at runtime
+                continue
+            if a_field is sentinel or b_field is sentinel:
+                # one of the node is missing a field
+                return False
             if not _compare(a_field, b_field):
                 return False
         else:
@@ -461,8 +469,11 @@ def compare(
             return False
         # Attributes are always ints.
         for attr in a._attributes:
-            a_attr = getattr(a, attr)
-            b_attr = getattr(b, attr)
+            a_attr = getattr(a, attr, sentinel)
+            b_attr = getattr(b, attr, sentinel)
+            if a_attr is sentinel and b_attr is sentinel:
+                # both nodes are missing an attribute at runtime
+                continue
             if a_attr != b_attr:
                 return False
         else: