Why Python?
In science and engineering, Python appears to be steadily eating other languages’ lunch and becoming the default language for student projects of all kinds.
The upshot appears to be that startups founded by recent graduates are using Python in increasing numbers. Among employers recently listed on the Silicon Milkroundabout website, experience in Python was more in demand than PHP or Java or Ruby, and–unlike in 2012–there is now apparently no significant difference in the demand for Django and Rails developers.
And to a close approximation, all computer courses on Udacity and all courses on Coursera that involve some computer programming, but not taught by Computer Science departments, use Python, so it has apparently become the default programming language of online learning, too.
JavaScript is still more widely used in frontend web development, because it is more-or-less the only language of front-end web development, and the emergence of Node.js on the backend is a very interesting development. PHP is more widely used in backend web development, but is not used in any other context. And in larger companies there are still more jobs for Java programmers.
But Python is becoming by an increasing margin the most widely-used general-purpose programming language in teaching, in science and engineering, and perhaps in startups, too. It is a great first programming language. And there are some excellent online courses that use it.