[IronPython] Debugging hosted python scripts

Jeff Hardy jdhardy at gmail.com
Thu Mar 31 18:02:36 CEST 2011

On Wed, Mar 30, 2011 at 8:52 PM, Keith Rome <rome at wintellect.com> wrote:
> We currently have a line of business application, written entirely in C#,
> that embeds the IronPython runtime. We offer a GUI script editing
> environment (using the SyntaxEditor control from Actipro Software, which
> works great for this). This script editor exists as just another dialog
> window in our application where the user can extend the business objects
> behind the application in various ways. The scripts are stored in a
> database, not in files on the local file system. We have great support for
> syntax highlighting, compiler error “squiggles”, even Intelliprompt
> functionality. I am now building live debugging support into our script
> editor GUI, which is where I have run into some difficulty.
> I have been going down the path of using ScriptEngine.SetTrace() and
> inspecting frames in the callback. This works fine if I am not doing
> anything interactive. For example, dumping some information to
> Debug.WriteLine(). However what I really need (I think?) is to be able to
> suspend the script execution during a trace callback. I don’t see a clear
> way to do this though. The script runtime simply continues execution when my
> callback returns. I have done some work around running the debugged script
> on a background thread, and then blocking it during “breakpoint” callbacks –
> but these scripts are normally run within the UI thread because they
> interact with data structures that are often databound to UI controls, and
> running them from a background thread is becoming a minefield of
> cross-thread violations. I cannot simply run the script in the UI thread,
> because blocking in the trace callback would make the application
> unresponsive.

I think this is going to be your biggest issue with debugging - AFAIK
the Python engine is not designed to be suspendable; it relies on
.NET's normal thread suspension mechanism to handle that case. If the
scripts are on the UI thread, and the debugger is on the UI thread,
that 's going to be an issue.

Now, if the engine were running in interpreted mode (i.e. it doesn't
try to convert Python to IL), it would probably be possible to suspend
it without suspending the thread, but I have no idea how much work
that would be. (Heck, it might already support it - Dino?) My hunch is
that it wouldn't be a huge amount of work, but I'm not familiar with
the interpreter loop at all.

- Jeff

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