From ce7bf5ce708d901e9677ca40e9802fd597a6ea3c Mon Sep 17 00:00:00 2001
From: Jan Petykiewicz <jan@mpxd.net>
Date: Wed, 1 Apr 2026 22:58:30 -0700
Subject: [PATCH] [ILibrary / LazyLibrary] raise a LibraryError instead of
 KeyError

---
 masque/library.py           | 4 ++++
 masque/test/test_library.py | 9 +++++++++
 2 files changed, 13 insertions(+)

diff --git a/masque/library.py b/masque/library.py
index 825dbf0..5a60e8d 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -826,6 +826,8 @@ class ILibrary(ILibraryView, MutableMapping[str, 'Pattern'], metaclass=ABCMeta):
         Returns:
             self
         """
+        if old_name not in self:
+            raise LibraryError(f'"{old_name}" does not exist in the library.')
         self[new_name] = self[old_name]
         del self[old_name]
         if move_references:
@@ -1479,6 +1481,8 @@ class LazyLibrary(ILibrary):
         Returns:
             self
         """
+        if old_name not in self.mapping:
+            raise LibraryError(f'"{old_name}" does not exist in the library.')
         self[new_name] = self.mapping[old_name]        # copy over function
         if old_name in self.cache:
             self.cache[new_name] = self.cache[old_name]
diff --git a/masque/test/test_library.py b/masque/test/test_library.py
index 3b731ad..621e703 100644
--- a/masque/test/test_library.py
+++ b/masque/test/test_library.py
@@ -221,6 +221,15 @@ def test_library_rename() -> None:
     assert "old" not in lib["parent"].refs
 
 
+
+
+@pytest.mark.parametrize("library_cls", (Library, LazyLibrary))
+def test_library_rename_missing_raises_library_error(library_cls: type[Library] | type[LazyLibrary]) -> None:
+    lib = library_cls()
+    lib["top"] = Pattern()
+
+    with pytest.raises(LibraryError, match="does not exist"):
+        lib.rename("missing", "new")
 def test_library_dfs_can_replace_existing_patterns() -> None:
     lib = Library()
     child = Pattern()