Re: [Python-ideas] Dataclasses, keyword args, and inheritance

What does attrs' solution for this problem look like? On Fri, Jan 26, 2018 at 11:11 AM, George Leslie-Waksman wrote:

Even if we could inherit the setting, I would think that we would still want to require the code be explicit. It seems worse to implicitly require keyword only arguments for a class without giving any indication in the code.

As it stands, the current implementation does not allow a later subclass to be declared without `keyword_only=True` so we could handle this case by adding a note to the `TypeError` message about considering the keyword_only flag.

How do I got about putting together a proposal to get this into 3.8?

--George

On Thu, Jan 25, 2018 at 5:12 AM Eric V. Smith wrote:

...

I'm not completely opposed to this feature. But there are some cases to consider. Here's the first one that occurs to me: note that due to the way dataclasses work, it would need to be used everywhere down an inheritance hierarchy. That is, if an intermediate base class required it, all class derived from that intermediate base would need to specify it, too. That's because each class just makes decisions based on its fields and its base classes' fields, and not on any flags attached to the base class. As it's currently implemented, a class doesn't remember any of the decorator's arguments, so there's no way to look for this information, anyway.

I think there are enough issues here that it's not going to make it in to 3.7. It would require getting a firm proposal together, selling the idea on python-dev, and completing the implementation before Monday. But if you want to try, I'd participate in the discussion.

Taking Ivan's suggestion one step further, a way to do this currently is to pass init=False and then write another decorator that adds the kw-only __init__. So the usage would be:

@dataclass class Foo: some_default: dict = field(default_factory=dict)

@kw_only_init @dataclass(init=False) class Bar(Foo): other_field: int

kw_only_init(cls) would look at fields(cls) and construct the __init__. It would be a hassle to re-implement dataclasses's _init_fn function, but it could be made to work (in reality, of course, you'd just copy it and hack it up to do what you want). You'd also need to use some private knowledge of InitVars if you wanted to support them (the stock fields(cls) doesn't return them).

For 3.8 we can consider changing dataclasses's APIs if we want to add this.

Eric.

...

It may be possible but it makes for pretty leaky abstractions and it's unclear what that custom __init__ should look like. How am I supposed to know what the replacement for default_factory is?

Moreover, suppose I want one base class with an optional argument and a half dozen subclasses each with their own required argument. At that point, I have to write the same __init__ function a half dozen times.

It feels rather burdensome for the user when an additional flag (say "kw_only=True") and a modification to: https://github.com/python/cpython/blob/master/Lib/dataclasses.py#L294

On 1/25/2018 1:38 AM, George Leslie-Waksman wrote: that

...

inserted `['*']` after `[self_name]` if the flag is specified could ameliorate this entire issue.

On Wed, Jan 24, 2018 at 3:22 PM Ivan Levkivskyi mailto:levkivskyi@gmail.com> wrote:

It is possible to pass init=False to the decorator on the subclass (and supply your own custom __init__, if necessary):

@dataclass class Foo: some_default: dict = field(default_factory=dict)

@dataclass(init=False) # This works class Bar(Foo): other_field: int

-- Ivan

On 23 January 2018 at 03:33, George Leslie-Waksman mailto:waksman@gmail.com> wrote:

The proposed implementation of dataclasses prevents defining fields with defaults before fields without defaults. This can create limitations on logical grouping of fields and on inheritance.

Take, for example, the case:

@dataclass class Foo: some_default: dict = field(default_factory=dict)

@dataclass class Bar(Foo): other_field: int

this results in the error:

5 @dataclass ----> 6 class Bar(Foo): 7 other_field: int 8

~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/ site-packages/dataclasses.py in dataclass(_cls, init, repr, eq, order, hash, frozen) 751 752 # We're called as @dataclass, with a class. --> 753 return wrap(_cls) 754 755

~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/ site-packages/dataclasses.py in wrap(cls) 743 744 def wrap(cls): --> 745 return _process_class(cls, repr, eq, order, hash, init, frozen) 746 747 # See if we're being called as @dataclass or @dataclass().

~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/ site-packages/dataclasses.py in _process_class(cls, repr, eq, order, hash, init, frozen) 675 # in __init__. Use "self" if possible. 676 '__dataclass_self__' if 'self' in fields --> 677 else 'self', 678 )) 679 if repr:

~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/ site-packages/dataclasses.py in _init_fn(fields, frozen, has_post_init, self_name) 422 seen_default = True 423 elif seen_default: --> 424 raise TypeError(f'non-default argument {f.name http://f.name!r} ' 425 'follows default argument') 426

TypeError: non-default argument 'other_field' follows default argument

I understand that this is a limitation of positional arguments because the effective __init__ signature is:

def __init__(self, some_default: dict = <something>, other_field: int):

However, keyword only arguments allow an entirely reasonable solution to this problem:

def __init__(self, *, some_default: dict = <something>, other_field: int):

And have the added benefit of making the fields in the __init__ call entirely explicit.

So, I propose the addition of a keyword_only flag to the @dataclass decorator that renders the __init__ method using keyword only arguments:

@dataclass(keyword_only=True) class Bar(Foo): other_field: int

--George Leslie-Waksman

_______________________________________________ Python-ideas mailing list Python-ideas@python.org mailto:Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/

_______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/

_______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/

-- --Guido van Rossum (python.org/~guido)