gh-138991: Update dataclass documentation for new eq behavior in Python 3.13 and add tests

Doc/library/dataclasses.rst

@@ -103,10 +103,16 @@ Module contents
      ignored.
 
    - *eq*: If true (the default), an :meth:`~object.__eq__` method will be
-     generated.  This method compares the class as if it were a tuple
-     of its fields, in order.  Both instances in the comparison must
+     generated.
+
+     .. versionchanged:: 3.13
+        The generated ``__eq__`` method now compares each field individually (e.g., ``self.a == other.a and self.b == other.b``), rather than comparing tuples of fields as in previous versions.
+
+     This method compares the class by comparing each field in order. Both instances in the comparison must
      be of the identical type.
 
+     In Python 3.12 and earlier, the comparison was performed by creating tuples of the fields and comparing them (e.g., ``(self.a, self.b) == (other.a, other.b)``).
+
      If the class already defines :meth:`!__eq__`, this parameter is
      ignored.
 

Lib/test/test_dataclasses/__init__.py

@@ -2597,6 +2597,45 @@ def __eq__(self, other):
         self.assertEqual(C(1), 5)
         self.assertNotEqual(C(1), 1)
 
+    def test_eq_field_by_field(self):
+        @dataclasses.dataclass
+        class Point:
+            x: int
+            y: int
+
+        p1 = Point(1, 2)
+        p2 = Point(1, 2)
+        p3 = Point(2, 1)
+        self.assertTrue(p1 == p2)
+        self.assertFalse(p1 == p3)
+
+    def test_eq_type_check(self):
+        @dataclasses.dataclass
+        class A:
+            x: int
+
+        @dataclasses.dataclass
+        class B:
+            x: int
+
+        a = A(1)
+        b = B(1)
+        self.assertFalse(a == b)
+
+    def test_eq_custom_field(self):
+        class AlwaysEqual(int):
+            def __eq__(self, other):
+                return True
+
+        @dataclasses.dataclass
+        class Foo:
+            x: AlwaysEqual
+            y: int
+
+        f1 = Foo(AlwaysEqual(1), 2)
+        f2 = Foo(AlwaysEqual(2), 2)
+        self.assertTrue(f1 == f2)
+
 
 class TestOrdering(unittest.TestCase):
     def test_functools_total_ordering(self):