title: New project: Python 2 or Python 3?
body: <p>I'm starting a new open-source software in Python, and I'm wondering whether I should use Python 2.x or Python 3.x.</p>
  
  <p>It will include a heavy GUI, complex scientific algorithms dealing with large amounts of data. I'll need at least Numpy, Scipy, PyQT4, PyOpenGL, h5py, optionaly Matplotlib. It should first be released in 2013, with updates and extensions during the next few years.</p>
  
  <p>It will be used by scientists that do not necessarily have a background in computer science, on a wide variety of computers (Linux, Windows, Mac...). Some machines may be old.</p>
  
  <p>If I choose Python 3.x, I fear that the software and the third-party libraries may be more complicated to install, especially on old systems where Python 2.x is already installed. It would be better if RPM-like packages (or Windows installers) for Python and the external libraries were available on most systems, such that one doesn't need to compile everything. Also, I don't know if the 3.x branch is mature enough.</p>
  
  <p>If I choose Python 2.x, I may need to port the software to Python 3.x in the near future if the 2.x branch becomes deprecated (when will that happen?).</p>
  
  <p>What would be the best option?</p>