From 2dca685a13c8824565554f612ab35d5d92531ff8 Mon Sep 17 00:00:00 2001
From: Claudiu Popa <pcmanticore@gmail.com>
Date: Mon, 2 Dec 2019 21:11:15 +0100
Subject: [PATCH] bpo-36470: Allow dataclasses.replace() to work InitVar with
 default values

If a dataclass contain an InitVar, that InitVar must be specified in the
call to replace(), unless it has a default value, in which case replace()
should not raise a `ValueError` if the value was not specified in the call
to replace(). Patch by Claudiu Popa.
---
 Lib/dataclasses.py                             |  2 +-
 Lib/test/test_dataclasses.py                   | 18 ++++++++++++++++++
 .../2019-12-02-21-03-41.bpo-36470.Gy9Sff.rst   |  1 +
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-12-02-21-03-41.bpo-36470.Gy9Sff.rst

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 91c1f6f80fc684..fdfbf34c0be2a8 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1249,7 +1249,7 @@ 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 238335e7d9edad..e11428f345141d 100644
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -3214,6 +3214,24 @@ 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/2019-12-02-21-03-41.bpo-36470.Gy9Sff.rst b/Misc/NEWS.d/next/Library/2019-12-02-21-03-41.bpo-36470.Gy9Sff.rst
new file mode 100644
index 00000000000000..a7b16eeaf0ad63
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-12-02-21-03-41.bpo-36470.Gy9Sff.rst
@@ -0,0 +1 @@
+Fix dataclasses with InitVars and replace(). Patch by Claudiu Popa.