dataclasses.replace broken if a class has any ClassVars

[ljodal]

BPO	33796
Nosy	@ericvsmith, @ned-deily, @miss-islington, @ljodal
PRs	bpo-33796: Ignore ClassVar for replace(). #7488 [3.7] bpo-33796: Ignore ClassVar for dataclasses.replace(). (GH-7488) #7493

^{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 2018-06-07.20:30:17.857>
created_at = <Date 2018-06-07.14:34:49.831>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'dataclasses.replace broken if a class has any ClassVars'
updated_at = <Date 2018-06-07.20:30:17.855>
user = 'https://github.com/ljodal'

bugs.python.org fields:

activity = <Date 2018-06-07.20:30:17.855>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2018-06-07.20:30:17.857>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2018-06-07.14:34:49.831>
creator = 'sigurd'
dependencies = []
files = []
hgrepos = []
issue_num = 33796
keywords = ['patch']
message_count = 9.0
messages = ['318942', '318948', '318949', '318950', '318955', '318959', '318966', '318970', '318971']
nosy_count = 4.0
nosy_names = ['eric.smith', 'ned.deily', 'miss-islington', 'sigurd']
pr_nums = ['7488', '7493']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33796'
versions = ['Python 3.7', 'Python 3.8']

[ljodal]

The dataclasses.replace function does not work for classes that have class variables. See the console output below for an example.

$ python
Python 3.7.0b5+ (heads/3.7:3c417610ad, Jun  7 2018, 16:21:29) 
[Clang 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import typing
>>> import dataclasses
>>> @dataclasses.dataclass(frozen=True)
... class Test:
...   a: int
...   class_var: typing.ClassVar[str] = 'foo'
... 
>>> obj = Test(a=1)
>>> dataclasses.replace(obj, a=2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/sigurdljodal/.pyenv/versions/3.7-dev/lib/python3.7/dataclasses.py", line 1179, in replace
    return obj.__class__(**changes)
TypeError: __init__() got an unexpected keyword argument 'class_var'

[ericvsmith]

Thanks for the report. This is the same error you get when using any non-field:

>>> @dataclass
... class C:
...   i: int
...
>>> c = C(4)
>>> replace(c, i=3)
C(i=3)
>>> replace(c, j=3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\home\eric\local\python\cpython\lib\dataclasses.py", line 1179, in replace
    return obj.__class__(**changes)
TypeError: __init__() got an unexpected keyword argument 'j'

I think the TypeError is correct in both cases. The error message might not be the best.

Are you suggesting that this shouldn't raise a TypeError? Since a ClassVar is not an instance variable, I don't think it makes any sense to replace() it.

[ljodal]

I think you misunderstood the issue here. I'm not trying to replace the class variable itself, I'm changing another field on a class that has a class variable. The problem is that the replace method includes the class variable in the change dict when copying fields that are not included in the function call.

My call is: dataclasses.replace(obj, a=2) where I try to replace a, but it fails because class_var is included in the attributes to __init__ by the replace function.

The problem is on this line: https://github.com/python/cpython/blob/master/Lib/dataclasses.py#L1162
The _FIELDS attribute includes the class variable as a field, and it is therefore added to the changes dict.

[ericvsmith]

Ah, you're right. I did misunderstand. Thanks for correcting me. PR soon.

[ericvsmith]

Ned: How do you feel about backporting this to 3.7?

It's an unfortunate bug, but it's your call for 3.7.0 vs. 3.7.1.

[ericvsmith]

New changeset e7adf2b by Eric V. Smith in branch 'master':
bpo-33796: Ignore ClassVar for dataclasses.replace(). (GH-7488)
e7adf2b

[ned-deily]

Let's fix it now.

[miss-islington]

New changeset 0aee3be by Miss Islington (bot) in branch '3.7':
bpo-33796: Ignore ClassVar for dataclasses.replace(). (GH-7488)
0aee3be

[ericvsmith]

Thanks, Ned. I've backported it.