lib.data: raise ValueError if initializer refers to nonexistent key.

Previously, `KeyError` was raised, which should not be escaping
the implementation (`Layout.const` is not a retrieval method).

Author: whitequark

amaranth/lib/data.py

@@ -219,12 +219,16 @@ def const(self, init):
         elif isinstance(init, Sequence):
             iterator = enumerate(init)
         else:
-            raise TypeError("Layout constant initializer must be a mapping or a sequence, not {!r}"
-                            .format(init))
+            raise TypeError(f"Layout constant initializer must be a mapping or a sequence, not "
+                            f"{init!r}")
 
         int_value = 0
         for key, key_value in iterator:
-            field = self[key]
+            try:
+                field = self[key]
+            except KeyError:
+                raise ValueError(f"Layout constant initializer refers to key {key!r}, which is not "
+                                 f"a part of the layout")
             cast_field_shape = Shape.cast(field.shape)
             if isinstance(field.shape, ShapeCastable):
                 key_value = hdl.Const.cast(hdl.Const(key_value, field.shape))

tests/test_lib_data.py

@@ -477,6 +477,10 @@ def test_const_wrong(self):
                 r"^Layout constant initializer must be a mapping or a sequence, not "
                 r"<.+?object.+?>$"):
             sl.const(object())
+        with self.assertRaisesRegex(ValueError,
+                r"^Layout constant initializer refers to key 'g', which is not a part "
+                r"of the layout$"):
+            sl.const({"g": 1})
         sl2 = data.StructLayout({"f": unsigned(2)})
         with self.assertRaisesRegex(ValueError,
                 r"^Const layout StructLayout.* differs from shape layout StructLayout.*$"):