bpo-36470: Allow dataclasses.replace() to handle InitVars with default values (GH-20867) (GH-25201)

miss-islington · ZackerySpytz · web-flow · commit bdee2a389e4b · 2021-04-05T16:07:29.000-04:00
Co-Authored-By: Claudiu Popa <pcmanticore@gmail.com> Automerge-Triggered-By: GH:ericvsmith (cherry picked from commit 7522067) Co-authored-by: Zackery Spytz <zspytz@gmail.com> Co-authored-by: Zackery Spytz <zspytz@gmail.com>
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
@@ -1280,7 +1280,7 @@ class C:
             continue
 
         if f.name not in changes:
-            if f._field_type is _FIELD_INITVAR:
+            if f._field_type is _FIELD_INITVAR and f.default is MISSING:
                 raise ValueError(f"InitVar {f.name!r} "
                                  'must be specified with replace()')
             changes[f.name] = getattr(obj, f.name)
diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
@@ -3252,6 +3252,24 @@ def __post_init__(self, y):
         c = replace(c, x=3, y=5)
         self.assertEqual(c.x, 15)
 
+    def test_initvar_with_default_value(self):
+        @dataclass
+        class C:
+            x: int
+            y: InitVar[int] = None
+            z: InitVar[int] = 42
+
+            def __post_init__(self, y, z):
+                if y is not None:
+                    self.x += y
+                if z is not None:
+                    self.x += z
+
+        c = C(x=1, y=10, z=1)
+        self.assertEqual(replace(c), C(x=12))
+        self.assertEqual(replace(c, y=4), C(x=12, y=4, z=42))
+        self.assertEqual(replace(c, y=4, z=1), C(x=12, y=4, z=1))
+
     def test_recursive_repr(self):
         @dataclass
         class C:
diff --git a/Misc/NEWS.d/next/Library/2020-06-13-23-33-32.bpo-36470.oi6Kdb.rst b/Misc/NEWS.d/next/Library/2020-06-13-23-33-32.bpo-36470.oi6Kdb.rst
@@ -0,0 +1,2 @@
+Fix dataclasses with ``InitVar``\s and :func:`~dataclasses.replace()`. Patch
+by Claudiu Popa.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Fix dataclasses with ``InitVar``\s and :func:`~dataclasses.replace()`. Patch
	`2`	`+by Claudiu Popa.`