Re: [Libguestfs] [PATCH nbdkit v2 03/10] python: Implement nbdkit API version 2.
On Fri, Nov 22, 2019 at 11:53 PM Nir Soffer <nsoffer(a)redhat.com> wrote:
On Fri, Nov 22, 2019 at 11:35 PM Eric Blake <eblake(a)redhat.com> wrote:
>
> On 11/22/19 3:20 PM, Nir Soffer wrote:
>
> >>> +# There are several variants of the API. nbdkit will call this
> >>> +# function first to determine which one you want to use. This is the
> >>> +# latest version at the time this example was written.
> >>> +def api_version():
> >>> + return 2
> >>
> >> Matches the C counterpart of #define NBDKIT_API_VERSION 2 at the top of
> >> a plugin.
> >
> > This is the same thing I was thinking about. This makes it more clear
> > that the api
> > version is constant, and not something the plugin should change while
> > it is being used.
>
> Hmm - api_version() really is constant for the entire life of nbdkit. We
> call it exactly once. Figuring out how to read a Python variable
> instead of calling a function would be slightly more in line with the
> fact that in C code it is a #define constant rather than a function
> pointer callback. But it is that much more glue code to figure out how
> to check for a python global variable, compared to the glue code we
> already have for calling a python function.
The extra glue code is (without error handling):
PyObject *d = PyModule_GetDict(PyObject *module);
PyObject *v = PyDict_GetItemString(d, "API_VERSION");
long api_version = PyLong_AsLong(v);
Or simpler (with error handling):
PyObject *v = PyObject_GetAttrString(m, "API_VERSION");
if (v == NULL)
return 1;
long value = PyLong_AsLong(v);
Py_DECREF(m);
return value;
On error -1 is returned and PyErr_Occurred() will return the exception type.