It's all about Numpy. Mayavi, IPython, mlab & friends are great for when I need to plot things or look at data, but Numpy is the workhorse I keep coming back to day in and day out. And also, the thing I wish for most when I have to use other languages. The combination of the speed of C and the elegance of, well, anything that is not C, is hard to beat. Once you get down the basics of array broadcasting, types, etc., it's possible to do some amazingly elegant things in Numpy, and quickly too. The numpy library has seemingly every array function I have ever wanted. If I were Matlab I'd be scared :-)