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Add keyword-only fields to dataclasses #87698

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Add keyword-only fields to dataclasses#87698
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ericvsmith
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3.10 (EOL)end of lifestdlibStandard Library Python modules in the Lib/ directorytype-featureA feature request or enhancement

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@ericvsmith
ericvsmith
opened on Mar 17, 2021
BPO 43532
Nosy @rhettinger, @ericvsmith, @ryanhiebert, @henryiii, @miss-islington, @tirkarthi, @EpicWink, @jack1142, @kamilturek, @cdce8p, @finite-state-machine
PRs
  • bpo-43532: Add kw_only to dataclass() and field(), and add KW_ONLY marker. #24909
  • bpo-43532: Add keyword-only fields to dataclasses. #25608
  • bpo-43532: add version added to KW_ONLY #31235
  • [3.10] bpo-43532: add version added to KW_ONLY (GH-31235) #31236
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    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 2022-02-09.21:49:39.659>
created_at = <Date 2021-03-17.17:39:21.912>
labels = ['type-feature', 'library', '3.10']
title = 'Add keyword-only fields to dataclasses'
updated_at = <Date 2022-02-09.21:49:39.658>
user = 'https://github.com/ericvsmith'

    bugs.python.org fields:

    activity = <Date 2022-02-09.21:49:39.658>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2022-02-09.21:49:39.659>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2021-03-17.17:39:21.912>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 43532
keywords = ['patch']
message_count = 11.0
messages = ['388949', '388951', '388984', '391920', '392018', '392115', '398856', '404609', '412943', '412944', '412946']
nosy_count = 11.0
nosy_names = ['rhettinger', 'eric.smith', 'ryanhiebert', 'Henry Schreiner', 'miss-islington', 'xtreak', 'Epic_Wink', 'jack1142', 'kamilturek', 'cdce8p', 'finite-state-machine']
pr_nums = ['24909', '25608', '31235', '31236']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43532'
versions = ['Python 3.10']

    Activity

    1. ericvsmith commented on Mar 17, 2021

      @ericvsmith
      ericvsmith
      on Mar 17, 2021
      MemberAuthor

      The idea is that a keyword-only field becomes a keyword-only argument to __init__().

      For the proposal and a discussion, see https://mail.python.org/archives/list/python-ideas@python.org/message/FI6KS4O67XDEIDYOFWCXMDLDOSCNSEYG/

      The @DataClass decorator will get a new parameter, kw_only, which defaults to False. If kw_only=True, all fields in the dataclass will be by efault keyword-only. In addition, field() will have a new kw_only parameter. If true, the field will be keyword-only. If false, it will not be keyword-only. If unspecified, it will use the value of dataclass's kw_only parameter.

      In addition, a module-level variable KW_ONLY will be added. If a field has this type, then all fields after it will default to kw_only=True. The field is otherwise completely ignored.

      Examples:

      @dataclasses.dataclass
class A:
    a: Any = field(kw_only=True)

      Will have __init__(self, *, a)

      @dataclasses.dataclass(kw_only=True)
class B:
    a: Any
    b: Any

      Will have __init__(self, *, a, b)

      @dataclasses.dataclass
class C:
    a: Any
    _: dataclasses.KW_ONLY
    b: Any
    c: Any

      Will have __init__(self, a, *, b, c)

      If any non-keyword-only parameters are present, they will be moved before all keyword-only parameters, only for the generated __init__. All other generated methods (repr, __lt__, etc.) will keep fields in the declared order, which is the case in versions 3.9 and earlier.

      @dataclasses.dataclass
class D:
    a: Any
    b: Any = field(kw_only=True)
    c: Any

      Will have __init__(self, a, c, *, b)

      PR to follow.

    2. ericvsmith
      added
      3.10 (EOL)end of life
      on Mar 17, 2021
    3. ericvsmith
      self-assigned this
      on Mar 17, 2021
    4. ericvsmith
      added
      stdlibStandard Library Python modules in the Lib/ directory
      type-featureA feature request or enhancement
      3.10 (EOL)end of life
      on Mar 17, 2021
    5. ericvsmith
      self-assigned this
      on Mar 17, 2021
    6. ericvsmith
      added
      stdlibStandard Library Python modules in the Lib/ directory
      type-featureA feature request or enhancement
      on Mar 17, 2021

    7. tirkarthi commented on Mar 17, 2021

      @tirkarthi
      tirkarthi
      on Mar 17, 2021
      Member

      This looks like a duplicate of https://bugs.python.org/issue33129 or that ticket can be closed.

    8. rhettinger commented on Mar 18, 2021

      @rhettinger
      rhettinger
      on Mar 18, 2021
      Contributor

      +1 for this idea. I don't see any downside.

    9. ericvsmith commented on Apr 26, 2021

      @ericvsmith
      ericvsmith
      on Apr 26, 2021
      MemberAuthor

      I've checked in the code, tests, and some basic documentation. I didn't finish the documentation in order to make sure I could get this in before beta 1. I'm going to leave this issue open until I have time for the rest of the documentation.

    10. EpicWink commented on Apr 27, 2021

      @EpicWink
      EpicWinkmannequin
      on Apr 27, 2021
      Mannequin

      Will this close https://bugs.python.org/issue36077 ?

    11. ericvsmith commented on Apr 27, 2021

      @ericvsmith
      ericvsmith
      on Apr 27, 2021
      MemberAuthor

      Will this close https://bugs.python.org/issue36077 ?

      No, not directly, although I'm going to close that issue as "won't fix".

      But you can specify that the child class requires all params as kw_only. Continuing the example:

      >>> @dataclass(kw_only=True)
... class Child(Parent):
...   y: int
...
>>> Child(0, y=1)
Child(x=0, y=1)

    12. jack1142 commented on Aug 4, 2021

      @jack1142
      jack1142mannequin
      on Aug 4, 2021
      Mannequin

      Does this change deserve an entry in the What's New in Python document in addition to the changelog entry?

    13. finite-state-machine commented on Oct 21, 2021

      @finite-state-machine
      finite-state-machinemannequin
      on Oct 21, 2021
      Mannequin

      I doubt it's worth opening a separate issue for this, so I'll mention it here:

      In the documentation (https://docs.python.org/3.10/library/dataclasses.html#dataclasses.KW_ONLY) nothing documents KW_ONLY as being new in Python 3.10.

    14. miss-islington commented on Feb 9, 2022

      @miss-islington
      miss-islington
      on Feb 9, 2022
      Contributor

      New changeset 5a3f972 by Henry Schreiner in branch 'main':
      bpo-43532: add version added to KW_ONLY (GH-31235)
      5a3f972

    15. miss-islington commented on Feb 9, 2022

      @miss-islington
      miss-islington
      on Feb 9, 2022
      Contributor

      New changeset 7445949 by Miss Islington (bot) in branch '3.10':
      bpo-43532: add version added to KW_ONLY (GH-31235)
      7445949

    16. ericvsmith commented on Feb 9, 2022

      @ericvsmith
      ericvsmith
      on Feb 9, 2022
      MemberAuthor

      Thanks, Henry Schreiner!

    17. ericvsmith
      closed this as completedon Feb 9, 2022
    18. ericvsmith
      closed this as completedon Feb 9, 2022
    19. ezio-melotti
      transferred this issue fromon Apr 10, 2022
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