Re: Python 3 bindings

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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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;
 
 /** 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.

> > 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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