Ian Bicking: the old part of his blog

Ideal webdev environment comment 000

I admit I couldn't figure out how to work with the breakpoints in Rails when I tried using them (some time ago now). I should try them again some time.

In Python I was able to implement interactivity without "breakpoints" per se. You can inspect the environment at the time of error, you just can't continue the request. But web request are so easy to restart that it didn't seem like a problem. Well, it is somewhat of a problem, because in the (fairly common) case where there is, say, a loop, and you know something is weird part way through the loop, it gets hard. With both Python (using that exception catcher) and ruby-breakpoint, you can put in something that gives you interactivity at some point in the code. In Python that is assert 0. But with ruby-breakpoint you can (I presume) continue from the breakpoint until you see the wonky situation. In Python you would have to, oh, put in an assert that identifies the situation (which, depending on how far into the looping it is, might be necessary anyway, as continuing over and over can be tedious). But even then, if you are just guessing that something is weird, it doesn't work well, because each guess is a different iteration of putting in a failing assert.

So, instead of breakpoints I think this can be resolved reasonably in Python with a capture-state call. This would capture the entire stack trace with all local variables and save it in a rendered form, and at the end of the request would display all those captured stacks at the bottom of the page. This gives you a complete view at multiple points during the request, but without interactivity. There are pluses and minuses to this. Anyway, I think it can give the same experience through a somewhat different path.

Comment on Re: An Ideal Web Development Environment
by Ian Bicking

Comments:

At work, I've been doing a thrice-perverse thing: using MS Access/VBA to prototype applications for SQLServer/VBScript. There is a Publish routine within Access that pushes out all of the code, with one hideously evil nested regex replacement function that papers-over a few irreconcilable differences between VBA/VBScript. One nice aspect of this approach is that I can run my library code within Access/VBA and leverage the IDE for all the usual reasons. Why I clutter your blog with this talk, though, is to pose the following question: can we factor the data, logic, and presentation of the idea web solution such that we can develop perfectly testable pieces using whatever we already use, then push out to a web server? Stated another way: why not just mock all of the web server objects, and factor out the network protocols, and see if changing how we think about the problem provides swell insights? Once all the technical hurdles are surmounted, we can figure out what to do about my company's cranial rectalitis. T-SQL has all the ugliness of Perl, without any power, to charm.

# Chris Smith

Wow, that's pretty cool.

Just wondering about how this works. In ruby-breakpoint I also define a special assert() statement that basically will just call breakpoint() when the specified condition isn't met.

Does this solution work on any kind of exception? How do you know at the point where the exception is raised that it will not be catched? (Or can you completely capture the environment in Python? I could try to manually grab information, but it would be no 100% solution.)

For Rails I also had a solution that would remember the location where an exception was caused. You could then click "Retry with Breakpoint" and the web page would be reloaded, but with a breakpoint placed just before the point where the exception occured before.

This only worked for some cases, of course, so I am wondering if there is something in Python that makes doing this easier.

ruby-breakpoint has similar trouble, by the way. There is no stepping in there.

In your sample things are still reasonable well (you will get an interactive session and when you're done with it you'll get the next one until the loop is exhausted), but there's still room of improvement there.

Adding stepping is on my list for the next significant release (as well as a GUI debugger client), but for now there is no really good way.

And wow, your Ajax client is very cool. All I have is a terminal one. Great work. :)

# Florian Gross

How it works: in Python you can do:

try:
    do stuff
except:
    exc_type, exc, tb = sys.exc_info()

That tb object is basically one line in the traceback, linked to the previous line by tb.tb_next. The tb object is pretty boring, but it has a link to the frame (tb.tb_frame). The frame object contains the local and global variables, so you can evaluate expressions in the context of those variables. I stuff the entire set of frames into a module-global variable, so that it doesn't disappear as long as you don't restart the server (this in effect leaks substantial memory, but this should only be enabled for debugging), and all the interaction is keyed off where I put the frames (each set gets an id) and the specific frame (each frame gets an id). The interpreter is a basic Ajax interpreter, nothing too fancy.

The system also includes a way to annotate tracebacks, which was taken from Zope (written by Shane Hathaway I think), which uses magic local variables and the same introspection to find the annotations. So if you just define a local variable like __traceback_info__ = 'index.html line 10' then that will show up in the traceback. Currently that is just used for templating languages, where there's typically a disconnect between the template source code and the code being run (both in the case of interpreters and compilers). Currently only ZPT uses this (and I guess DTML) but it would be good if more did -- hopefully Kid will soon too.

With all of these, all the work is done only when an exception goes all the way up to the exception catcher (which is an application wrapper, aka middleware). So there's no real overhead to using these either. And they are complimentary to the normal way exceptions work, so show a very accurate indication of the state and real context of the exception -- a lot of other attempts to make exceptions better for template code covers up errors above or below the template.

Anyway, that's how it works. There's also lots of people that have run their applications under pdb, which may be more similar to how ruby-breakpoint works. Pdb is a traditional debugger and uses a special mode of the Python interpreter to do its thing (sys.settrace()). I've used pdb here and there, but somehow it could never catch for me; winpdb looks considerably easier to use, but I haven't tried it.

# Ian Bicking

Ah, Ruby doesn't have a way of going from an exception to the frames it passed through (called bindings in Ruby).

Having that makes all this quite straightforward. Still quite cool stuff.

What ruby-breakpoint uses is a hackish way of getting the breakpoint() caller's binding which will then be used for the debugging session.

pdb seems similar to debug.rb that comes with Ruby. It's more of a traditional debugger, but I found an interactive environment very natural to do this kind of thing.

Currently ruby-breakpoint doesn't require a permanent global trace function which is a good thing because those are quite slow in Ruby.

WinPDB looks nice. Already has some of the features I would like my GUI client to have.

Thanks for all the information. This has been very insightful!

# Florian Gross