[issue33129] Add kwarg-only option to dataclass

[Eric V. Smith]

Eric V. Smith  added the comment:

Closing this in favor of issue 43532, which has a slightly elaborated approach.

--
resolution:  -> duplicate
stage: patch review -> resolved
status: open -> closed
superseder:  -> Add keyword-only fields to dataclasses

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[issue33129] Add kwarg-only option to dataclass

[Paul Bryan]

Change by Paul Bryan :


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[issue33129] Add kwarg-only option to dataclass

[conchylicultor]

conchylicultor  added the comment:

For the API, I think we could add this feature with a single new 
`dataclass(kw_only: bool | str)`

```
@dataclasses.dataclass(kw_only=True)
class A:
  a: int
  b: int
  c: int

# A(*, a, b, c)


@dataclasses.dataclass(kw_only='b')
class A:
  a: int
  b: int
  c: int

# A(a, *, b, c)
```

I think this would be flexible enough for all use cases, while avoiding the 
boilerplate from the above proposals.

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[issue33129] Add kwarg-only option to dataclass

[conchylicultor]

conchylicultor  added the comment:

Solving this would significantly improve @dataclass inheritance 
(https://bugs.python.org/issue39300). Any idea when this will land ?

Currently, it is not possible to add required argument to a child dataclass 
(unless hacks like duplicating attributes):

```
import dataclasses

@dataclasses.dataclass
class A:
  x: int = 1


@dataclasses.dataclass
class B(A):
  y: int  # ERROR: non-default argument follows default argument
```

Keywords-only would solve this issue:

```
@dataclasses.dataclass
class B(A):
  y: int = dataclasses.field(kw_only=True)


B(123, y=456)
```

Note: Attrs already support this: 
https://www.attrs.org/en/stable/examples.html#keyword-only-attributes


For the behavior, I think there are two options:

```
class A:
  a0: int
  a1: int = field(kw_only=True)
  a2: int


class B(A):
  b0: int
  b1: int = field(kw_only=True)
  b2: int
```


Option 1: All attributes following `kw_only` are `kw_only` (kw_only indicates 
where to place '*')

A(a0, *, a1, a2)
B(a0, b0, *, a1, a2, b1, b2)

Option 2: `kw_only` are individually moved to the end

A(a0, a2, *, a1)
B(a0, a2, b0, b2, *, a1, b1)

I personally prefer Option 1, as it makes it easier to place the `*` with less 
boilerplate.

```
class Request:
  url: str
  timeout: int = field(default=-1, kw_only=True)
  verify: bool = False
  params: Optional[Dict[str, str]] = None
  cookies: Optional[Dict[str, str]] = None

Request(url, *, timeout=-1, verify=False, params=None, cookies=None)
```

Which looks better than:

```
class Request:
  url: str
  timeout: int = field(default=-1, kw_only=True)
  verify: bool = field(default=False, kw_only=True)
  params: Optional[Dict[str, str]] = field(default=None, kw_only=True)
  cookies: Optional[Dict[str, str]] = field(default=None, kw_only=True)
```

--
nosy: +conchylicultor

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[issue33129] Add kwarg-only option to dataclass

[Ryan Hiebert]

Change by Ryan Hiebert :


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nosy: +ryanhiebert

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[issue33129] Add kwarg-only option to dataclass

[wyz23x2]

wyz23x2  added the comment:

Since '/' was introduced in Python 3.8, support for positional parameters 
should be supported too.

--
nosy: +wyz23x2
versions: +Python 3.9

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[issue33129] Add kwarg-only option to dataclass

[pmp-p]

Change by pmp-p :


--
nosy: +pmpp

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[issue33129] Add kwarg-only option to dataclass

[Prakhar Goel]

Prakhar Goel  added the comment:

Hi,

As another piece of evidence: I run into this problem _all the time_. Whenever 
I need a field in the parent with a default, I either need to add bogus default 
values for every field in every subclass or just skip the default in the parent.

I'd like to suggest a middle ground:
1. A field level keyword_only option which forces just that field to be a 
kw-only field (for every subclass as well).
2. A class level keyword_only option that does:
2a. False (default) means current behavior where classes are not keyword only 
(but may be forced to be by the field specific arg).
2b. True means all the fields in this class (but not the subclasses!) Would be 
marked as keyword only. This does affect the order of arguments in the init 
method but that seems acceptable given that keyword only parameters are 
conceptually order independent.
2c. None implies that precisely the fields with a default (or default_factory) 
should be keyword only fields.

Thoughts?

--
nosy: +Prakhar Goel

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[issue33129] Add kwarg-only option to dataclass

[Michael Lee]

Michael Lee  added the comment:

Hello! I've submitted a PR based on this issue, since it seemed the most 
relevant from my searching of the backlog.

Here's a link: https://github.com/python/cpython/pull/19206

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[issue33129] Add kwarg-only option to dataclass

[Michael Lee]

Change by Michael Lee :


--
nosy: +michael.lee
nosy_count: 5.0 -> 6.0
pull_requests: +18569
pull_request: https://github.com/python/cpython/pull/19206

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[issue33129] Add kwarg-only option to dataclass

[Jimbo Jim]

Jimbo Jim  added the comment:

first post in bugs.python.org... are people normally scary around here?

Another usecase of keyword-only arguments is that I can mix default and 
non-default fields in any order. This could be achieved by either marking all 
fields as read-only, or by implicitly adding an asterisk before the first 
"conflicting" non-default argument (or by translating .../etc into an asterisk).

(Is it a bad idea to create a private fork of `pip install dataclasses` with 
implicit asterisk, purely for my own project?)

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nosy: +Jimbo Jim

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[issue33129] Add kwarg-only option to dataclass

[Raymond Hettinger]

Raymond Hettinger  added the comment:

+1 from me -- think would occasionally be nice to have.

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[issue33129] Add kwarg-only option to dataclass

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

There is an open PR and some discussion about this feature in attrs repo : 
https://github.com/python-attrs/attrs/pull/281

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[issue33129] Add kwarg-only option to dataclass

[Eric V. Smith]

Eric V. Smith  added the comment:

I'd forgotten about this issue and created #33493. I'll close it. Copied here 
is my comment from that issue:

I've had several requests for keyword-only arguments. This is a placeholder to 
remind me to work on it. I have not decided if it's a good idea or not.

I propose adding a keyword_only argument to field(), defaulting to False.

I'm thinking that the basic idea would be to put all keyword-only fields at the 
end of the arguments to __init__, but for all other uses, leave them where they 
appear in the class definition. That way comparison operations, in particular, 
would use the fields as they appear in the class definition (which is the 
current behavior). Since they'd be at the end of __init__, and since order 
doesn't matter (they're keyword-only, after all), then this would work as 
expected for base classes.

That is, given:

@dataclass
class B:
a: field(type=int, keyword_only=True)
b: int

@dataclass
class C(B):
c: int
d: field(type=int, keyword_only=True)

Then B's __init__ would take (b, c, *, a, d) as its arguments, but its 
comparison functions would compare the tuples as (a, b, c, d).

It would be an error for a ClassVar field to be keyword-only. I think it would 
be okay if an InitVar field were keyword-only, but I haven't given it a lot of 
thought.

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[issue33129] Add kwarg-only option to dataclass

[Alan Du]

Change by Alan  Du :


--
keywords: +patch
pull_requests: +5975
stage:  -> patch review

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[issue33129] Add kwarg-only option to dataclass

[Eric V. Smith]

Eric V. Smith  added the comment:

Changing this to 3.8: there's not enough time to add this to 3.7.

--
versions:  -Python 3.7

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[issue33129] Add kwarg-only option to dataclass

[Eric V. Smith]

Change by Eric V. Smith :


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assignee:  -> eric.smith

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[issue33129] Add kwarg-only option to dataclass

[Alan Du]

Alan  Du  added the comment:

Err... the right link would actually be 
https://github.com/alanhdu/cpython/tree/bpo-33129.

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[issue33129] Add kwarg-only option to dataclass

[Alan Du]

Alan  Du  added the comment:

If this would be accepted, I'd be happy to contribute a patch adding a 
`kwarg_only` option to `dataclass` (although it might take me a while since I'm 
not super familiar with the Python development workflow). I believe I already 
have the code change necessary at 
https://github.com/alanhdu/cpython/commit/fa35b39e5204845690ff774812f882f40b0e5f90
 (although it still needs tests and documentation).

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[issue33129] Add kwarg-only option to dataclass

[Alan Du]

New submission from Alan  Du :

I'd like to request a new option to the `dataclasses.dataclass` decorator to 
make the `__init__` keyword-only.

The two use-cases I have in mind are:

(1) Using as a dataclass big-bag-of-config. In this scenario, forcing the user 
to specify the keywords is a lot nicer than passing in a dozen positional 
parameters.

(2) Having kwarg-only parameters means that inheritance and default parameters 
play nicely with each other again instead of raising a TypeError.

--
components: Library (Lib)
messages: 314341
nosy: alan_du, eric.smith
priority: normal
severity: normal
status: open
title: Add kwarg-only option to dataclass
type: enhancement
versions: Python 3.7, Python 3.8

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