Using the same script on installs with different EPD versions

In the newest version of Enthought’s Python Distribution (EPD) on Ubuntu, the plotting package has been moved from enthought.mayavi.mlab to the shorter and more general mayavi.mlab. This does however mean that if you, like me, need to work with different versions of EPD on multiple systems, will experience the following error from time to time:

ImportError: No module named enthought.mayavi.mlab

Now, to avoid switching the import statement every time you switch systems, you can make Python check if one of the versions is installed during import. If it is not, we’ll tell it to try the other. This is done in this simple command:

try:
    from enthought.mayavi.mlab import *
except ImportError:
    from mayavi.mlab import *

Just replace any other similar import statements the same way and your code should once again be working across all your installations.

Fixing “undefined reference to `vtable for …”

These annoying errors have been haunting me the last couple of days, so I figured I should share the most common reason for their occurrence. That is in my projects at least.

This error is caused because the linker in gcc is unable to find the functions you have defined in your headers in your actual code. So if you have a header which looks like this:

#ifndef MESH_H
#define MESH_H

class Mesh
{
public:
    Mesh();
    ~Mesh();
    virtual void draw();
};

#endif // MESH_H

You must at least have these functions defined in your .cpp file:

#include "mesh.h"

Mesh::Mesh() {
}

Mesh::~Mesh() {
}

void Mesh::draw() {
}

After this, make sure you clean your compile environment to make sure no object files are being misinterpreted by the compiler. If you are using Qt or a project with a Makefile, you could just run these three commands (the first only applies to Qt projects).

qmake
make clean
make

Should you still have trouble, make sure that qmake is actually generating your .moc files for any objects that need them. Sometimes it might even be necessary to empty the build directory completely yourself to make sure there are no files left behind that are not cleaned by make clean.