Thursday, February 13, 2014

Configuring Kivy on PyDev

Kivy installation on PyDev is a bit more involved because it requires some environment variables to be set in order to work properly, so, I thought I'd give a step-by-step on where exactly to configure that on PyDev (especially now that LiClipse: adds support to the Kivy Language).

Note: The details below use paths based on windows, but it should be similar in other platforms.

1. Download/extract Kivy (for this example D:\bin\Kivy-1.7.2-w32\)

2. Add interpreter in Window > Preferences > Pydev > Interpreters > Python Interpreter (point to: D:\bin\Kivy-1.7.2-w32\Python\python.exe).

Note: For PyDev 3.3.3 onwards, the easier way to go there is doing: Ctrl+3 and writing 'Python interpreter' to open that preferences page (and the same thing can be used to go to a view or even activate some action).

3. Add the Kivy directory to the PYTHONPATH for this interpreter (in the same Python Interpreter page > libraries > add folder > D:\bin\Kivy-1.7.2-w32\kivy)

4. Add 'kivy' to the 'forced builtins' (again in that same page > forced builtins).

5. Add the needed environment variables (in that same page > environment):

GST_REGISTRY = D:\bin\Kivy-1.7.2-w32\gstreamer\registry.bin

GST_PLUGIN_PATH = D:\bin\Kivy-1.7.2-w32\gstreamer\lib\gstreamer-0.10

PATH = D:\bin\Kivy-1.7.2-w32;D:\bin\Kivy-1.7.2-w32\Python;D:\bin\Kivy-1.7.2-w32\gstreamer\bin;D:\bin\Kivy-1.7.2-w32\MinGW\bin;%PATH%

Alternatively, instead of adding those manually to the environment, open a cmd.exe, execute D:\bin\Kivy-1.7.2-w32\kivyenv.bat and then start Eclipse (but then you have to remember to do that manually every time -- or add it to the system environment variables -- note that you have to remember to update it if you move it or upgrade kivy).

After that, it should be possible to go to the pydev package explorer, expand the interpreter node in the tree > system libs > examples, open some example and open it, then, with the editor opened used F9 to run the example (you may have to select which project should be used to get the information on the PYTHONPATH to be used as it's running as an external file).

Tuesday, February 04, 2014

Changing the locals of a frame (frame.f_locals) and persisting results (with ctypes)

Up until now I didn't know of a proper way to change the locals of a frame (out of the normal execution flow in Python), so, when in the PyDev debugger changing a variable wouldn't always work.

So, for instance, if you have a frame (which you could get from a traceback, sys._getframe().f_back, etc), you could get its locals with frame.f_locals, but changing the frame.f_locals (which gives you a dictionary) wouldn't apply the results back to the frame.

This is mostly due to how CPython works: frame.f_locals actually creates a dictionary using PyFrame_FastToLocals, but changes to the dictionary aren't applied back.

Some years ago I had found a way to make it work (see: through a CPython function: PyFrame_FastToLocals, but up until recently, I thought it needed a modified version of CPython in order to work, now, recently I discovered ctypes can access a lot from the python api (through ctypes.pythonapi):

So, after changing frame.f_locals, it's possible to use ctypes to call PyFrame_LocalsToFast doing:

import ctypes

ctypes.pythonapi.PyFrame_LocalsToFast(ctypes.py_object(frame), ctypes.c_int(0))


A note: the second parameter (which may be 0 or 1) defines whether we want to erase variables removed from the dict (which would require 1) or not.

So,  the PyDev debugger now incorporates this utility so that if you're running in CPython, it will properly change the variable in a scope when you change a variable :)

Note that this isn't compatible with other Python implementations (this is not something the language dictates how it should work -- probably the ideal would be making frame.f_locals writable).