[ILibraryView] don't fail on nested dangling ref

masque/library.py

@@ -294,8 +294,9 @@ class ILibraryView(Mapping[str, 'Pattern'], metaclass=ABCMeta):
         def flatten_single(name: str) -> None:
             flattened[name] = None
             pat = self[name].deepcopy()
+            refs_by_target = tuple((target, tuple(refs)) for target, refs in pat.refs.items())
 
-            for target in pat.refs:
+            for target, refs in refs_by_target:
                 if target is None:
                     continue
                 if dangling_ok and target not in self:
@@ -310,7 +311,7 @@ class ILibraryView(Mapping[str, 'Pattern'], metaclass=ABCMeta):
                 if target_pat.is_empty() and not ports_only:        # avoid some extra allocations
                     continue
 
-                for ref in pat.refs[target]:
+                for ref in refs:
                     if flatten_ports and ref.repetition is not None and target_pat.ports:
                         raise PatternError(
                             f'Cannot flatten ports from repeated ref to {target!r}; '

masque/test/test_library.py

@@ -166,6 +166,24 @@ def test_library_flatten_repeated_ref_with_ports_raises() -> None:
         lib.flatten("parent", flatten_ports=True)
 
 
+def test_library_flatten_dangling_ok_nested_preserves_dangling_refs() -> None:
+    lib = Library()
+    child = Pattern()
+    child.ref("missing")
+    lib["child"] = child
+
+    parent = Pattern()
+    parent.ref("child")
+    lib["parent"] = parent
+
+    flat = lib.flatten("parent", dangling_ok=True)
+
+    assert set(flat["child"].refs) == {"missing"}
+    assert flat["child"].has_refs()
+    assert set(flat["parent"].refs) == {"missing"}
+    assert flat["parent"].has_refs()
+
+
 def test_lazy_library() -> None:
     lib = LazyLibrary()
     called = 0