Python reigns on library support, and it is usually faster than implementing your own procedures, but I found out that when you are trying to solve a complex problem where you are both learning it and solving it simultaneously, having a repl like the one in common lisp really helps you get into the zone and deeply understand your software.
Sometimes it makes sense to pays the price to implement a library just for this.
ipython can handle this situation fine, however, cl repl leaves ipython in the dust for more complex programs
also python is fine when all you are after is the end result. however if you try to understand what is going on at a deeper level in your program you will quickly discover that python is just a front end to some "black box" (how black it is will depend on your understanding of python's ffi, c, c++, fortran)
If you compare try to understand actually JavaScript has the most accessible (for normal mortal) on the state of the system. IPython you have to type and sort of print. Python on code might help as you can checkpoint and get some state during that checkpoint.
If there is no problem I am not sure lisp give that much (for our mortal).
Obviously when problem occur in unexpected place and suddenly the availability of the stack is great. And that is lisp winning.
Not sure for general understanding as usually you have a case in mind and want to trace the state. In that case still think JavaScript, Python with code, iPython for document still ok.
