[Python-checkins] gh-102578: Optimise setting and deleting mutable attributes on non-dataclass subclasses of frozen dataclasses (gh-102573)

[ericvsmith]

https://github.com/python/cpython/commit/ee6f8413a99d0ee4828e1c81911e203d3fff85d5
commit: ee6f8413a99d0ee4828e1c81911e203d3fff85d5
branch: main
author: Xuehai Pan <XuehaiPan at outlook.com>
committer: ericvsmith <ericvsmith at users.noreply.github.com>
date: 2023-03-10T19:21:22-05:00
summary:

gh-102578: Optimise setting and deleting mutable attributes on non-dataclass subclasses of frozen dataclasses (gh-102573)

files:
A Misc/NEWS.d/next/Library/2023-03-10-13-21-16.gh-issue-102578.-gujoI.rst
M Lib/dataclasses.py
M Lib/test/test_dataclasses.py

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 8bc8594d674b..78a126f051e2 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -616,21 +616,19 @@ def _repr_fn(fields, globals):
 def _frozen_get_del_attr(cls, fields, globals):
     locals = {'cls': cls,
               'FrozenInstanceError': FrozenInstanceError}
+    condition = 'type(self) is cls'
     if fields:
-        fields_str = '(' + ','.join(repr(f.name) for f in fields) + ',)'
-    else:
-        # Special case for the zero-length tuple.
-        fields_str = '()'
+        condition += ' or name in {' + ', '.join(repr(f.name) for f in fields) + '}'
     return (_create_fn('__setattr__',
                       ('self', 'name', 'value'),
-                      (f'if type(self) is cls or name in {fields_str}:',
+                      (f'if {condition}:',
                         ' raise FrozenInstanceError(f"cannot assign to field {name!r}")',
                        f'super(cls, self).__setattr__(name, value)'),
                        locals=locals,
                        globals=globals),
             _create_fn('__delattr__',
                       ('self', 'name'),
-                      (f'if type(self) is cls or name in {fields_str}:',
+                      (f'if {condition}:',
                         ' raise FrozenInstanceError(f"cannot delete field {name!r}")',
                        f'super(cls, self).__delattr__(name)'),
                        locals=locals,
diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
index 589f229f4623..5486b2ef3f47 100644
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -2767,6 +2767,19 @@ class C:
             c.i = 5
         self.assertEqual(c.i, 10)
 
+    def test_frozen_empty(self):
+        @dataclass(frozen=True)
+        class C:
+            pass
+
+        c = C()
+        self.assertFalse(hasattr(c, 'i'))
+        with self.assertRaises(FrozenInstanceError):
+            c.i = 5
+        self.assertFalse(hasattr(c, 'i'))
+        with self.assertRaises(FrozenInstanceError):
+            del c.i
+
     def test_inherit(self):
         @dataclass(frozen=True)
         class C:
@@ -2890,6 +2903,37 @@ class S(D):
         self.assertEqual(s.y, 10)
         self.assertEqual(s.cached, True)
 
+        with self.assertRaises(FrozenInstanceError):
+            del s.x
+        self.assertEqual(s.x, 3)
+        with self.assertRaises(FrozenInstanceError):
+            del s.y
+        self.assertEqual(s.y, 10)
+        del s.cached
+        self.assertFalse(hasattr(s, 'cached'))
+        with self.assertRaises(AttributeError) as cm:
+            del s.cached
+        self.assertNotIsInstance(cm.exception, FrozenInstanceError)
+
+    def test_non_frozen_normal_derived_from_empty_frozen(self):
+        @dataclass(frozen=True)
+        class D:
+            pass
+
+        class S(D):
+            pass
+
+        s = S()
+        self.assertFalse(hasattr(s, 'x'))
+        s.x = 5
+        self.assertEqual(s.x, 5)
+
+        del s.x
+        self.assertFalse(hasattr(s, 'x'))
+        with self.assertRaises(AttributeError) as cm:
+            del s.x
+        self.assertNotIsInstance(cm.exception, FrozenInstanceError)
+
     def test_overwriting_frozen(self):
         # frozen uses __setattr__ and __delattr__.
         with self.assertRaisesRegex(TypeError,
diff --git a/Misc/NEWS.d/next/Library/2023-03-10-13-21-16.gh-issue-102578.-gujoI.rst b/Misc/NEWS.d/next/Library/2023-03-10-13-21-16.gh-issue-102578.-gujoI.rst
new file mode 100644
index 000000000000..7307148d9a81
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-03-10-13-21-16.gh-issue-102578.-gujoI.rst
@@ -0,0 +1,4 @@
+Speed up setting or deleting mutable attributes on non-dataclass subclasses of
+frozen dataclasses. Due to the implementation of ``__setattr__`` and
+``__delattr__`` for frozen dataclasses, this previously had a time complexity
+of ``O(n)``. It now has a time complexity of ``O(1)``.