Make tf2_py compatible with python3.

[de-vri-es]

This PR makes tf2_py compatible with python3. I compiled the package for both python 3.5 and 2.7 (which worked), but I did not test the resulting python module.

There were three issues to fix:

• Python3 uses unicode strings for text. The old python2 string type no longer exists.
• Module initialization has been overhauled for python3.
• Type definitions changed slightly.

I put wrapper functions for the correct string types in python_compat.h (and one convenience function for importing modules). Something similar could be done for the type definition and the module initialization, but those would involve writing new macros, which would not necessarily make the code easier to maintain (especially in the case of module initialization).

I did not use py3c (a C API compatibility layer for python2/3) to avoid adding a new dependency. If a new dependency is acceptable, py3c would be a good alternative. It would involve adding a new rosdep key though, I assume.

If this is the way to go, PyObject_BorrowAttrString could also be moved to python_compat.h, to group python convenience functions together. (Though I would rename it so it doesn't look like a real python function. It took me a while of digging through the python C API before realising it is defined in the source file itself).

[de-vri-es]

This should fix #50, btw.

[de-vri-es]

One more addendum: Rewriting the whole thing in Cython seems like a good idea to me, perhabs even with a more full-featured API (while maintaining backwards compatibility of course).

However, I wanted python3 compatibility fast, and this was the quickest, least intrusive way to get that.

[tfoote]

@ros-pull-request-builder retest this please

Thanks for the contribution. I'll try to review it tomorrow. Python3 support will be great.

[tfoote]

Thanks for taking the time to do this. It looks good to me. Bumping PyObject_BorrowAttrString into the python_compat.h makes sense to me.

Thanks for working around it without the extra dependency. If this is enough lets stick with this and not completely refactor it to Cython.

[de-vri-es]

Moved PyObject_BorrowAtrrString to python_compat.h and renamed it to pythonBorrowAttrString to avoid looking like it is part of the official Python API.

(Update: And made it inline, since it is defined in a header.)

Thanks for the review.

[tfoote]

Thanks again!

[spaghetti-]

Thank you for this patch!

[koniarik]

Hi folks, will there be anytime soon release of geometry2 with this patch?

[nstiurca]

@SquirrelCZE I'm wondering the same thing.

In the meantime, I built from source based on the latest version I just pulled (439e235), but I can't figure out how to actually use TF2 in Python 3. Here's what I did:

mkdir -p overlay_ws/src
cd overlay_ws/src
git clone https://github.com/ros/geometry2.git
catkin_init_workspace
cd ..
rosdep install --from-paths src --os=ubuntu:xenial
catkin_make --use-ninja --cmake-args -DCMAKE_BUILD_TYPE=Release
source devel/setup.bash

Then I tried to use it with

$ python3
Python 3.5.2 (default, Sep 10 2016, 08:21:44) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import tf2_py
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/nicu/dbot/overlay_ws/devel/lib/python2.7/dist-packages/tf2_py/__init__.py", line 35, in <module>
    exec(__fh.read())
  File "<string>", line 37, in <module>
ImportError: No module named '_tf2'
>>> 

[nstiurca]

I should say the import error is the same (except for paths) I get when only working with the latest version installed from APT.

[MohamadHalwani]

@SquirrelCZE I'm wondering the same thing.

In the meantime, I built from source based on the latest version I just pulled (439e235), but I can't figure out how to actually use TF2 in Python 3. Here's what I did:

mkdir -p overlay_ws/src
cd overlay_ws/src
git clone https://github.com/ros/geometry2.git
catkin_init_workspace
cd ..
rosdep install --from-paths src --os=ubuntu:xenial
catkin_make --use-ninja --cmake-args -DCMAKE_BUILD_TYPE=Release
source devel/setup.bash

Then I tried to use it with

$ python3
Python 3.5.2 (default, Sep 10 2016, 08:21:44) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import tf2_py
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/nicu/dbot/overlay_ws/devel/lib/python2.7/dist-packages/tf2_py/__init__.py", line 35, in <module>
    exec(__fh.read())
  File "<string>", line 37, in <module>
ImportError: No module named '_tf2'
>>> 

Have you solved this ??? Coming from the future :D