Description

assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2021-11-29.18:34:32.678>
created_at = <Date 2021-04-21.12:19:17.095>
labels = ['3.11', 'type-bug', '3.9', '3.10', 'docs']
title = 'dataclasses.astuple (and .asdict) do deepcopy on all fields'
updated_at = <Date 2021-11-29.18:44:26.912>
user = 'https://bugs.python.org/mandolaerik'

activity = <Date 2021-11-29.18:44:26.912>
actor = 'andrei.avk'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2021-11-29.18:34:32.678>
closer = 'eric.smith'
components = ['Documentation']
creation = <Date 2021-04-21.12:19:17.095>
creator = 'mandolaerik'
dependencies = []
files = []
hgrepos = []
issue_num = 43905
keywords = ['patch']
message_count = 14.0
messages = ['391516', '391518', '391519', '391520', '394015', '394019', '396284', '407241', '407243', '407306', '407310', '407311', '407314', '407316']
nosy_count = 5.0
nosy_names = ['eric.smith', 'serhiy.storchaka', 'miss-islington', 'andrei.avk', 'mandolaerik']
pr_nums = ['26154', '29851', '29852']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43905'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

Activity

1. mandolaerik commented on Apr 21, 2021

   

   mandolaerikmannequin

   on Apr 21, 2021

   MannequinAuthor

   More actions

   It seems that the 'dataclass.astuple' function does a deepcopy of all fields. This is not documented. Two problems:

   1. Dictionary keys that rely on object identity are ruined:

      import dataclasses
      @dataclasses.dataclass
      class Foo:
          key: object
      key = object()
      lut = {key: 5}
      (y,) = dataclasses.astuple(Foo(x))
      \# KeyError
      lut[y]

   2. dataclasses can only be converted to a tuple if all fields are serializable:

      import dataclasses
      @dataclasses.dataclass
      class Foo:
          f: object
      foo = Foo(open('test.py'))
      dataclasses.astuple(foo)

   ->

   TypeError: cannot pickle '_io.TextIOWrapper' object
   

   In my use case, I just want a list of all fields. I can do the following as a workaround:

   (getattr(foo, field.name) for field in dataclasses.fields(foo))

   Tested on Python 3.8.7 and 3.7.9.
2. mandolaerik

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   3.7 (EOL)end of lifeend of life

   3.8 (EOL)end of lifeend of life

   on Apr 21, 2021
3. ericvsmith commented on Apr 21, 2021

   

   ericvsmith

   on Apr 21, 2021

   Member

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   Unfortunately this can't be changed, although I suppose it should be documented.

   In general I think this API was a mistake, and should not have been added. There are just too many cases where it doesn't do what you want, or where it fails.

   I'd like to deprecate it and remove it (along with asdict), but I fear that would be too disruptive.

   Your approach seems reasonable to me for your use case.
4. ericvsmith

   assigned

   ericvsmith

   on Apr 21, 2021
5. serhiy-storchaka commented on Apr 21, 2021

   

   serhiy-storchaka

   on Apr 21, 2021

   Member

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   Why deepcopy is used at all? It is a very specific feature which should not be used by default. If you want to make a deep copy of fields, you can call copy.deepcopy() explicitly.

       copy.deepcopy(dataclasses.astuple(obj))

   or

       dataclasses.astuple(copy.deepcopy(obj))
6. ericvsmith commented on Apr 21, 2021

   

   ericvsmith

   on Apr 21, 2021

   Member

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   The reason for the deep copying was to support changing a hierarchy of dataclasses into something that could be JSON serialized. But it didn't really work out. It recurses into dataclasses, namedtuples, lists, tuples, and dicts, and deep copies everything else.

   As I said, it's a design flaw.
7. mandolaerik commented on May 20, 2021

   

   mandolaerikmannequin

   on May 20, 2021

   MannequinAuthor

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   Would it make sense to make dataclasses iterable, like so?

       def __iter__(self):
           return (getattr(self, field.name) for field in fields(self))

   With that in place, deprecating astuple would maybe be less disruptive?
8. ericvsmith commented on May 20, 2021

   

   ericvsmith

   on May 20, 2021

   Member

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   No, iteration is explicitly a non-goal of PEP-557. See the section on namedtuple for why: https://www.python.org/dev/peps/pep-0557/#why-not-just-use-namedtuple
9. akulakov commented on Jun 21, 2021

   

   akulakov

   on Jun 21, 2021

   Contributor

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   I've added a PR here: #26154
10. 15 remaining items
11. ericvsmith

    added

    3.11only security fixesonly security fixes

    and removed

    3.7 (EOL)end of lifeend of life

    3.8 (EOL)end of lifeend of life

    on Nov 29, 2021
12. miss-islington commented on Nov 29, 2021

    

    miss-islington

    on Nov 29, 2021

    Contributor

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    New changeset c1f93f0 by andrei kulakov in branch 'main':
    bpo-43905: Expand dataclasses.astuple() and asdict() docs (GH-26154)
    c1f93f0
13. miss-islington commented on Nov 29, 2021

    

    miss-islington

    on Nov 29, 2021

    Contributor

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    New changeset 376b24e by Miss Islington (bot) in branch '3.9':
    bpo-43905: Expand dataclasses.astuple() and asdict() docs (GH-26154)
    376b24e
14. miss-islington commented on Nov 29, 2021

    

    miss-islington

    on Nov 29, 2021

    Contributor

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    New changeset 32f1491 by Miss Islington (bot) in branch '3.10':
    bpo-43905: Expand dataclasses.astuple() and asdict() docs (GH-26154)
    32f1491
15. ericvsmith commented on Nov 29, 2021

    

    ericvsmith

    on Nov 29, 2021

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    Thanks, @andrei.avk!
16. ericvsmith

    closed this as completedon Nov 29, 2021
17. ericvsmith

    added

    type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

    on Nov 29, 2021
18. ericvsmith

    closed this as completedon Nov 29, 2021
19. ericvsmith

    added

    type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

    on Nov 29, 2021
20. akulakov commented on Nov 29, 2021

    

    akulakov

    on Nov 29, 2021

    Contributor

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    Thank you for reviewing Eric!
21. ezio-melotti

    transferred this issue fromon Apr 10, 2022
22. carljm

    added a commit that references this issue on Mar 11, 2023

    gh-88071: Update docstrings of dataclass' astuple and asdict (#101806)

    767d3a8
23. iritkatriel

    added a commit that references this issue on Mar 12, 2023

    pythongh-88071: Update docstrings of dataclass' astuple and asdict (p…

    f8d4afb

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