Re: [Xen-devel] Python 3 bindings

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Feb 21, 2017 at 02:03:00PM +0100, Marek Marczykowski-Górecki wrote:
> On Mon, Feb 20, 2017 at 05:18:44PM +0000, Wei Liu wrote:
> > On Fri, Feb 17, 2017 at 01:36:01PM +0100, Marek Marczykowski-Górecki wrote:
> > > Hi,
> > > 
> > > I'm adjusting python bindings to work on python3 too. This will require
> > > few #if in the code (to compile for both python2 and python3), but it
> > > isn't that bad. But there are some major changes in python3, which
> > > require some decision about the bindings API:
> > > 
> > > 1. Python3 has no longer separate 'int' and 'long' type - old 'long'
> > > type was renamed to 'int' (but on C-API level, it uses PyLong_*). I see
> > > two options:
> > >   - switch to PyLong_* everywhere, including python2 bindings - this
> > >     makes the code much cleaner, but it is an API change in python2
> > >   - switch to PyLong_* only for python3 - this will introduce some
> > >     #ifdefs, but python2 API will be unchanged
> > 
> > Could you be more specific? Like, provide a code snippet?
> 
> Here is compile tested only version:
> https://github.com/marmarek/xen/tree/python3
> 
> It uses PyLong_* only for python3, here is how it looks in code (I've
> skipped s/PyInt_/PyLongOrInt_/ for readability):
> 
> -----8<-----
> --- a/tools/python/xen/lowlevel/xc/xc.c
> +++ b/tools/python/xen/lowlevel/xc/xc.c
> @@ -34,6 +34,17 @@
>  
>  #define FLASK_CTX_LEN 1024
>  
> +/* Python 2 compatibility */
> +#if PY_VERSION_HEX >= 0x03000000
> +#define PyLongOrInt_FromLong PyLong_FromLong
> +#define PyLongOrInt_Check PyLong_Check
> +#define PyLongOrInt_AsLong PyLong_AsLong
> +#else
> +#define PyLongOrInt_FromLong PyInt_FromLong
> +#define PyLongOrInt_Check PyInt_Check
> +#define PyLongOrInt_AsLong PyInt_AsLong
> +#endif
> +
>  static PyObject *xc_error_obj, *zero;
>  
>  typedef struct {
> --- a/tools/python/xen/lowlevel/xs/xs.c
> +++ b/tools/python/xen/lowlevel/xs/xs.c
> @@ -43,6 +43,14 @@
>  #define PKG "xen.lowlevel.xs"
>  #define CLS "xs"
>  
> +#if PY_VERSION_HEX < 0x03000000
> +/* Python 2 compatibility */
> +#define PyLong_FromLong PyInt_FromLong
> +#undef PyLong_Check
> +#define PyLong_Check PyInt_Check
> +#define PyLong_AsLong PyInt_AsLong
> +#endif
> +
>  static PyObject *xs_error;
>  

If this is the recommended practice, then that's fine. I can't seem to
find such practice in https://docs.python.org/3/howto/cporting.html but
I'm no python binding expert.

BTW, I went through your python3 branch. It seems that some patches can
be submitted independently.


>  /** Python wrapper round an xs handle.
> -----8<-----
> 
> > 
> > > 
> > > 2. Python3 has no longer separate 'str' and 'unicode' type, new 'str' is
> > > the same as 'unicode' (PyUnicode_* at C-API level). For things not
> > > really unicode-aware, 'bytes' type should be used. On the other hand, in
> > > python2 'bytes' type was the same as 'str'.
> > > This affects various places, where in most cases 'bytes' type is
> > > appropriate (for example cpuid). But I'm not sure about xenstore paths -
> > > those should also be 'bytes', or maybe 'unicode' (which is implicitly
> > > using 'utf-8' encoding)? I think the only reason to use 'unicode' is
> > 
> > According to docs/txt/misc/xenstore.txt, paths should be ASCII
> > alphanumerics plus four punctuation characters. Not sure if this is
> > relevant to what you describe.
> 
> It's easy to make function accept both 'bytes' and 'unicode'. The
> question is what should be return type (read_watch, ls etc) - given
> limited character set used there, I'm in favor of 'unicode' - easier to
> handle, but we shouldn't hit any unicode decoding problems.
> Maybe the same should apply to path arguments (use 'unicode')? Most
> file-handling methods in python3 use 'unicode' for paths, if that
> matters.
> 

OK. Using unicode makes sense to me. Again, I'm no python expert and I
trust what you said. :-)

> > > convenience for API users - in python3 if you write 'some string' it
> > > will be unicode type, to create bytes data you need to write b'some
> > > string'.
> > > As for python2, it should definitely be still 'str'/'bytes' type.
> > > 
> > > There is one more little detail - build process. Here I'm going to
> > > follow popular standard - use $(PYTHON) variable - if that points to
> > > python3, build for python3. Actually this means no change in the current
> > > makefile. If someone want to build for both python2 and python3, will
> > > need to call the build twice - at packaging level.
> 
> -- 
> Best Regards,
> Marek Marczykowski-Górecki
> Invisible Things Lab
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?



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