Allowing only a single instance of a program to open using Qt

Many text editors open a new tab and not a new editor window when a second (etc. etc.) document is opened. For my program that displays curves of simulation results I wanted to have similar behaviour. This program is written in Qt using Qwt (also see this earlier post). This program already supports drag and drop and file association (i.e. double clicking on a file to open it with this program). To prevent a second program from opening when a second file associated with this program is double clicked I only had to follow this example.

Python profiling trials

Python comes with some default profiling capabilities. However, as also noted here, this profiling can slow down your program to a crawl making the gathering of useful statistics very tedious. Although it is very easy to get the proposed alternative "plop" running the output it generates is very limited and I did not find a description to get a more traditional output from this profiler. Although statprof does generate traditional output without slowing down running your program very much, it does not really seem to deal correctly with wrapper functions which severely limits its applicability to my typical programs. Thus, unfortunately, I seem to be stuck with using CProfile for a while.

A nice introduction to CProfile is given on slippen's blog.

To turn on profiling of Cython code add the line

#cython: profile=True

to the top of your .pyx file (including the hash).

using SWIG to connect python and a c++ dll

Although I am a great fan of Cython when you already have a class defined in C++ code you do not want to rewrite it for Cython. Already a long time ago I once used SWIGg to create a link between C++ classes and python. This is fairly easy as long as you have clear interface functions which you can use to pass data to and from your class. Nevertheless, it took me some effort to get things running for my new class.

Following the SWIG tutorial I started with something like this myExample.i

%module myExample
 %{
 /* Includes the header in the wrapper code */
 #include "myModule.h"
 #include "myModule2.h"
 %}
 
 /* Parse the header file to generate wrappers */
 #include "myModule.h"
 #include "myModule2.h"

After compiling this I got a module that I could import in Python. However, the module did not contain the classes I had defined in my C++ code. Because there are not some many clear examples of SWIG and C++ I could not really find an explanation for this. Only after recreating the exact example of the tutorial (using copy/paste from the web) I found that I made a simple typing error: the #include in the second part of the myExample.i should be %include!

%module myExample
 %{
 /* Includes the header in the wrapper code */
 #include "myModule.h"
 #include "myModule2.h"
 %}
 
 /* Parse the header file to generate wrappers */
 %include "myModule.h"
 %include "myModule2.h"

Now everything works nicely. Compiling is not very difficult. I have setup a Makefile for this:

CPP=g++

CFLAGS=  -Wunused  -O3

PYTH_INCL = /c/Python27/include
PYTH_LIB = /c/Python27/Libs

PYLIB = -lpython27

OBJS= myModule.o
modOBJS= myModule2.o
SRC_DIR = ..
all: myExample_mod

myExample_mod: $(modOBJS) myExample.i
 swig -I.. -python -c++ -shadow myExample.i
 $(CPP) -I$(PYTH_INCL) $(INCLUDE) -c myExample_wrap.cxx
 $(CPP) -shared -L$(PYTH_LIB) myExample_wrap.o $(OBJS) $(PYLIB) $(LIBS) -o _myExample.pyd

%.o: $(SRC_DIR)/%.cpp $(SRC_DIR)/%.h
 $(CPP) -c $(CFLAGS) $(INCLUDE)  $< -o $@

With this Makefile you can use import myExample in your python code to import the module. Thus far I have only used simple interface methods in my classed that work with integers and floats as input and output variables. This makes communication between the module and python very simple (no special effort seems to be required). I assume that if you want to pass arrays, strings, etc. things will become more difficult.

cython -mno-cygwin problems

The command I use to compile a cython extension on windows:

python setup.py build_ext --inplace --compiler=mingw32

(see also these two posts) has problems with newer mingw/cygwin installations. Already for some to the compiler option -mno-cygwin is no longer used and the mingw compiler should be called directly. When you run the setup.py script with the compiler flag as shown, however, python automatically adds the -mno-cygwin flag. This then stops the compiler. The best solution for this I have found so far is to change the

c:\Python27\Lib\distutils\cygwinccompiler.py

file. Simply remove the -mno-cygwin option everywhere in the definition of the Mingw32CCompiler class (just search for no-cygwin in this file and you will find it).

Looks like I also could have googled this instead of finding out again how to use grep to search in subdirectories which is not as simple as adding the -r option ( find ./* -type f -exec grep -l "no-cygwin" {} \;).

Location drive specification MinGWPortable

A long time ago I found a portable version of MinGW somewhere on the web. It is sometimes very convenient to have a gcc compiler ready to run on a computer that is not your own. However, I used it mainly on one computer and when I finally did try to run it on a different machine it didn't work. The problem was that it was looking for the MinGW and msys stuff on the wrong drive. Although not very portable it is easy to change this as long as you remember that msys emulated a unix environment! Just change the drive in

/etc/fstab

It took me some time to find this. But instead of searching myself I could have looked here.

Somehow I am no longer able to find my original MinWGPortable on the web anymore. That is a pity because the msys terminal is extremely nice: you can resize it in any direction! Perhaps I should have another look here.

Windows application icon in Qt Creator

In an earlier post I described how to add icons to a Qt application using Qt Creator. With the procedure described there, the application shows the icon in the window title bar and on the taskbar. However, when you browse to the folder containing the application in windows, the executable will not be shown with the icon. The same is true when you create a shortcut to the executable (windows will also say that the executable does not contain an icon).

As described here, there is a simple windows specific method to add an application icon that will change the appearance of the executable when browsing under windows (and when creating a shortcut to the executable). You must first create a windows specific resource file which has the extension .rc e.g. myapp.rc. This simple text file then needs to contain the (single) line:

IDI_ICON1 ICON DISCARDABLE "images/myapp.ico"

The final step is then to add this resource file in the Qt project (.pro) file:

RC_FILE = myapp.rc

This works for me when I put the myapp.rc in the same folder as where the Qt project file is located (and that contains the images folder). I did not try any other file locations.

pythonxy cython mingw problems

Again I was having issues using a cython compiled package with pythonxy. For some reason when running from a ipython terminal started from pythonxy, I would get a DLL load error. If I would run the python/cython program directly (i.e. from a dos box: python my_program.py) then it would run fine. By default if you compile a program with mingw you need a dll with the std c or c++ library (typically this dll has a difficult name as libgcc_s_dw2-1.dll). As I have mingw installed in more than one location (32bit, 64bit, QtSDK, Python(x,y)) I think it cannot find the right dll (although I could not detect which dll it was using). The problem can be most easily solved if you link the std library statically. This you can do by simply adding "stdc++" to your libraries field in setup.py.

So my new setup.py looks now something like

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext
import numpy

ext_modules = [Extension("BvL_cython",["model_cython.pyx"],\
    libraries=["m","stdc++"],include_dirs=[numpy.get_include()])]

setup(
    name= 'Model class',
    cmdclass = {'build_ext': build_ext},
    ext_modules = ext_modules
)

(This also shows how to automatically detect the location of the numpy include directories)