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dataclasses slots with init=False field raises AttributeException #88815

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tefra mannequin opened this issue Jul 15, 2021 · 7 comments
Closed

dataclasses slots with init=False field raises AttributeException #88815

tefra mannequin opened this issue Jul 15, 2021 · 7 comments
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@ericvsmith
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3.10 only security fixes 3.11 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error

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@tefra
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tefra mannequin commented Jul 15, 2021

BPO 44649
Nosy @ericvsmith, @miss-islington, @tefra
PRs
  • bpo-44649: Fix dataclasses(slots=True) with a field with a default, but init=False #29692
  • [3.10] bpo-44649: Fix dataclasses(slots=True) with a field with a default, but init=False (GH-29692) #29704
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2021-11-22.13:29:26.395>
created_at = <Date 2021-07-15.19:06:03.054>
labels = ['type-bug', 'library', '3.10', '3.11']
title = 'dataclasses slots with init=False field raises AttributeException'
updated_at = <Date 2021-11-22.13:47:49.731>
user = 'https://github.com/tefra'

    bugs.python.org fields:

    activity = <Date 2021-11-22.13:47:49.731>
actor = 'miss-islington'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2021-11-22.13:29:26.395>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2021-07-15.19:06:03.054>
creator = 'tefra'
dependencies = []
files = []
hgrepos = []
issue_num = 44649
keywords = ['patch']
message_count = 7.0
messages = ['397575', '397653', '397678', '401039', '406772', '406774', '406776']
nosy_count = 3.0
nosy_names = ['eric.smith', 'miss-islington', 'tefra']
pr_nums = ['29692', '29704']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44649'
versions = ['Python 3.10', 'Python 3.11']
    @tefra
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    tefra mannequin commented Jul 15, 2021

    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+

    @tefra tefra mannequin added 3.10 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error labels Jul 15, 2021
    @ericvsmith
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    I created bpo-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'.

    @ericvsmith
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    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__.

    @tefra
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    tefra mannequin commented Sep 4, 2021

    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.

    @ericvsmith ericvsmith added 3.11 only security fixes labels Nov 22, 2021
    @ericvsmith
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    New changeset d3062f6 by Eric V. Smith in branch 'main':
    bpo-44649: Fix dataclasses(slots=True) with a field with a default, but init=False (GH-29692)
    d3062f6

    @ericvsmith
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    Thanks for the bug report!

    @miss-islington
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    New changeset 10343bd 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)
    10343bd

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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