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bpo-33100: Dataclasses now handles __slots__ and default values correctly. #6152

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Merged
ericvsmith merged 2 commits into from
Mar 20, 2018
Merged

bpo-33100: Dataclasses now handles __slots__ and default values correctly. #6152

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99afb95
If the class has a member that's a MemberDescriptorType, it's not
ericvsmith Mar 20, 2018
3112e2f
Added blurb.
ericvsmith Mar 20, 2018
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3 changes: 3 additions & 0 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -519,6 +519,9 @@ def _get_field(cls, a_name, a_type):
if isinstance(default, Field):
f = default
else:
if isinstance(default, types.MemberDescriptorType):
# This is a field in __slots__, so it has no default value.
default = MISSING
f = field(default=default)

# Assume it's a normal field until proven otherwise.
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42 changes: 42 additions & 0 deletions Lib/test/test_dataclasses.py
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Expand Up @@ -2564,5 +2564,47 @@ class S(D):
self.assertEqual(s.cached, True)


class TestSlots(unittest.TestCase):
def test_simple(self):
@dataclass
class C:
__slots__ = ('x',)
x: Any

# There was a bug where a variable in a slot was assumed
# to also have a default value (of type types.MemberDescriptorType).
with self.assertRaisesRegex(TypeError,
"__init__\(\) missing 1 required positional argument: 'x'"):
C()

# We can create an instance, and assign to x.
c = C(10)
self.assertEqual(c.x, 10)
c.x = 5
self.assertEqual(c.x, 5)

# We can't assign to anything else.
with self.assertRaisesRegex(AttributeError, "'C' object has no attribute 'y'"):
c.y = 5

def test_derived_added_field(self):
# See bpo-33100.
@dataclass
class Base:
__slots__ = ('x',)
x: Any

@dataclass
class Derived(Base):
x: int
y: int

d = Derived(1, 2)
self.assertEqual((d.x, d.y), (1, 2))

# We can add a new field to the derived instance.
d.z = 10


if __name__ == '__main__':
unittest.main()
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2018-03-19-20-47-00.bpo-33100.chyIO4.rst
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@@ -0,0 +1,2 @@
Dataclasses: If a field has a default value that's a MemberDescriptorType,
then it's from that field being in __slots__, not an actual default value.