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bpo-33175: dataclasses should look up __set_name__ on class, not instance #6305

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Merged
ericvsmith merged 1 commit into from
Mar 29, 2018
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bpo-33175: dataclasses should look up __set_name__ on class, not instance #6305

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4 changes: 2 additions & 2 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -248,11 +248,11 @@ def __repr__(self):
# the default value, so the end result is a descriptor that had
# __set_name__ called on it at the right time.
def __set_name__(self, owner, name):
func = getattr(self.default, '__set_name__', None)
func = getattr(type(self.default), '__set_name__', None)
if func:
# There is a __set_name__ method on the descriptor,
# call it.
func(owner, name)
func(self.default, owner, name)


class _DataclassParams:
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39 changes: 34 additions & 5 deletions Lib/test/test_dataclasses.py
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Expand Up @@ -2705,7 +2705,7 @@ def test_set_name(self):
# Create a descriptor.
class D:
def __set_name__(self, owner, name):
self.name = name
self.name = name + 'x'
def __get__(self, instance, owner):
if instance is not None:
return 1
Expand All @@ -2716,15 +2716,15 @@ def __get__(self, instance, owner):
@dataclass
class C:
c: int=D()
self.assertEqual(C.c.name, 'c')
self.assertEqual(C.c.name, 'cx')

# Now test with a default value and init=False, which is the
# only time this is really meaningful. If not using
# init=False, then the descriptor will be overwritten, anyway.
@dataclass
class C:
c: int=field(default=D(), init=False)
self.assertEqual(C.c.name, 'c')
self.assertEqual(C.c.name, 'cx')
self.assertEqual(C().c, 1)

def test_non_descriptor(self):
Expand All @@ -2733,12 +2733,41 @@ def test_non_descriptor(self):

class D:
def __set_name__(self, owner, name):
self.name = name
self.name = name + 'x'

@dataclass
class C:
c: int=field(default=D(), init=False)
self.assertEqual(C.c.name, 'c')
self.assertEqual(C.c.name, 'cx')

def test_lookup_on_instance(self):
# See bpo-33175.
class D:
pass

d = D()
# Create an attribute on the instance, not type.
d.__set_name__ = Mock()

# Make sure d.__set_name__ is not called.
@dataclass
class C:
i: int=field(default=d, init=False)

self.assertEqual(d.__set_name__.call_count, 0)

def test_lookup_on_class(self):
# See bpo-33175.
class D:
pass
D.__set_name__ = Mock()

# Make sure D.__set_name__ is called.
@dataclass
class C:
i: int=field(default=D(), init=False)

self.assertEqual(D.__set_name__.call_count, 1)


if __name__ == '__main__':
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2018-03-29-04-32-25.bpo-33175._zs1yM.rst
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@@ -0,0 +1,2 @@
In dataclasses, Field.__set_name__ now looks up the __set_name__ special

@JelleZijlstra JelleZijlstra Mar 29, 2018

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Does this need a NEWS entry if there was no release with the buggy code? (I'm mostly curious; you'll know the norms better than I do.)

@ericvsmith ericvsmith Mar 29, 2018

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I thought about this, but unfortunately I think it does need the NEWS blurb. Ned has already cut 3.7.0b3, so the bug will be in that release. Thanks again for catching this.

method on the class, not the instance, of the default value.