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Tools/gdb/libpython.py is currently Python 3 incompatible. Unfortunately recent versions of gdb (such as the one provided in Ubuntu 13.10) may be linked against Python 3 rather than Python 2, breaking debugging support.
Most of the issues appear to be trivial issues such as the print statement syntax and removal of functions like xrange, unichr and long.

should go into 2.7 as well.

Do you want to provide a patch? It's hard for someone with a py2-linked gcc to make sure that the libpython pluging works properly.

Sure thing. I've got a patch attached to https://bugs.launchpad.net/ubuntu/+source/gdb/+bug/1241668 but want to make sure that it doesn't break things on the py2-linked version.

Can you please try the attached updated patch?

I ended up with a very similar looking patch - and so merged my changes into Pitrou's patch.
I also had a go at fixing up the `test_gdb` unit test, which revealed a few more string/unicode issues. Finally tracked them down to the `write_unicode` function. Sadly the `write` function in GDB's Python API only accepts unicode strings, hence we must pre-backslash escape for C-string encoding (either Python's default encoding or utf-8 in Python 3.1+) then turn it back into a unicode string again to avoid a TypeError. Yuck.
The patch almost applies on Python 2.7, but needs a little bit of massaging due to slight differences in Tools/gdb/libpython.py. Need to test the new unicode changes haven't broken anything and then I'll upload a patch for Python 2.7.

And here's the patch for Python 2.7.
The result of testing is as follows:
- python (default) against py2-linked gdb: All tests pass
- python (2.7) against py2-linked gdb: `test_long` fails.
- python (default) against py3-linked gdb: All tests pass
- python (2.7) against py3-linked gdb: `test_long`, `test_strings` and `test_unicode` fail.
`test_long` has been removed from the default branch, in addition gdb doesn't seem to distinguish printing 0 from 0L. `test_strings` likely fails since Python 3 will print printable unicode characters (like "\xff") while Python 2 will not. `test_unicode` likely fails due to the 'u' prefix on unicode strings in Python 2. This kind of behavior is pretty much expected when cross-testing gdb.

I've added a review comment to the 3.3 version of the patch.

I tested the latest patch (python27-gdb_py3.patch) with ubuntu 13.10 gdb compiled against python3.3, while it fixes the syntax errors it does not fix the functionality.
E.g. one gets this error on breakpoints:
Python Exception <class 'gdb.error'> There is no member named length.: 
Breakpoint 3, PyTuple_Size (op=) at ../Objects/tupleobject.c:127
and the objects are not printed in their string representation as they should be with the plugin.

on further investigation I seem to have screwed up patching the files. Patching properly they do work. Sorry for the noise.

New changeset 9f2a0043396b by Antoine Pitrou in branch 'default':
Issue #19308: fix the gdb plugin on gdbs linked with Python 3
http://hg.python.org/cpython/rev/9f2a0043396b 

New changeset 752db82b7933 by Antoine Pitrou in branch '3.3':
Issue #19308: fix the gdb plugin on gdbs linked with Python 3
http://hg.python.org/cpython/rev/752db82b7933 

New changeset ef4636faf8bd by Antoine Pitrou in branch '2.7':
Issue #19308: fix the gdb plugin on gdbs linked with Python 3
http://hg.python.org/cpython/rev/ef4636faf8bd 

I've committed updated patches to all 3 branches (+ some later fixups). Gonna wait for the buildbots' outcome.

The buildbots look ok now.