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bpo-44015: dataclasses should allow KW_ONLY to be specified only once per class #25841

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May 3, 2021
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bpo-44015: dataclasses should allow KW_ONLY to be specified only once per class #25841

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3c5eda4
bpo-44015: Raise an error if KW_ONLY is specified more than once.
ericvsmith May 3, 2021
28636f6
Add blurb.
ericvsmith May 3, 2021
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5 changes: 5 additions & 0 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -930,6 +930,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
# we can.
cls_fields = []
# Get a reference to this module for the _is_kw_only() test.
KW_ONLY_seen = False
dataclasses = sys.modules[__name__]
for name, type in cls_annotations.items():
# See if this is a marker to change the value of kw_only.
Expand All @@ -939,6 +940,10 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
_is_kw_only))):
# Switch the default to kw_only=True, and ignore this
# annotation: it's not a real field.
if KW_ONLY_seen:
raise TypeError(f'{name!r} is KW_ONLY, but KW_ONLY '
'has already been specified')
KW_ONLY_seen = True
kw_only = True
else:
# Otherwise it's a field of some type.
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77 changes: 77 additions & 0 deletions Lib/test/test_dataclasses.py
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Expand Up @@ -3699,6 +3699,83 @@ class C:
self.assertEqual(c.b, 3)
self.assertEqual(c.c, 2)

def test_KW_ONLY_as_string(self):
@dataclass
class A:
a: int
_: 'dataclasses.KW_ONLY'
b: int
c: int
A(3, c=5, b=4)
msg = "takes 2 positional arguments but 4 were given"
with self.assertRaisesRegex(TypeError, msg):
A(3, 4, 5)

def test_KW_ONLY_twice(self):
msg = "'Y' is KW_ONLY, but KW_ONLY has already been specified"

with self.assertRaisesRegex(TypeError, msg):
@dataclass
class A:
a: int
X: KW_ONLY
Y: KW_ONLY
b: int
c: int

with self.assertRaisesRegex(TypeError, msg):
@dataclass
class A:
a: int
X: KW_ONLY
b: int
Y: KW_ONLY
c: int

with self.assertRaisesRegex(TypeError, msg):
@dataclass
class A:
a: int
X: KW_ONLY
b: int
c: int
Y: KW_ONLY

# But this usage is okay, since it's not using KW_ONLY.
@dataclass
class A:
a: int
_: KW_ONLY
b: int
c: int = field(kw_only=True)

# And if inheriting, it's okay.
@dataclass
class A:
a: int
_: KW_ONLY
b: int
c: int
@dataclass
class B(A):
_: KW_ONLY
d: int

# Make sure the error is raised in a derived class.
with self.assertRaisesRegex(TypeError, msg):
@dataclass
class A:
a: int
_: KW_ONLY
b: int
c: int
@dataclass
class B(A):
X: KW_ONLY
d: int
Y: KW_ONLY


def test_post_init(self):
@dataclass
class A:
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2021-05-03-03-03-49.bpo-44015.V5936k.rst
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@@ -0,0 +1 @@
In @dataclass(), raise a TypeError if KW_ONLY is specified more than once.