[Python-ideas] RFC: PEP: Add dict.__version__

[Neil Girdhar]

On Sun, Jan 10, 2016 at 12:57 PM, Steven D'Aprano <steve at pearwood.info>
wrote:

> On Sun, Jan 10, 2016 at 11:48:35AM -0500, Neil Girdhar wrote:
>
> [...]
> > > v = mydict.__version__
> > > maybe_modify(mydict)
> > > if v != mydict.__version__:
> > >     print("dict has changed")
> >
> >
> > This is exactly what I want to avoid.  If you want to do something like
> > this, I think you should do it in regular Python by subclassing dict and
> > overriding the mutating methods.
>
> That doesn't help Victor, because exec need an actual dict, not
> subclasses. Victor's PEP says this is a blocker.
>

No, he can still do what he wants transparently in the interpreter.  What I
want to avoid is Python users using __version__ in their own code.

>
> I can already subclass dict to do that now. But if Victor's suggestion
> is accepted, then I don't need to. The functionality will already exist.
> Why shouldn't I use it?
>

Because people write code for the abc "Mapping".  What you are suggesting
is then to add "__version__" to the abc Mapping, which I am against.
Mapping provides the minimum interface to be a mapping; there is no reason
that every Mapping should have a "__version__".

>
> > What happens if someone uses a custom Mapping?
>
> If they inherit from dict or UserDict, they get this functionality for
> free. If they don't, they're responsible for implementing it if they
> want it.
>

But they shouldn't have to implement it just so that code written for
Mappings works — as it does now.

>
>
> > Do all custom Mappings need to implement __version__?
>
> I believe the answer to that is No, but the PEP probably should clarify
> that.
>

If the answer is "no" then honestly no user should write code counting on
the existence of __version__.

>
>
> --
> Steve
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