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bpo-36470: Allow dataclasses.replace() to work InitVar with default values #17441

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bpo-36470: Allow dataclasses.replace() to work InitVar with default values #17441

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2 changes: 1 addition & 1 deletion Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
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18 changes: 18 additions & 0 deletions Lib/test/test_dataclasses.py
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Expand Up @@ -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):
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@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:
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2019-12-02-21-03-41.bpo-36470.Gy9Sff.rst
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@@ -0,0 +1 @@
Fix dataclasses with InitVars and replace(). Patch by Claudiu Popa.