The inherent limitation here is that Online Python Tutor is built upon a line-oriented single-step debugger (Python's bdb module), so there's no way for it to step inside of subexpressions (e.g., in your list comprehension example).
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Online Python Tutor isn't meant for running large examples. Think of it as a "digital whiteboard"; I don't think anyone would expect to visualize a 100,000-step program with tons of pointers on a whiteboard :)
My motivation has been to ease the learning path for an existing popular language (Python).
Here is some context behind the current incarnation of Online Python Tutor:
I have no idea -- that's why I put that parenthetical caveat in that paragraph :)
just to provide some context, this page is the epilogue of a ~100-page memoir of my Ph.D. experiences [1], so parts of it might not make any sense as a standalone article.
also, that's sort of why i had the parenthetical following that statement ...
(Two obvious caveats: Not every Ph.D. student received these benefits—many grew jaded and burned-out from their struggles. Also, lots of people cultivate these positive traits without going through a Ph.D. program.)
maybe i didn't make the disclaimer big enough, though :)
I don't know whether that's the convention in your field (or with your advisor), so I don't want to make any generalizations. It's certainly possible to graduate with one big idea -> single paper, but that's just not what I ended up doing.
re: Onward! yes, i've heard good things about it; will keep a look-out.
re: focus -- yes, that's key. but of course, it's bad to be blindly focused down a bad direction, too. don't put too much pressure on yourself early on, since nearly everyone flops and flounders.
re: what to do over the summer -- i don't know if there's much you can do to prepare :) maybe reading your professor's past papers? again, i wouldn't stress about it too much.
re: distilling -- my main intention in writing this memoir wasn't to provide advice, since it's hard for me to determine how much facets of my experience generalize. i do have plans to convert the "lessons learned" section from the epilogue into a talk, though.
i have this other article from the beginning of my third year that's more of the "advice" type, although again, I'm wary of making generalizations about Ph.D. life: http://pgbovine.net/PhD-life.htm
(other fun fact - i used one of your blog quotes as the title slide of my Ph.D. oral defense)
here's an HTML version too ... http://pgbovine.net/PhD-memoir.htm