fallback: match the C extension's strict_map_key error message

Author: Vansh8803

msgpack/fallback.py

@@ -539,15 +539,21 @@ def _unpack(self, execute=EX_CONSTRUCT):
                 for _ in range(n):
                     key = self._unpack(EX_CONSTRUCT)
                     if self._strict_map_key and type(key) not in (str, bytes):
-                        raise ValueError("%s is not allowed for map key" % str(type(key)))
+                        raise ValueError(
+                            "%s is not allowed for map key when strict_map_key=True"
+                            % type(key).__name__
+                        )
                     pairs.append((key, self._unpack(EX_CONSTRUCT)))
                 ret = self._object_pairs_hook(pairs)
             else:
                 ret = {}
                 for _ in range(n):
                     key = self._unpack(EX_CONSTRUCT)
                     if self._strict_map_key and type(key) not in (str, bytes):
-                        raise ValueError("%s is not allowed for map key" % str(type(key)))
+                        raise ValueError(
+                            "%s is not allowed for map key when strict_map_key=True"
+                            % type(key).__name__
+                        )
                     if isinstance(key, str):
                         key = sys.intern(key)
                     ret[key] = self._unpack(EX_CONSTRUCT)

test/test_except.py

@@ -147,6 +147,14 @@ def test_strict_map_key():
         unpackb(packed, raw=False, strict_map_key=True)
 
 
+def test_strict_map_key_error_message():
+    # The pure-Python fallback should report the same message as the C
+    # extension when a map key has a disallowed type.
+    packed = packb({42: 1}, use_bin_type=True)
+    with raises(ValueError, match="int is not allowed for map key when strict_map_key=True"):
+        unpackb(packed, raw=False, strict_map_key=True)
+
+
 def test_strict_map_key_with_object_pairs_hook():
     # strict_map_key should be enforced even when object_pairs_hook is set
     invalid = {42: "value"}