gh-102947: Improve traceback when calling fields() on a non-dataclass (#102948)

AlexWaygood · web-flow · commit baf4eb083c09 · 2023-03-23T14:17:54.000Z
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
@@ -1248,7 +1248,7 @@ def fields(class_or_instance):
     try:
         fields = getattr(class_or_instance, _FIELDS)
     except AttributeError:
-        raise TypeError('must be called with a dataclass type or instance')
+        raise TypeError('must be called with a dataclass type or instance') from None
 
     # Exclude pseudo-fields.  Note that fields is sorted by insertion
     # order, so the order of the tuple is as the fields were defined.
diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
@@ -5,11 +5,13 @@
 from dataclasses import *
 
 import abc
+import io
 import pickle
 import inspect
 import builtins
 import types
 import weakref
+import traceback
 import unittest
 from unittest.mock import Mock
 from typing import ClassVar, Any, List, Union, Tuple, Dict, Generic, TypeVar, Optional, Protocol, DefaultDict
@@ -1526,6 +1528,16 @@ class C: pass
         with self.assertRaisesRegex(TypeError, 'dataclass type or instance'):
             fields(C())
 
+    def test_clean_traceback_from_fields_exception(self):
+        stdout = io.StringIO()
+        try:
+            fields(object)
+        except TypeError as exc:
+            traceback.print_exception(exc, file=stdout)
+        printed_traceback = stdout.getvalue()
+        self.assertNotIn("AttributeError", printed_traceback)
+        self.assertNotIn("__dataclass_fields__", printed_traceback)
+
     def test_helper_asdict(self):
         # Basic tests for asdict(), it should return a new dictionary.
         @dataclass
diff --git a/Misc/NEWS.d/next/Library/2023-03-23-13-34-33.gh-issue-102947.cTwcpU.rst b/Misc/NEWS.d/next/Library/2023-03-23-13-34-33.gh-issue-102947.cTwcpU.rst
@@ -0,0 +1,2 @@
+Improve traceback when :func:`dataclasses.fields` is called on a
+non-dataclass. Patch by Alex Waygood

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Improve traceback when :func:`dataclasses.fields` is called on a
	`2`	`+non-dataclass. Patch by Alex Waygood`