fix: simplify UnsupportedTypeError message

bigframes/functions/function_typing.py

@@ -60,8 +60,22 @@ class UnsupportedTypeError(ValueError):
     def __init__(self, type_, supported_types):
         self.type = type_
         self.supported_types = supported_types
+
+        types_to_format = supported_types
+        if isinstance(supported_types, dict):
+            types_to_format = supported_types.keys()
+
+        supported_types_str = ", ".join(
+            sorted(
+                [
+                    getattr(supported, "__name__", supported)
+                    for supported in types_to_format
+                ]
+            )
+        )
+
         super().__init__(
-            f"'{type_}' must be one of the supported types ({supported_types}) "
+            f"'{getattr(type_, '__name__', type_)}' must be one of the supported types ({supported_types_str}) "
             "or a list of one of those types."
         )
 

tests/unit/functions/test_function_typing.py

@@ -0,0 +1,50 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import datetime
+import decimal
+
+import pytest
+
+from bigframes.functions import function_typing
+
+
+def test_unsupported_type_error_init_with_dict():
+    err = function_typing.UnsupportedTypeError(
+        decimal.Decimal, {int: "INT64", float: "FLOAT64"}
+    )
+
+    message = str(err)
+
+    assert "Decimal" in message
+    assert "float, int" in message
+
+
+def test_unsupported_type_error_init_with_set():
+    err = function_typing.UnsupportedTypeError(decimal.Decimal, {int, float})
+
+    message = str(err)
+
+    assert "Decimal" in message
+    assert "float, int" in message
+
+
+def test_sdk_type_from_python_type_raises_unsupported_type_error():
+    with pytest.raises(function_typing.UnsupportedTypeError) as excinfo:
+        function_typing.sdk_type_from_python_type(datetime.datetime)
+
+    message = str(excinfo.value)
+
+    assert "datetime" in message
+    assert "bool, bytes, float, int, str" in message