The immediate use case I can think of this for is to make it possible to
just do:
__len__ = instancemethod(operator.attrgetter('_length'))
__hash__ = instancemethod(operator.attrgetter('_cached_hash'))
and stuff like that. Mostly just a minor performance optimization to avoid
the overhead of Python level function calls (and in the case of stuff like
__hash__, reduce the probability of the GIL releasing during a dict/set
operation, though you'd still need some way to move __eq__ to the C layer
to allow stuff like setdefault to be truly atomic).
This was possible in Python 2 with types.MethodType (since it could make
unbound methods of a class for you; unbound methods no longer exist). Now
the solution is... considerably uglier (requires ctypes):
https://stackoverflow.com/q/40120596/364696
On Sun, Sep 13, 2020 at 5:24 AM Steven D'Aprano <[email protected]> wrote:
> On Sun, Sep 13, 2020 at 12:32:54AM -0400, Random832 wrote:
>
> > This isn't what I was suggesting - I meant something like this:
> >
> > class instancemethod:
> > def __init__(self, wrapped):
> > self.wrapped = wrapped
> > def __get__(self, obj, objtype):
> > if obj is None: return self.wrapped
> > else: return MethodType(self.wrapped, obj)
> >
> > this wouldn't be useful for functions, but would give other callables
> > the same functionality as functions, automatically creating the bound
> > method object, e.g.:
> >
> > class D:
> > def __init__(self, obj, *args): ...
> >
> > class C:
> > foo = instancemethod(D)
>
> You want a method which does absolutely nothing at all but delegate to a
> class constructor or callable object (but not a function), with no
> docstring and no pre-processing of arguments or post-processing of the
> result.
>
> Seems like an awfully small niche for this to be in the stdlib.
>
> But having said that, I might have a use for that too. Except... I would
> need a docstring. And pre- and post-processing. Hmmm.
>
> *shrug*
>
> Seems to me that this might be useful in theory, but in practice we
> might never use it, preferring this instead:
>
> class C:
> def foo(self, *args):
> """Doc string."""
> return D(self, *args)
>
> with appropriate pre- and post-processing as needed.
>
> Interesting suggestion though, I may have to play around with it.
>
>
>
> --
> Steve
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