gh-107838: In dataclasses, improve error message when a non-default field follows a default field. (gh-107842)

Add the name of the previous default argument field in an error message.

Lib/dataclasses.py

@@ -575,15 +575,15 @@ def _init_fn(fields, std_fields, kw_only_fields, frozen, has_post_init,
     # message, and future-proofs us in case we build up the function
     # using ast.
 
-    seen_default = False
+    seen_default = None
     for f in std_fields:
         # Only consider the non-kw-only fields in the __init__ call.
         if f.init:
             if not (f.default is MISSING and f.default_factory is MISSING):
-                seen_default = True
+                seen_default = f
             elif seen_default:
                 raise TypeError(f'non-default argument {f.name!r} '
-                                'follows default argument')
+                                f'follows default argument {seen_default.name!r}')
 
     locals = {f'__dataclass_type_{f.name}__': f.type for f in fields}
     locals.update({

Lib/test/test_dataclasses.py

@@ -134,7 +134,7 @@ class C:
         # Non-defaults following defaults.
         with self.assertRaisesRegex(TypeError,
                                     "non-default argument 'y' follows "
-                                    "default argument"):
+                                    "default argument 'x'"):
             @dataclass
             class C:
                 x: int = 0
@@ -143,7 +143,7 @@ class C:
         # A derived class adds a non-default field after a default one.
         with self.assertRaisesRegex(TypeError,
                                     "non-default argument 'y' follows "
-                                    "default argument"):
+                                    "default argument 'x'"):
             @dataclass
             class B:
                 x: int = 0
@@ -156,7 +156,7 @@ class C(B):
         #  a field which didn't use to have a default.
         with self.assertRaisesRegex(TypeError,
                                     "non-default argument 'y' follows "
-                                    "default argument"):
+                                    "default argument 'x'"):
             @dataclass
             class B:
                 x: int
@@ -4521,7 +4521,7 @@ class A:
 
         # Make sure we still check for non-kwarg non-defaults not following
         # defaults.
-        err_regex = "non-default argument 'z' follows default argument"
+        err_regex = "non-default argument 'z' follows default argument 'a'"
         with self.assertRaisesRegex(TypeError, err_regex):
             @dataclass
             class A: