[Libguestfs] [PATCH nbdkit v2 03/10] python: Implement nbdkit API version 2.

[Nir Soffer]

On Fri, Nov 22, 2019 at 11:55 PM Eric Blake <eblake at redhat.com> wrote:
>
> On 11/22/19 3:46 PM, Nir Soffer wrote:
>
> >>>> and for zero (once fast zero is supported later in the series), it could
> >>>> look like:
> >>>>
> >>>> def zero(h, count, offset, may_trim=False, fua=False, fast=False):
> >>>
> >>> This is nicer - but not extensible.
> >>
> >> Why not?  Any future flag additions would still appear as new key=value
> >> kwargs.
> >
> > Because there is no **kwargs argument, you will get TypeError when adding new
> > argument. We will need new api version whenever we add new argument.
>
> Not if you use PyObject_Call(fn, args, kwargs) or similar to pass
> 'may_trim=False' as a kwarg instead of a positional.
> As long as new flags are always passed via keyword instead of position, and we don't
> pass any keywords that the plugin has not opted into,
>  then we can add
> new keywords and call plugins with fewer flags than what our current
> nbdkit supports.
>
> > This is extensible:
> >
> >      def zero(h, count, offset, may_trim=False, fua=False, fast=False, **kwargs):
>
> But that is also called via PyObject_Call(fn, args, kwargs) - as long as
> we pass kwargs correctly, the C code doesn't care whether the Python
> definition has a ** catchall or not, because the rules for translating
> the C code's two lists (one of positional, the other of kwargs) into the
> Python signature work for multiple different python signatures.

I see, this will work and is extensible, but it may be harder to write
correct plugin
code. You may forget to define can_foo() and then define foo=None or define
can_foo() and forget to define the foo argument.

Nir