dataclasses.astuple (and .asdict) do deepcopy on all fields

[mandolaerik]

BPO	43905
Nosy	@ericvsmith, @serhiy-storchaka, @miss-islington, @akulakov
PRs	bpo-43905: Expand dataclasses.astuple() and asdict() docs #26154 [3.10] bpo-43905: Expand dataclasses.astuple() and asdict() docs (GH-26154) #29851 [3.9] bpo-43905: Expand dataclasses.astuple() and asdict() docs (GH-26154) #29852

^{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 2021-11-29.18:34:32.678>
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'

bugs.python.org fields:

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']

Linked PRs

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

[mandolaerik]

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.

[ericvsmith]

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.

[serhiy-storchaka]

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))

[ericvsmith]

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.

[mandolaerik]

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?

[ericvsmith]

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

[akulakov]

I've added a PR here: #26154