On Mon, Feb 19, 2018 at 5:06 PM, Chris Barker - NOAA Federal <
chris.barker at noaa.gov> wrote:
> If I have this right, on the discussion about frozen and hash, a use
> case was brought up for taking a few steps to create an instance (and
> thus wanting it not frozen) and then wanting it hashable.
Chris Barker - NOAA Federal wrote:
If I have this right, on the discussion about frozen and hash, a use
case was brought up for taking a few steps to create an instance (and
thus wanting it not frozen) and then wanting it hashable.
Which pointed to the idea of a “ freeze this from now on” method
Steven D'Aprano wrote:
So in principle, we could have a mutable class, and a immutable one, and
when you flick the switch, the instance.__class__ changes from mutable
to frozen.
That seems unfriendly to subclasses as well. To extend a class
you now need to subclass both the mutable and immutab
> On Feb 20, 2018, at 2:38 PM, Guido van Rossum wrote:
>
> But then the class would also inherit a bunch of misfeatures from tuple (like
> being indexable and having a length). It would be nicer if it used __slots__
> instead.
FWIW, George Sakkis made a tool like this about nine years ago.
> On Feb 20, 2018, at 5:38 PM, Guido van Rossum wrote:
>
>> On Tue, Feb 20, 2018 at 1:37 PM, Eric V. Smith wrote:
>>> On 2/17/2018 2:35 PM, Guido van Rossum wrote:
>>> PS. I have to ponder why frozen dataclasses don't use `__new__`.
>>
>> As I'm sure everyone is now aware after the rest of thi
On Tue, Feb 20, 2018 at 1:37 PM, Eric V. Smith wrote:
> On 2/17/2018 2:35 PM, Guido van Rossum wrote:
>
>> PS. I have to ponder why frozen dataclasses don't use `__new__`.
>>
>
> As I'm sure everyone is now aware after the rest of this discussion: it's
> because the returned object isn't really i
On Tue, Feb 20, 2018 at 12:50 PM, Chris Barker
wrote:
> Is that one attribute that big a deal? I suppose that simple dataclasses
> with only two or so attributes would see a significant change, but if
> you're really worried about space, wouldn't you use a namedtuple or simply
> a tuple anyway?
>
On 2/17/2018 2:35 PM, Guido van Rossum wrote:
PS. I have to ponder why frozen dataclasses don't use `__new__`.
As I'm sure everyone is now aware after the rest of this discussion:
it's because the returned object isn't really immutable.
That said, I have threatened to create a decorator simi
On Tue, Feb 20, 2018 at 8:45 AM, Guido van Rossum wrote:
> TBH, I don't hate Nick's solution that assigns to __class__ in
> __post_init__. You can probably come up with a class decorator that hides
> construction of the frozen subclass. I do think that it should be used very
> sparingly, as the e
On Mon, Feb 19, 2018 at 8:16 PM, Glenn Linderman
wrote:
> On 2/19/2018 7:02 PM, Guido van Rossum wrote:
>
> But how?
>
> On Mon, Feb 19, 2018 at 5:06 PM, Chris Barker - NOAA Federal <
> chris.bar...@noaa.gov> wrote:
>
>> ... maybe it would be helpful to be able to
>> freeze an instance after crea
On 20 February 2018 at 15:11, Steven D'Aprano wrote:
> So in principle, we could have a mutable class, and a immutable one, and
> when you flick the switch, the instance.__class__ changes from mutable
> to frozen.
>
> If you don't hate this, we can think about the details needed to get
> it work i
On Mon, Feb 19, 2018 at 07:02:42PM -0800, Guido van Rossum wrote:
> On Mon, Feb 19, 2018 at 5:06 PM, Chris Barker - NOAA Federal <
> chris.bar...@noaa.gov> wrote:
[...]
> > This seems another use case — maybe it would be helpful to be able to
> > freeze an instance after creation for multiple use-
On 2/19/2018 7:02 PM, Guido van Rossum wrote:
But how?
On Mon, Feb 19, 2018 at 5:06 PM, Chris Barker - NOAA Federal
mailto:chris.bar...@noaa.gov>> wrote:
... maybe it would be helpful to be able to
freeze an instance after creation for multiple use-cases?
And there's the crux of th
But how?
Well, I hadn’t thought that far ;-)
But it would make frozen an instance level property, rather than a
class-level one — some instances would be frozen, some not.
Which would be kinda compatible with the idea of hashability being a
property of values, rather than type. Frozen-ness woul
But how?
On Mon, Feb 19, 2018 at 5:06 PM, Chris Barker - NOAA Federal <
chris.bar...@noaa.gov> wrote:
> If I have this right, on the discussion about frozen and hash, a use
> case was brought up for taking a few steps to create an instance (and
> thus wanting it not frozen) and then wanting it ha
If I have this right, on the discussion about frozen and hash, a use
case was brought up for taking a few steps to create an instance (and
thus wanting it not frozen) and then wanting it hashable.
Which pointed to the idea of a “ freeze this from now on” method.
This seems another use case — mayb
The underlying issue here is that we don't want an extra state flag in the
object to indicate "this object is currently [im]mutable". Using __class__
assignment to signal this is clever way to add this state, though not
without risks.
On Sun, Feb 18, 2018 at 4:34 PM, Nick Coghlan wrote:
> On 18
On 18 February 2018 at 14:10, Guido van Rossum wrote:
> Agreed the __pre_init__ idea is no improvement. I think we're back where you
> started -- just use `object.__setattr__` to set the attribute in
> `__post_init__`. That's what the PEP says is used by the generated
> `__init__`, so I think it i
Agreed the __pre_init__ idea is no improvement. I think we're back where
you started -- just use `object.__setattr__` to set the attribute in
`__post_init__`. That's what the PEP says is used by the generated
`__init__`, so I think it is reasonable to copy that pattern. Presumably
the situation doe
>
> Why can'y you make `name` on `NamedObjectItem` a property that returns `
> self.obj.name`? Why store a duplicate copy of the name?
>
Agreed, it's probably a better design not to store a duplicate reference to
name. But when I tried that, the property clashed with the inherited field.
This caus
Why can'y you make `name` on `NamedObjectItem` a property that returns `
self.obj.name`? Why store a duplicate copy of the name?
PS. I have to ponder why frozen dataclasses don't use `__new__`.
On Fri, Feb 16, 2018 at 11:43 PM, Ben Lewis wrote:
> On Sat, Feb 17, 2018 at 6:40 PM, Guido van Rossu
On Sat, Feb 17, 2018 at 6:40 PM, Guido van Rossum wrote:
>
>
> That's a pretty tricky proposal, and one that's been debated on and off
> for a long time in other contexts. And that flag would somehow have to be
> part of every instance's state.
>
> In general the right way to initialize an immuta
On Fri, Feb 16, 2018 at 3:38 PM, Ben Lewis wrote:
> I have been using dataclasses package in a pet project of mine. I'm sorry
> if this issue has already been raised. I came across a situation where I
> wanted to use the __post_init__ function to initialise some inherited
> fields from a dataclas
Hello
I have been using dataclasses package in a pet project of mine. I'm sorry
if this issue has already been raised. I came across a situation where I
wanted to use the __post_init__ function to initialise some inherited
fields from a dataclass with frozen=True. The problem is that because it is
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