From 75220674c07abfc90c2cd7862d04cfa2e2354450 Mon Sep 17 00:00:00 2001
From: Zackery Spytz <zspytz@gmail.com>
Date: Mon, 5 Apr 2021 13:41:01 -0600
Subject: [PATCH] bpo-36470: Allow dataclasses.replace() to handle InitVars
 with default values (GH-20867)

Co-Authored-By: Claudiu Popa <pcmanticore@gmail.com>

Automerge-Triggered-By: GH:ericvsmith
---
 Lib/dataclasses.py                             |  2 +-
 Lib/test/test_dataclasses.py                   | 18 ++++++++++++++++++
 .../2020-06-13-23-33-32.bpo-36470.oi6Kdb.rst   |  2 ++
 3 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-06-13-23-33-32.bpo-36470.oi6Kdb.rst

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 422a95cebe8534..3de2ec04ad3708 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1300,7 +1300,7 @@ 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
index 0bfed41b369d19..4f5c3c8aab167b 100644
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -3251,6 +3251,24 @@ 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
new file mode 100644
index 00000000000000..9b6ab994f3b6a3
--- /dev/null
+++ 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.