gh-142214: Fix two regressions in dataclasses

Author: JelleZijlstra

Lib/dataclasses.py

@@ -1,6 +1,7 @@
 import re
 import sys
 import copy
+import trace
 import types
 import inspect
 import keyword
@@ -550,7 +551,10 @@ def __annotate__(format, /):
 
                 new_annotations = {}
                 for k in annotation_fields:
-                    new_annotations[k] = cls_annotations[k]
+                    # gh-142214: The annotation may be missing in unusual dynamic cases.
+                    # If so, just skip it.
+                    if k in cls_annotations:
+                        new_annotations[k] = cls_annotations[k]
 
                 if return_type is not MISSING:
                     if format == Format.STRING:
@@ -1399,9 +1403,11 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
             f.type = ann
 
     # Fix the class reference in the __annotate__ method
-    init_annotate = newcls.__init__.__annotate__
-    if getattr(init_annotate, "__generated_by_dataclasses__", False):
-        _update_func_cell_for__class__(init_annotate, cls, newcls)
+    init = newcls.__init__
+    if hasattr(init, "__annotate__"):
+        init_annotate = init.__annotate__
+        if getattr(init_annotate, "__generated_by_dataclasses__", False):
+            _update_func_cell_for__class__(init_annotate, cls, newcls)
 
     return newcls
 

Lib/test/test_dataclasses/__init__.py

@@ -927,6 +927,20 @@ class C:
 
         validate_class(C)
 
+    def test_incomplete_annotations(self):
+        # gh-142214
+        @dataclass
+        class C:
+            "doc"  # needed because otherwise we fetch the annotations at the wrong time
+            x: int
+
+        C.__annotate__ = lambda _: {}
+
+        self.assertEqual(
+            annotationlib.get_annotations(C.__init__),
+            {"return": None}
+        )
+
     def test_missing_default(self):
         # Test that MISSING works the same as a default not being
         #  specified.
@@ -2578,6 +2592,20 @@ def __init__(self, x: int) -> None:
 
         self.assertFalse(hasattr(E.__init__.__annotate__, "__generated_by_dataclasses__"))
 
+    def test_slots_true_init_false(self):
+        # Test that slots=True and init=False work together and
+        #  that __annotate__ is not added to __init__.
+
+        @dataclass(slots=True, init=False)
+        class F:
+            x: int
+
+        f = F()
+        f.x = 10
+        self.assertEqual(f.x, 10)
+
+        self.assertFalse(hasattr(F.__init__, "__annotate__"))
+
     def test_init_false_forwardref(self):
         # Test forward references in fields not required for __init__ annotations.
 

Misc/NEWS.d/next/Library/2025-12-03-06-12-39.gh-issue-142214.appYNZ.rst

@@ -0,0 +1,8 @@
+Fix two regressions in :mod:`dataclasses` related to annotations:
+
+- An exception is no longer raised if ``slots=True`` is used and the
+``__init__`` method does not   have an ``__annotate__`` attribute (likely
+because ``init=False`` was used). - An exception is no longer raised if
+annotations are requested on the ``__init__`` method   and one of the fields
+is not present in the class annotations. This can occur in certain   dynamic
+scenarios.