Tuesday, September 22, 2015

Python Tutor

Read This: http://radar.oreilly.com/2015/08/learning-programming-at-scale.html

The core visualization tool (pythontutor.com) can be helpful for many people. The shared environments seem like a cool idea, also, but I don't have any specific comments on the other tools.

While this looks very cool, I'm not a huge fan of this kind of step-by-step visualization. This uses very clear graphics, and looks very clever, it has some limitations. I think that some aspects of "visualization" can be misleading. Following an execution path for a specific initial condition can obscure events and conditions that aren't on the happy path. It's not clear how a group of statements establish a more general condition.

I'm a fan of formal post-conditions. From these, we can postulate a statement, and work out the weakest precondition for the statement. As we work through this exercise, we create a formal proof and a program. It's very elegant. And it covers the general case, not specific examples.

Most importantly, this effort depends on having formal semantics for each statement. To write code, we need to have a concise definition of the general state change is made by each statement in a language. We're looking at the general case for each statement rather than following a specific initial condition through a statement.

In C, what does this do? a[i++] = ++i; There is no formal definition. The statement has three state changes stated. But how are they ordered? No matter what initial values for a[] and i we provide, this is still pretty murky. A debugger only reveals the specific implementation being debugged.
Visualization may help some people understand the state change created by a statement. Some people do learn things by watching this kind of "debugger" mode. In particular, this may help because it has much better graphics than the built-in character-mode debugger.

This idea works best with programs that already make sense: programs that are well designed. Programs that make orderly progress from some initial state to the desired final state.

Programs written by learners may not be all that clean. Realistically, they may be inept. They may even reach the far end of the spectrum and be downright bad.

While this tool is graphically gorgeous, it's still a debugger. It wallows around in an internal world in which the formal semantics can get obscured. The general case can't easily be shown.

We have a forest and trees problem here. A debugger (or other statement-by-statement visualization tool) emphasizes each individual tree. The larger structures of glades, thickets, groves, stands, brakes, and coppices are lost to view.

The humble while statement (especially one with an internal if-break) can be extremely difficult to understand as a single statement. If we break down the statement-by-statement execution, the presence of two termination conditions (one on the while clause and one on the if clause) can be obscured because a visualization must follow a specific initial condition.

With really well-written tutorials -- and some necessary metadata -- a super-visualizer might be able to highlight the non-happy-path logic that exists.  This alternate path viewing could be helpful for showing how complex logic works (and doesn't work.)

With programs written by learners -- programs which are inept and won't have appropriate metadata -- a super-visualizer would need to reason very carefully about the code to determine what happy path and non-happy-path kinds of logic are present. It would have to locate and highlight

  • contradictory elif clauses, 
  • gaps among elif clauses, 
  • missing else clauses, 
  • hellishly complex else clauses, 
  • break conditions, 
  • continue conditions, as well as 
  • exception handling.

For truly bad programs, the super-visualizer may be stumped as to what is intended. Indeed, it may be impossible to determine how it can be displayed meaningfully to show alternatives and show how the specific code generalizes into a final outcome.

def this_program_terminates(some_code):
    # details omitted

def demo():
    while this_program_terminates(demo):
        print("w00t w00t")

What does this do? How can any visualizer aid the student to show problems?

To take this one step further, I think this kind of thing might also be hazardous to learning how the functional programming feature of Python work.  I think that exposing the underlying mechanics of a generator expression might be more confusing than simply treating it as a "lazy list."

It's very nice. But it isn't perfect. Something that supports reasoning about the general post-conditions established by a statement would be more useful than a step-by-step debugger with great graphics.

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