[New-bugs-announce] [issue45366] dataclass init=False field with default works but default_factory does not

simple_coder878 report at bugs.python.org
Mon Oct 4 16:17:33 EDT 2021


New submission from simple_coder878 <junk757 at gmail.com>:

Simple example
from dataclasses import dataclass, field

@dataclass(init=False)
class TestObject(object):
    m: str = field(default='hi')
    k: list = field(default_factory=list)

    def test(self):
        print(f'm is {self.m} ')
        print(f'k is {self.k}')

if __name__ == '__main__':
    myobject = TestObject()
    myobject.test()


Produces:

Traceback (most recent call last):
  File "H:\unit_test\tests_dataclass.py", line 81, in <module>
    myobject.test()
  File "H:\unit_test\tests_dataclass.py", line 76, in test
    print(f'k is {self.k}')
AttributeError: 'TestObject' object has no attribute 'k'
m is hi 

So m is initialized to hi but k just disappears

But wait there's more!

If i do 

from dataclasses import dataclass, field

@dataclass(init=False)
class TestObject(object):
    m: str = field(default='hi')
    k: list = field(default_factory=list)

    def test(self):
        print(f'm is {self.m} ')
        print(f'k is {self.k}')


@dataclass
class InheritedTestObject(TestObject):

    def __post_init__(self):
        super().__init__()
        print(f'Inherited m is {self.m} ')
        print(f'Inherited k is {self.k}')
        print(f'Inherited g is {self.k}')


if __name__ == '__main__':
    myobject = InheritedTestObject()
    myobject.test()

It produces:

Inherited m is hi 
Inherited k is []
Inherited g is []
m is hi 
k is []

Process finished with exit code 0

NO ERRORS!

It seems like a bug to me, but what is the expected behavior in this case? I would expect the first case to not error out and should have an empty list.

I've only tested this on Python 3.9 so far.

----------
components: ctypes
messages: 403182
nosy: simple_coder878
priority: normal
severity: normal
status: open
title: dataclass init=False field with default works but default_factory does not
type: behavior
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45366>
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