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&gt; wrote: > > On Fri, Nov 22, 2019 at 11:35 PM Eric Blake <eblake(a)redhat.com&gt; 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.