That's fair. Let me then qualify my statement with "in the initial
release". The initial release has enough functionality to deal with without
considering your rather esoteric use case. (And I consider it esoteric
because attrs has apparently never seen the need to solve it either.) We
can reconsider for 3.8.
On Mon, Jan 29, 2018 at 11:38 AM, George Leslie-Waksman <waks...@gmail.com>
wrote:
> Given I started this thread from a perspective of this is a feature that I
> would like because I need it, it feels a little dismissive to take attrs
> not having the feature to mean "there's no reason to try to implement this."
>
> On Mon, Jan 29, 2018 at 11:05 AM Guido van Rossum <gu...@python.org>
> wrote:
>
>> I think that settles it -- there's no reason to try to implement this.
>>
>> On Mon, Jan 29, 2018 at 10:51 AM, George Leslie-Waksman <
>> waks...@gmail.com> wrote:
>>
>>> attrs' seems to also not allow mandatory attributes to follow optional
>>> one:
>>>
>>> In [14]: @attr.s
>>> ...: class Baz:
>>> ...: a = attr.ib(default=attr.Factory(list))
>>> ...: b = attr.ib()
>>> ...:
>>> ------------------------------------------------------------
>>> ---------------
>>> ValueError Traceback (most recent call
>>> last)
>>> <ipython-input-14-2c63f3f229a5> in <module>()
>>> ----> 1 @attr.s
>>> 2 class Baz:
>>> 3 a = attr.ib(default=attr.Factory(list))
>>> 4 b = attr.ib()
>>> 5
>>>
>>> /Users/waksman/.pyenv/versions/3.6.1/envs/temp/lib/
>>> python3.6/site-packages/attr/_make.py in attrs(maybe_cls, these,
>>> repr_ns, repr, cmp, hash, init, slots, frozen, str, auto_attribs)
>>> 700 return wrap
>>> 701 else:
>>> --> 702 return wrap(maybe_cls)
>>> 703
>>> 704
>>>
>>> /Users/waksman/.pyenv/versions/3.6.1/envs/temp/lib/
>>> python3.6/site-packages/attr/_make.py in wrap(cls)
>>> 669 raise TypeError("attrs only works with new-style
>>> classes.")
>>> 670
>>> --> 671 builder = _ClassBuilder(cls, these, slots, frozen,
>>> auto_attribs)
>>> 672
>>> 673 if repr is True:
>>>
>>> /Users/waksman/.pyenv/versions/3.6.1/envs/temp/lib/
>>> python3.6/site-packages/attr/_make.py in __init__(self, cls, these,
>>> slots, frozen, auto_attribs)
>>> 369
>>> 370 def __init__(self, cls, these, slots, frozen, auto_attribs):
>>> --> 371 attrs, super_attrs = _transform_attrs(cls, these,
>>> auto_attribs)
>>> 372
>>> 373 self._cls = cls
>>>
>>> /Users/waksman/.pyenv/versions/3.6.1/envs/temp/lib/
>>> python3.6/site-packages/attr/_make.py in _transform_attrs(cls, these,
>>> auto_attribs)
>>> 335 "No mandatory attributes allowed after an
>>> attribute with a "
>>> 336 "default value or factory. Attribute in
>>> question: {a!r}"
>>> --> 337 .format(a=a)
>>> 338 )
>>> 339 elif had_default is False and \
>>>
>>> ValueError: No mandatory attributes allowed after an attribute with a
>>> default value or factory. Attribute in question: Attribute(name='b',
>>> default=NOTHING, validator=None, repr=True, cmp=True, hash=None, init=True,
>>> metadata=mappingproxy({}), type=None, converter=None)
>>>
>>>
>>> On Fri, Jan 26, 2018 at 1:44 PM Guido van Rossum <gu...@python.org>
>>> wrote:
>>>
>>>> What does attrs' solution for this problem look like?
>>>>
>>>> On Fri, Jan 26, 2018 at 11:11 AM, George Leslie-Waksman <
>>>> waks...@gmail.com> 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 <e...@trueblade.com>
>>>>> 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.
>>>>>>
>>>>>> On 1/25/2018 1:38 AM, George Leslie-Waksman wrote:
>>>>>> > 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 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 <
>>>>>> levkivs...@gmail.com
>>>>>> > <mailto:levkivs...@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
>>>>>> > <waks...@gmail.com <mailto:waks...@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)
>>>>
>>>
>>
>>
>> --
>> --Guido van Rossum (python.org/~guido)
>>
>
--
--Guido van Rossum (python.org/~guido)
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