dataclasses: replace() give poor error message if using InitVar

[ericvsmith]

BPO	33805
Nosy	@ericvsmith, @corona10, @miss-islington
PRs	bpo-33805: Improve error message of dataclasses.replace() #7580 [3.7] bpo-33805: Improve error message of dataclasses.replace() (GH-7580) #7876

^{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 2018-06-23.15:28:07.455>
created_at = <Date 2018-06-08.08:51:39.491>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'dataclasses: replace() give poor error message if using InitVar'
updated_at = <Date 2018-06-23.15:28:07.454>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2018-06-23.15:28:07.454>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2018-06-23.15:28:07.455>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2018-06-08.08:51:39.491>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 33805
keywords = ['patch']
message_count = 4.0
messages = ['319036', '319198', '320309', '320311']
nosy_count = 3.0
nosy_names = ['eric.smith', 'corona10', 'miss-islington']
pr_nums = ['7580', '7876']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33805'
versions = ['Python 3.7', 'Python 3.8']

[ericvsmith]

If a dataclass contains an InitVar without a default value, that InitVar must be specified in the call to replace(). This is because replace() works by first creating a new object, and InitVars without defaults, by definition, must be specified when creating the object. There is no other source for the value of the InitVar to use.

However, the exception you get is confusing:

>>> from dataclasses import *
>>> @dataclass
... class C:
...   i: int
...   j: InitVar[int]
...
>>> c = C(1, 2)
>>> replace(c, i=3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\home\eric\local\python\cpython\lib\dataclasses.py", line 1176, in replace
    changes[f.name] = getattr(obj, f.name)
AttributeError: 'C' object has no attribute 'j'
>>>

This message really should say something like "InitVar 'j' must be specified".

[corona10]

@eric.smith
Can I take a look this issue?

[ericvsmith]

New changeset 3d70f7a by Eric V. Smith (Dong-hee Na) in branch 'master':
bpo-33805: Improve error message of dataclasses.replace() (GH-7580)
3d70f7a

[miss-islington]

New changeset bbef7ab by Miss Islington (bot) in branch '3.7':
bpo-33805: Improve error message of dataclasses.replace() (GH-7580)
bbef7ab