Allow dataclass subclasses to define required fields after optional base fields

Author: LukeGrochocki

Lib/dataclasses.py

@@ -409,12 +409,21 @@ def field(*, default=MISSING, default_factory=MISSING, init=True, repr=True,
 
 
 def _fields_in_init_order(fields):
-    # Returns the fields as __init__ will output them.  It returns 2 tuples:
-    # the first for normal args, and the second for keyword args.
+    required_fields = []
+    optional_fields = []
+    kw_fields = []
+    for f in fields:
+        if not f.init:
+            continue
+        if f.kw_only:
+            kw_fields.append(f)
+        elif f.default is MISSING and f.default_factory is MISSING:
+            required_fields.append(f)
+        else:
+            optional_fields.append(f)
+
+    return tuple(required_fields + optional_fields), tuple(kw_fields)
 
-    return (tuple(f for f in fields if f.init and not f.kw_only),
-            tuple(f for f in fields if f.init and f.kw_only)
-            )
 
 
 def _tuple_str(obj_name, fields):

Lib/test/test_dataclass.py

@@ -0,0 +1,26 @@
+import unittest
+from dataclasses import dataclass
+
+class TestDataclassInheritanceFieldOrder(unittest.TestCase):
+    def test_required_after_optional_in_subclass(self):
+        @dataclass
+        class Base:
+            x: int = 10  # optional field with default
+
+        @dataclass
+        class Sub(Base):
+            y: int  # required field in subclass
+
+        # Should require y but allow optional x to be omitted
+        obj = Sub(y=5)
+        self.assertEqual(obj.x, 10)
+        self.assertEqual(obj.y, 5)
+
+        # Should allow overriding x
+        obj2 = Sub(x=42, y=9)
+        self.assertEqual(obj2.x, 42)
+        self.assertEqual(obj2.y, 9)
+
+        # Missing y should raise TypeError
+        with self.assertRaises(TypeError):
+            Sub()