dataclasses.replace() fails with the field named "obj"

[serhiy-storchaka]

BPO	37163
Nosy	@brettcannon, @ericvsmith, @serhiy-storchaka, @pablogsal
PRs	[3.8] bpo-37163: Deprecate passing argument obj of dataclasses.replace() by keyword. #13877 bpo-37163: Make the obj argument of dataclasses.replace() a positional-only. #14390 [3.7] bpo-37163: dataclasses.replace() now supports the field named "obj". (GH-13877) #14405

^{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 = None
closed_at = <Date 2019-06-26.20:16:55.693>
created_at = <Date 2019-06-05.11:13:04.903>
labels = ['3.7', '3.8', 'type-bug', 'library', '3.9']
title = 'dataclasses.replace() fails with the field named "obj"'
updated_at = <Date 2019-06-26.20:16:55.692>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2019-06-26.20:16:55.692>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2019-06-26.20:16:55.693>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2019-06-05.11:13:04.903>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 37163
keywords = ['patch']
message_count = 8.0
messages = ['344704', '344707', '344741', '344751', '344896', '346022', '346647', '346674']
nosy_count = 4.0
nosy_names = ['brett.cannon', 'eric.smith', 'serhiy.storchaka', 'pablogsal']
pr_nums = ['13877', '14390', '14405']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37163'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[serhiy-storchaka]

>>> from dataclasses import *
>>> @dataclass
... class D:
...     obj: object
... 
>>> replace(D(123), obj='abc')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: replace() got multiple values for argument 'obj'

[ericvsmith]

Possibly a use for positional-only parameters. Backward compatibility is the question, of course.

[serhiy-storchaka]

I propose to add a deprecation warning in 3.8 and use the PEP-570 syntax in 3.9 (which means a TypeError if obj is passed by keyword argument).

[brettcannon]

+1 for Serhiy's suggestion

[pablogsal]

+1 to Serhiy's proposal

[serhiy-storchaka]

New changeset f5b89af by Serhiy Storchaka in branch '3.8':
bpo-37163: Deprecate passing argument obj of dataclasses.replace() by keyword. (GH-13877)
f5b89af

[serhiy-storchaka]

New changeset 2d88e63 by Serhiy Storchaka in branch 'master':
bpo-37163: Make the obj argument of dataclasses.replace() a positional-only. (GH-14390)
2d88e63

[serhiy-storchaka]

New changeset 6ef103f by Serhiy Storchaka in branch '3.7':
[3.7] bpo-37163: dataclasses.replace() now supports the field named "obj". (GH-13877) (GH-14405)
6ef103f