I am trying the new slots directive but I get an AttributeError when I try to access a field with init=False


>>> from dataclasses import dataclass, field
>>> 
>>> @dataclass(slots=True)
... class Example:
...     a: str
...     b: str = field(default="b", init=False)
... 
>>> obj = Example("a")
>>> obj.__slots__
('a', 'b')
>>> obj.
obj.a  obj.b  
>>> obj.a
'a'
>>> obj.b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: b. Did you mean: 'b'?
>>> 



❯ python --version
Python 3.10.0b4+

I created issue 44655 for the confusing error message.

The problem with dataclasses is that the instance variable 'b' needs to be initialized, instead of the current dataclasses behavior where it relies on the class variable when reading 'b'.

With Pablo's changes, the error now reads (in 3.11):

    obj.b
    ^^^^^
AttributeError: 'Example' object has no attribute 'b'

Which is a vast improvement!

I'm working on a PR to initialize obj.b in __init__.

Thanks for the update eric, yeah the new error messages are much more comprehensive and I am glad to hear the original issue will be addressed as well.

New changeset d3062f672c92855b7e9e962ad4bf1a67abd4589b by Eric V. Smith in branch 'main':
bpo-44649: Fix dataclasses(slots=True) with a field with a default, but init=False (GH-29692)
https://github.com/python/cpython/commit/d3062f672c92855b7e9e962ad4bf1a67abd4589b

Thanks for the bug report!

New changeset 10343bd98390ef15909e3a19f26a6178162996fd by Miss Islington (bot) in branch '3.10':
bpo-44649: Fix dataclasses(slots=True) with a field with a default, but init=False (GH-29692)
https://github.com/python/cpython/commit/10343bd98390ef15909e3a19f26a6178162996fd