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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.
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.
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.
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.
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?).
What would be the best option?
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While Python 2 had its heyday in the early 2000s, Python 3 is the best choice to learn in 2022. You might find some circumstances in which you need to use Python 2, but on the whole, Python 3 is the most popular language. Since Python 2 was sunsetted in 2020, Python 3 has dominated the programming world.
We have decided that January 1, 2020, was the day that we sunset Python 2. That means that we will not improve it anymore after that day, even if someone finds a security problem in it. You should upgrade to Python 3 as soon as you can.
Python 2 finally reached end of life (EOL) at the start of 2020. Python 2 EOL means that version of the language will no longer receive any sort of updates, including security updates.
Python version 3 is not backwardly compatible with Python 2. Many recent developers are creating libraries which you can only use with Python 3. Many older libraries created for Python 2 is not forward-compatible.
UPDATE: My original answer was given in 2012. However, now, years later, the answer should and must be Python 3.
This wiki discusses exactly your question: Should I use Python 2 or Python 3 for my development activity?
This is a very large subjective part to this question which depends on exactly your specific situation and constraints.
IMO, however, if you can't be sure that all 3rd party libraries you need will work with Python v3.x I would stick with Python 2.x but try to write code to be as compatible with Python 3.x as much possible (e.g., print() etc) so that if you ever need or want to migrate to v3 your work won't be so challenging. And of course there are documentation and tools on moving from version 2 to 3. And this on porting v2 code to v3.
This will especially be the case on systems where you don't have control or can't install newer versions of Python or customize the installation of 3rd party software/libraries.
Given that your software will be run on a wide variety of systems (Linux, Windows, Mac ..) and by a number of different users I still think that v2 is more of a common denominator these days.
So, short-attention-span summary: Use version 2.x but perpare your code for v3.x
Finally, if you put this search string "[python] version 3 or 2" into the SO search box, you'll find a number of related posts:
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